Cell Structure - Cell Organelles
1. What is cell theory?
Cell theory asserts that the cell is the constituent unit of living beings.
Before the discovery of the cell, it was not recognized that living beings were made of building blocks like cells.
The cell theory is one of the basic theories of Biology.
2. Are there living beings without cells?
Viruses are considered the only living beings that do not have cells. Viruses are constituted by genetic material (DNA or RNA) enwrapped by a protein capsule. They do not have membranes and cell organelles nor do they have self-metabolism.
3. In 1665 Robert Hooke, an English scientist, published his book Micrographia, in which he described that pieces of cork viewed under the microscope presented small cavities similar to pores which were filled with air. Based on later knowledge, of what were the walls of those cavities constituted? What is the historical importance of that observation?
The walls of the cavities observed by Hooke were the walls of the plant cells that form the tissue. The observation led to the discovery of the cells, a fact only possible after the invention of the microscope. In that work, Hooke established the term “cell”, now widely used in Biology, to designate those cavities seen under the microscope.
Cell Structure Review - Image Diversity: Hooke's cell
4. What are the two big groups into which cells are classified?
Cells can be classified as eukaryotic or prokaryotic.
Prokaryotic cell is that without a delimited nucleus. Eukaryotic cells are those with nucleus delimited by membrane.
Cell Structure Review - Image Diversity: eukaryotic cell prokaryotic cell
5. Do bacteria cells have a nucleus?
In bacteria the genetic material is dispersed in the cytosol and there is no internal membrane that delimits a nucleus.
6. Are there any bacteria made of more than one cell?
There are no pluricellular bacteria. All bacteria are unicellular prokaryotic.
7. What is the plasma membrane of the cell? What are its main functions?
The plasma membrane is the outer membrane of the cell, it delimits the cell itself and a cell interior with specific conditions for the cellular function. Since it is selectively permeable, theplasma membrane has an important role for the passage of substances inwards or outwards.
Cell Structure Review - Image Diversity: cell membrane
8. What are the chemical substances that compose the plasma membrane?
The main constituents of the plasma membrane are phospholipids, proteins and carbohydrates. The phospholipds, amphipathic molecules, are regularly organized in the membrane according to their polarity: two layers of phospholipids form the lipid bilayer with the polar part of the phospholipids pointing to the exterior of the layer and the non-polar phospholipid chains in the interior. Proteins can be found embedded in the lipid bilayer and there are also some carbohydrates bound to proteins and to phospholipids in the outer face of the membrane.
9. What is the difference between plasma membrane and cell wall?
Plasma membrane and cell wall are not the same thing. Plasma membrane, also called cell membrane, is the outer membrane common to all living cells and it is made of a phospholipid bilayer, embedded proteins and some appended carbohydrates.
Because cell membranes are fragile, in some types of cells there are even outer structures that support and protect the membrane, like the cellulose wall of plant cells and the chitin wall of some fungi cells. Most bacteria also present an outer cell wall made of peptidoglycans and other organic substances.
Cell Structure Review - Image Diversity: cell wall
10. What are the main respective constituents of cell walls in bacteria, protists, fungi and plants?
In bacteria the cell wall is made of peptidoglycans; among protists algae have cell walls made of cellulose; in fungi, the cell wall is made of chitin (the same substance that makes the exoskeleton of arthropods); in plants, the cell wall is made of cellulose too.
11. Do membranes form only the outer wrapping of cells?
Lipid membranes do not form only the outer cover of cells. Cell organelles, such as the Golgi complex, mitochondria, chloroplasts, lysosomes, the endoplasmic reticula and the nucleus, are delimited by membranes too.
Cell Structure Review - Image Diversity: cell nucleus
12. Which type of cell came first in evolution - the eukaryotic cell or the prokaryotic cell?
This is an interesting problem of biological evolution. The most accepted hypothesis asserts that the more simple cell, the prokaryotic cell, appeared earlier in evolution than the more complex eukaryotic cell. The endosymbiotic hypothesis, for example, affirms that aerobic eukaryotic cells appeared from the mutualist ecological interaction between aerobic prokaryotes and primitive anaerobic eukaryotes.
13. Concerning the presence of the nucleus what is the difference between animal and bacterial cells?
Animal cells (cells of living beings of the kingdom Animalia) have an interior membrane that delimits a cell nucleus and thus they are eukaryotic cells; in these cells thegenetic material is located within the nucleus. Bacterial cells (cells of living beings of the kingdom Monera) do not have organized cellular nuclei and so they are prokaryotic cells and theirgenetic material is found dispersed in the cytosol.
14. What are the three main parts of a eukaryotic cell?
The eukaryotic cell can be divided into two main portions: the cell membrane that separates the intracellular space from the outer space physically delimiting the cell; the cytoplasm, the interior portion filled with cytosol (the aqueous fluid inside the cell); and the nucleus, the membrane-delimited internal region that contains thegenetic material.
15. What are the main structures within the cell nucleus?
Within the cell nucleus the main structures are: the nucleolus, an optically dense region, spherical shaped, where there are concentrated ribosomal RNA (rRNA) associated to proteins (there may be more than one nucleolus in a nucleus); the chromatin, made of DNA molecules dispersed in the nuclear matrix during the cell interphase; the karyotecha, or nuclear membrane, the membrane that delimits the nucleus.
16. What are the substances that constitute the chromatin? What is the difference between chromatin and chromosome?
The chromatin, dispersed in the nucleus, is a set of filamentous DNA molecules associated to nuclear proteins called histones. Each DNA filament is a double helix of DNA and thus a chromosome.
17. What is the fluid that fills the nucleus called?
The aqueous fluid that fills the nuclear region is called karyolymph, or nucleoplasm. In the fluid there are proteins, enzymes and other important substances for the nuclear metabolism.
18. Of what substances is the nucleolus made? Is there a membrane around the nucleolus?
Nucleolus is a region within the nucleus made of ribosomal RNA (rRNA) and proteins. It is not delimited by membrane.
19. What is the name of the membrane that delimits the nucleus? To which component of the cell structure is that membrane contiguous?
The nuclear membrane is also called karyotheca. The nuclear membrane is continuous to the endoplasmic reticulum membrane.
20. What are the main cytoplasmic structures present in animal cells?
The main cytoplasmic structures of the cell are the centrioles, the cytoskeleton, lysosomes, mitochondria, peroxisomes, the Golgi apparatus, the endoplasmic reticula and ribosomes.
Cell Structure Review - Image Diversity: cell organelles
21. What are cytoplasmic inclusions?
Cytoplasmic inclusions are cytoplasmic molecular aggregates, such as pigments, organic polymers and crystals. They are not considered cell organelles.
Fat droplets and glycogen granules are examples of cytoplasmic inclusions.
Cell Structure Review - Image Diversity: cytoplasmic inclusions
22. Where in the cell can ribosomes be found? What is the main biological function of ribosomes?
Ribosomes can be found free in the cytoplasm, adhered to the outer side of the nuclear membrane or associated to the endoplasmic reticulum membrane defining the rough endoplasmic reticulum.Ribosomes are the structures where protein synthesis takes place.
Cell Structure Review - Image Diversity: ribosomes
23. What is the difference between smooth and rough endoplasmic reticulum?
The endoplasmic reticulum is a delicate membranous structure contiguous to the nuclear membrane and present in the cytoplasm. It forms an extensive net of channels throughout the cell and is classified into rough or smooth types.
The rough endoplasmic reticulum has a great number of ribosomes attached to the external side of its membrane. The smooth endoplasmic reticulum does not have ribosomes attached to its membrane.
The main functions of the rough endoplasmic reticulum are synthesis and storage of proteins made in the ribosomes. The smooth endoplasmic reticulum plays a role in the lipid synthesis and, in muscle cells, it is important in the conduction of the contraction stimulus.
Cell Structure Review - Image Diversity: endoplasmic reticulum
24. A netlike membranous complex of superposed flat saccules with vesicles detaching from the extremities seen in electronic microscopy. What is the observed structure? What is its biological function?
What is being observed is the Golgi complex, or Golgi apparatus. This cytoplasmic organelle is associated with chemical processing and modification of proteins made by the cell and with storage and branding of these proteins for posterior use or secretion. Vesicles seen under the electronic microscope contain material already processed, ready to be exported (secreted) by the cell. The vesicles detach from the Golgi apparatus, travel across the cytoplasm and fuse with the plasma membrane then secreting their substances to the exterior.
Cell Structure Review - Image Diversity: Golgi apparatus
25. On which organelle of the cell structure does intracellular digestion depend? What is the chemical content of those organelles?
Intracellular digestion occurs by the action of lysosomes. Lysosomes have digestive enzymes (hydrolases) that are made in the rough endoplasmic reticulum and stored in the Golgi apparatus. Lysosomes are hydrolase-containing vesicles that detach from the Golgi apparatus.
26. Why are lysosomes known as “the cleaners” of the cell waste?
Lysosomes carry out autophagic and heterophagic digestion: autophagic digestion by digesting residual substances from the cellular metabolism; heterophagic digestion by digesting substances that enter the cell. Lysosomes enfold the substances to be degraded forming digestive vacuoles, or residual vacuoles, that later migrate toward the plasma membrane fusing with it and liberating (exocytosis) the digested material to the exterior.
Cell Structure Review - Image Diversity: lysosomes
27. Which are the cell organelles that participate in cell division and in the formation of cillia and flagella of some eukaryotic cells?
The organelles that participate in the cell division and in the formation of cilia and flagella of some eukaryotic cells are the centrioles. Some cells have cillia (paramecium, the bronchial ciliated epithelium, etc.) or flagella (flagellate protists, sperm cells, etc.); these cell structures are composed of microtubules originated from the centrioles. Centrioles also make the aster microtubules that are very important for cell division.
Cell Structure Review - Image Diversity: centrioles
28. What are the morphological, chemical and functional similarities and differences between lysosomes and peroxisomes?
Similarities: lysosomes and peroxisomes are small membranous vesicles that contain enzymes and enclose residual substances from internal or external origin degrading them. Differences: lysosomes have digestive enzymes (hydrolases) that break substances to be digested into small molecules; peroxisomes contain enzymes that degrade mainly long-chained fatty acids and amino acids and that inactivate toxic agents including ethanol; within peroxisomes there is the enzyme catalase, responsible for the oxidation of organic compounds by hydrogen peroxide (H2O2) and, when this substance is in excess, by the degradation of the peroxide into water and molecular oxygen.
29. What are mitochondria? What is the basic morphology of these organelles and in which cells can they be found?
Mitochondria are the organelles in which the most important part of the cellular respiration occurs: the ATP production.
Mitochondria are organelles delimited by two lipid membranes. The inner membrane invaginates to the interior of the organelle forming cristae that delimitate the internal space known as mitochondrial matrix and where mitochondrial DNA (mtDNA), mitochondrial RNA (mt RNA), mitochondrial ribosomes and respiratory enzymes can be found. Mitochondria are numerous in eukaryotic cells and they are even more abundant in those cells that use more energy, like muscle cells. Because they have their own DNA, RNA and ribosomes, mitochondria can self-replicate.
Cell Structure Review - Image Diversity: mitochondria
30. Why can mitochondria be considered the power plants of the aerobic cells?
Mitochondria are the “power plants” of aerobic cells because within them the final stages of the cellular respiration process occurs. Cellular respiration is the process of using organic molecule (mainly glucose) and oxygen to produce carbon dioxide and energy. The energy is stored in the form of ATP (adenosine triphosphate) molecules and later used in other cellular metabolic reactions. In mitochondria the two last steps of the cellular respiration take place: the Krebs cycle and the respiratory chain.
31. What is the endosymbiotic hypothesis about the origin of mitochondria? What are the molecular facts that support the hypothesis? To which other cellular organelles can the hypothesis also be applied?
It is presumed that mitochondria were primitive aerobic prokaryotes that were engulfed in mutualism by primitive anaerobic eukaryotes, receiving protection from these beings and offering energy to them. This hypothesis is called the endosymbiotic hypothesis on the origin of mitochondria.
The hypothesis is strengthened by some molecular evidence such as the fact that mitochondria have their own independent DNA and protein synthesis machinery, with their own RNA and ribosomes, and that they can self-replicate.
The endosymbiotic theory can be applied to chloroplasts too. It is supposed that these organelles were primitive photosynthetic prokaryotes because they have their own DNA, RNA and ribosomes and they can self-replicate too.
32. What are the main components of the cytoskeleton?
The cytoskeleton is a network of very small tubules and filaments distributed throughout the cytoplasm of eukaryotic cells. It is made of microtubules, microfilaments and intermediate filaments.
Microtubules are formed by molecules of a protein called tubulin. Microfilaments are made of actin, the same protein that participates in the contraction of muscle cells. Intermediate filaments are made of protein too.
Cell Structure Review - Image Diversity: cytoskeleton
33. What are the functions of the cytoskeleton?
As the name indicates, the cytoskeleton is responsible for the support of the normal shape of the cell; it also acts as a facilitator for substance transport across the cell and for the movement of cellular organelles. For example, the sliding between actin-containing filaments and the protein myosin creates pseudopods. In cells of the phagocytic defense system, like macrophages, cytoskeleton is responsible for the plasma membrane projections that engulf the external material to be interiorized and attacked by the cell.
34. What are chloroplasts? What are the main function of chloroplasts?
Chloroplasts are organelles present in the cytoplasm of plant and algae cells. Like mitochondria, chloroplasts have two boundary membranes and many internal membranous sacs. Within the organelle there are DNA, RNA and ribosomes and also the pigment chlorophyll, responsible for absorption of photic energy that is used in photosynthesis.
The main function of chloroplasts is photosynthesis: the production of highly energetic organic molecules (glucose) from carbon dioxide, water and light.
Cell Structure Review - Image Diversity: chloroplasts
35. What is the molecule responsible for the absorption of photic energy for photosynthesis? Where is that molecule located in photosynthetic cells?
The chlorophyll molecules are responsible for the absorption of light energy for photosynthesis. These molecules are found in the internal membranes of chloroplasts.
36. What are the colors (of the electromagnetic spectrum) absorbed by plants? What would happen to photosynthesis if the green light waves that reach a vegetable were blocked?
Chlorophyll absorbs all other colors of the electromagnetic spectrum but it practically does not absorb the green. The green color is reflected and such reflection provides the characteristic color of plants. If the green light that reaches a plant is blocked and exposure of the plant to other colors is maintained there would be no harm to the photosynthesis process. Apparent paradox: the green light is not important for photosynthesis.
There is a difference between the optimum color frequency for the two main types of chlorophyll, the chlorophyll A and the chlorophyll B. Chlorophyll A has an absorption peak at approximately 420 nm wavelength (anil) and chlorophyll B has its major absorption in 450 nm wavelength (blue).
Cell Structure Review - Image Diversity: electromagnetic spectrum
37. What is the path followed by the energy absorbed by plants to be used in photosynthesis?
The energy source of photosynthesis is the sun, the unique and central star of our planetary system. In photosynthesis the solar energy is transformed into chemical energy, the energy of the chemical bonds of the produced glucose molecules (and of the released molecular oxygen). The energy of glucose is then stored as starch (a glucose polymer) or it is used in the cellular respiration process and transferred to ATP molecules. ATP is consumed in metabolic processes that spend energy (for example, in active transport across membranes).
38. Of what substance is the plant cell wall made? Of which monomer is it made?
The plant cell wall is made of cellulose. Cellulose is a polymer whose monomer is glucose. There are other polymers of glucose, like glycogen and starch.
Cell Structure Review - Image Diversity: plant cell wall
39. What is the function of the plant cell wall?
The plant cell wall has structural and protective functions. It plays an important role in the constraint of the cell size, preventing the cell to break when it absorbs a lot of water.
40. What are plant cell vacuoles? What are their functions? What is the covering membrane of the vacuoles called?
Plant cell vacuoles are cell structures delimited by membranes within which there is an aqueous solution made of various substances like carbohydrates and proteins. In young plant cells many small vacuoles can be seen; within adult cells the most part of the internal area of the cell is occupied by a central vacuole.
The main function of the vacuoles is the osmotic balance of the intracellular space. They act as “an external space” inside the cell. Vacuoles absorb or release water in response to the cellular metabolic necessities by increasing or lowering the concentration of osmotic particles dissolved in the cytosol. Vacuoles also serve as a storage place for some substances.
The membrane that delimits the vacuoles is called tonoplast, named after the osmotic function of the structure.
Cell Membrane - Biology Q&A
1. What is a membrane?
Membrane is any delicate sheet that separates one region from another blocking or permitting (selectively or completely) the passage of substances. The skin, for example, can be considered amembrane that separates the exterior from the interior of the body; cellophane, used in chemical laboratories to separate solutions, acts as amembrane too.
2. Concerning their permeability how are membranes classified?
Membranes can be classified as impermeable, permeable, semipermeable or selectively permeable.
An impermeable membrane is that through which no substance can pass. Semipermeable membranes are those that let only solvents, like water, to pass through it. Permeable membranes are those that let solvent and solutes, like ions and molecules, to pass across it. There are also selectively permeable membranes, i.e., membranes that besides allowing the passage of solvent, let only some specific solutes to pass while blocking others.
3. What is diffusion?
Diffusion is the spreading of substance molecules from a region where the substance is more concentrated to another region where it is less concentrated. For example, during the boiling of water in a kitchen gaseous water particles tend to uniformly spread in the air by diffusion.
4. What is meant by concentration gradient? Is it correct to refer to “concentration gradient of water”?
Concentration gradient is the difference of concentration of a substance between two regions.
Concentration is a term used to designate the quantity of a solute divided by the total quantity of the solution. Since water in general is the solvent in this situation it is not correct to refer to “concentration of water” in a given solution.
5. What is the difference between osmosis and diffusion?
Osmosis is the phenomenon of movement of solvent particles (in general, water) from a region of lower solute concentration to a region of higher solute concentration. Diffusion, on the other hand, is the movement of solutes from a region of higher solute concentration to a region of lower solute concentration.
One can consider osmosis as movement of water (solvent) and diffusion as movement of solutes, both concentration gradient-driven.
6. What is osmotic pressure?
Osmotic pressure is the pressure created in an aqueous solution by a region of lower solute concentration upon a region of higher solute concentration forcing the passage of water from that to this more concentrated region. The intensity of theosmotic pressure (in units of pressure) is equal to the pressure that is necessary to apply in the solution to prevent its dilution by the entering of water by osmosis.
It is possible to apply in the solution another pressure in the contrary way to the osmotic pressure, like the hydrostatic pressure of the liquid or the atmospheric pressure. In plant cells, for example, the rigid cell wall makes opposite pressure against the tendency of water to enter when the cell is put under a hypotonic environment. Microscopically, the pressure contrary to theosmotic pressure does not forbid water to pass through a semipermeable membrane but it creates a compensatory flux of water in the opposite way.
7. Can solutions with the same concentration of different solutes have different osmotic pressures?
The osmotic pressure of a solution does not depend on the nature of the solute, it depends only on the quantity of molecules (particles) in relation to the total solution volume. Solutions with same concentration of particles even containing different solutes exert the sameosmotic pressure.
Even when the solution contains a mixture of different solutes its osmotic pressure depends only on its total particle concentration regardless of the nature of the solutes.
8. How are solutions classified according to their comparative tonicity?
Comparative to another, a solution can be hypotonic (or hyposmotic), isotonic (or isosmotic) or hypertonic (or hyperosmotic).
When a solution is less concentrated than another the adjective hypotonic is given and the more concentrated is called hypertonic. When two compared solutions have the same concentration both receive the adjective isotonic. So this classification makes sense only for comparison of solutions.
9. Concerning permeability what type of membrane is the cell membrane?
The cell membrane is a selectively permeable membrane, i.e., it allows the passage of water and some selected solutes.
Cell Membrane Review - Image Diversity: cell membrane
10. What are the basic constituents of the cell membrane?
The cell membrane is formed of lipids, proteins and carbohydrates.
The membrane lipids are phospholipids, a special type of lipid to which one extremity a phosphate group is bound thus assigning electrical charge to this region of the molecule. Since phospholipids have one electrically charged extremity and a long neutral organic chain they can organize themselves in two layers of associated molecules: the hydrophilic portion (polar) of each layer faces outwards in contact with water (a polar molecule too) of the extracellular and the intracellular space and the hydrophobic chains (non polar) face inwards isolated from the water. Because this type ofmembrane is made of two phospolipid layers it is also called a bilipid membrane.
Membrane proteins are embedded and dispersed in the compact bilipid structure. Carbohydrates appear in the outer surface of the membrane associated to some of those proteins under the form of glycoproteins or bound to phospholipids forming glycolipids. The membrane carbohydrates form the glycocalix of the membrane.
This description (with further explanations) is known as the fluid mosaic model about the structure of the cell membrane.
Cell Membrane Review - Image Diversity: phospholipid bilayer membrane proteins glycocalyx
11. What are the respective functions of phospholipids, proteins and carbohydrates of the cell membrane?
Membrane phospholipids have a structural function, they form the bilipid membrane that constitutes the cell membrane itself.
Membrane proteins have several specialized functions. Some of them are channels for substances to pass through the membrane, others are receptors and signalers of information, others are enzymes, others are cell identifiers (cellular labels) and there are still those that participate in the adhesion complexes between cells or between the internal surface of themembrane and the cytosketeleton.
Membrane carbohydrates, associated to proteins or to lipids, are found in the outer surface of the cell membrane and they have in general labeling functions for recognition of the cell by other cells and substances (for example, they differentiate red blood cells in relation to the ABO blood group system), immune modulation functions, pathogen sensitization functions, etc.
12. What are differentiations of the cell membrane?
In some types of cells, the cell membrane presents differentiations that are necessary for the specific functions of the cells. The main differentiations are the microvilli and the structures for reinforcement of adhesion or union between cells (cell junctions).
Microvilli are multiple external projections of the membrane resembling glove fingers. This differentiation is found in cells of tissues where it is advantageous to increase the size of the surface area in contact with the exterior, for example, in the enteric (intestinal) epithelium for absorption of nutrients.
Membrane differentiations for reinforcement of adhesion between cells occur mainly in epithelial tissues where the need for coverage and impermeability requires cells to be “glued” to neighboring cells. These differentiations can be interdigitations, desmosomes, tight junctions (zonula occludens), zonula adherens (adherens junctions) and gap junctions.
13. What is the relationship between concentration gradient and active and passive transport?
Passive transport is the movement of substances across membranes in favor of their concentration gradient, i.e., from a more concentrated region to a less concentrated region. Active transport, on the other hand, is the transport of substances across membranes against their concentration gradient, from a less concentrated to a more concentrated region. In passive transport, because it is spontaneous, there is no energy spent; the active transport however requires energy (work) to occur.
Active transport works to maintain or increase the concentration gradient of a substance between two regions while passive transport acts in a manner to reduce the concentration gradient.
14. What are the three main types of passive transport?
The three main types of passive transport are simple diffusion, osmosis and facilitated diffusion.
Cell Membrane Review - Image Diversity: passive transport
15. What is the energy source used in active transport through biological membranes?
The energy necessary for active transport (against the concentration gradient of the transported substance) to occur comes from ATP molecules. The active transportation uses chemical energy from ATP.
16. What is the difference between simple and facilitated diffusion? Facilitated by which type of molecule does the term “facilitated” mean?
Simple diffusion is the direct passage of substances across the membrane in favor of their concentration gradient. In facilitated diffusion the movement of substances is also in favor of their concentration gradient but the substances move bound to specific molecules that act as “permeabilizers”, i.e., facilitators of their passage through the membrane.
Cell Membrane Review - Image Diversity: facilitated diffusion
17. How does the intensity of simple diffusion vary in relation to the concentration gradient of the moved substance?
The higher the concentration gradient of a substance the more intense its simple diffusion will be. If the concentration gradient diminishes the intensity of simple diffusion diminishes too.
18. How does the intensity of facilitated diffusion vary in relation to the concentration of the moved substance? What is the limiting factor?
Like simple diffusion facilitated diffusion is more intense when the concentration gradient of the substance increases and less intense when the gradient lessens. In facilitated diffusion however there is a limiting factor: the quantity of the permeases that facilitate the transport through the membrane. Even in a situation in which the concentration gradient of the diffusing substance increases, if there are not enough permeases to perform the transport there will be no increase in the intensity of the diffusion. This situation is called saturation of the transport proteins and it represents the point at which the maximum transport capacity of the substance across the membrane is achieved.
19. Without saturation of transport proteins and under the same concentration gradient how can the speed of simple diffusion be compared to the speed of facilitated diffusion?
The action of facilitator proteins in facilitated diffusion makes this type of diffusion faster than simple diffusion under equal concentration gradients of the moved substance.
20. How does facilitated diffusion present similarities with enzymatic chemical reactions?
One of the main examples of facilitated transport is the entrance of glucose from the blood into cells. Glucose from blood binds to specific permeases (hexose-transporting permeases) present in the cell membrane and by diffusion facilitated by these proteins it enters the cell to play its metabolic functions.
Facilitated diffusion resembles chemical catalysis because the transported substances bind to permeases like substrates bind to enzymes and in addition, after one transport job is concluded, the permease is not consumed and can perform other successive transports.
21. What are some examples of biological activities in which osmosis plays an important role?
Hemolysis (destruction of red blood cells) by entrance of water, the hydric regulation in plants and the entrance of water in the xylem of vascular plants are all examples of biological phenomena caused by osmosis.
Excessive dilution of the blood plasma causes, by osmosis, the entrance of too much water into red blood cells and then the destruction of these cells (hemolysis). Osmosis is also the main process for maintenance of the flaccid, turgid or plasmolytic states of plant cells. Osmosis is one of the forces responsible for the entrance of water into plant roots since root cells are hypertonic in comparison to the soil.
22. What do facilitated diffusion and active transport have in common? What are the differences between them?
Facilitated diffusion can be confused with active transport because in both processes there is participation of membrane proteins.
In active transport however the transported substance moves against its concentration gradient and with energy spent. Facilitated diffusion is a passive transport in favor of the concentration gradient and it does not require energy.
Cell Membrane Review - Image Diversity: active transport
23. Which are the molecules that make possible active transport through membranes?
Active transport is made by specific membrane proteins. These proteins are called “pumps” because they “pump” the moving substance through the membrane using energy from ATP molecules.
24. How does the sodium-potassium pump present in the cell membrane work? What is the importance of this protein for the cell?
The sodium-potassium pump is the transport protein that maintains the concentration gradient of these ions between the intra and the extracellular spaces. This protein is phosphorylated in each pumping cycle and then it pumps three sodium ions outside the cell and puts two potassium ions inwards. The phosphorylation is made by the binding of a phosphate donated by one ATP molecule that then is converted into ADP (adenosine diphosphate).
The job of the sodium-potassium pump, also known as sodium-potassium ATPase, is fundamental to keep the characteristic negative electrical charge in the intracellular side of the membrane of the resting cell and to create adequate conditions of sodium and potassium concentrations inside and outside the cell to maintain the cellular metabolism.
Cell Membrane Review - Image Diversity: sodium-potassium pump
25. What is mass transportation across the cell membrane?
Mass transportation is the entrance or the exiting of substances in or from the cell engulfed by portions of membrane. The fusion of internal substance-containing membranous vesicles with the cell membrane is called exocytosis. The entrance of substances into the cell after they have been engulfed by projections of the membrane is called endocytosis.
26. What are the two main types of endocytosis?
Endocytosis is the entrance of material in the cell engulfed by portions of the cell membrane.
Endocytosis can be classified as pinocytosis or phagocytosis. In pinocytosis small particles on the external surface of the membrane stimulate the invagination of the membrane inwards and vesicles full of that particles then detach from the membrane and enter the cytoplasm. In phagocytosis bigger particles on the external surface of the membrane induce the projection of pseudopods outwards enclosing the particles; the vesicle then detaches from the membrane and enters the cytoplasm receiving the name phagosome.
Cell Membrane Review - Image Diversity: pynocitosis phagocytosis
27. How does the plant cell wall react when it is placed under hypotonic medium?
The plant cell wall (the covering of the cell external to the cell membrane) is made of cellulose, a polymer of glucose.
When the cell is put under hypotonic medium it absorbs too much water through osmosis. In that situation the cell wall pressure acts to compensate the osmotic pressure thus forbidding excessive increase of the cellular volume and the cell lysis.
28. What is meant by suction force of the plant cell? Does the suction force facilitate or make difficult the entrance of water into the cell?
The suction force (SF) is the osmotic pressure of the plant cell vacuole, i.e., of the vacuolar internal solution.
Since the vacuolar solution is hypertonic in comparison to cytosol it attracts water thus increasing the cytosol concentration. With the osmotic action of the vacuole the cytosol becomes hypertonic in relation to the exterior and more water enters the cell.
29. What is the wall resistance of plant cells? Does this resistance facilitate or make difficult the entrance of water into the cell?
Wall resistance, or turgor pressure (TP), is the pressure made by the distension of the plant cell wall in opposition to the increase of the cell volume. The wall resistance works against the entrance of water in the cell, i.e., it acts forcing the exiting of water and compensating the entrance of the solvent by osmosis.
30. What does the formula DPD = SF – TP mean?
DPD is the abbreviation of diffusion pressure deficit, SF (suction force) is the vacuolar osmotic pressure and TP is the turgor pressure.
The difference between SF and TP determines whether water tends or not to enter the cell. If SF > TP, DPD > 0 and water tends to enter the cell by osmosis. If TP > SF, DPD < 0 and water cannot enter the cell by osmosis.
31. What are the values of DPD for plant cells under hypertonic, isotonic and hypotonic media?
In plant cells under hypertonic medium there is loss of water for the exterior, SF > 0 (the vacuolar pressure is high because it is concentrated) and TP = 0 (there is no distension of the cell wall since the cellular volume is reduced) so DPD = SF. These cells are called plasmolysed cells, situation characterized by the retraction of the cell membrane that detach from the cell wall.
In plant cells under isotonic medium there is no increase of the internal water volume, SF > 0 and TP = 0 (since the cell wall is not distended). The cell membrane slightly touches the cell wall and in this situation the cell is called a flaccid cell.
In plant cells under hypotonic medium there is tendency of water to enter, SF = TP (since the osmotic pressure is totally compensated by the distension of the cell wall) and DPD = 0. The cell that has expanded itself to this point is called a turgid cell.
Cell Membrane Review - Image Diversity: plasmolysed cell flaccid cell turgid cell
32. What is the formula of the DPD for withered (shrunken) plant cells? How is that situation possible?
Withered plant cells are those that have shrunk due to loss of water by evaporation without enough replacement. In this situation the cell membrane retracts and detaches from the cell wall. The cell wall moreover expands in length to stimulate the entrance of water making TP < 0. Since DPD = SF – TP and TP is negative (< 0) its formula becomes DPD = SF +
TP
.
33. What is deplasmolysis of plant cells?
The plant cell when placed under hypertonic medium loses a great amount of water and its cell membrane detaches from the cell wall. In that situation the cell is called a plasmolysed cell. When the plasmolysed cell is placed under hypertonic medium it absorbs water and becomes a turgid cell. This phenomenon is called deplasmolysis.
34. Why are salt and sugar used in the production of dried meat and dried fruits?
Substances that maintain a highly hypertonic environment, like sugar and salt, are used in the production of dried meat, fruits or fish (for example, cod) because the material to be conserved is then dehydrated and the resulting dryness prevents the growth of populations of decomposer beings (since these beings also lose water and die).
Cell Skeleton and Cell Movement - Q&A
1. What is a cytoskeleton? What are its main constituents in animal cells?
Cytoskeleton is the cytoplasmic structure that supports the cell, keeps its shape and fixates and moves the cell organelles. It is made of an extensive network of fibers dispersed in the cytoplasm and anchored in the plasma membrane. Its components are microtubules, microfilaments and intermediate filaments.
Cell Skeleton and Cell Movement - Image Diversity: the "cell skeleton"
2. Of which substance are microtubules made? In which structures and cellular processes do microtubules participate?
Microtubules are made of consecutive dimers of the protein tubulin (each dimer has an alpha and a beta tubulin associated). Microtubules participate in cell division, they are constituents of cilia and flagella and they also form the centrioles.
Cytoskeleton and Cell Movement - Image Diversity: microtubules tubulin
3. Of which substance are microfilaments made? What are the properties of these elements that give motility to cells?
Microfilaments are made of actin (a protein). The contractile association of actin with myosin and other cytoplasmic proteins give to microfilaments the ability to promote cell movement.
Cytoskeleton and Cell Movement - Image Diversity: microfilaments actin and myosin intermediate filaments
4. What are cell movements? How are these movements created?
Cell movements are movements performed by cell structures, like the movements of cilia and flagella, the pseudopod movements (in amoeba, macrophages, etc.), the cyclosis of the cytoplasm and the sarcomere contraction in muscle cells.
Cell movements can be created by the cytoskeleton action, by differences of viscosity among cytoplasmic regions and by intracellular contraction systems.
5. What are cilia and flagella? How do these structures acquire movement? What are some examples of ciliated and flagellated cells in humans?
Cilia and flagella are structures found in some prokaryotes as well in some eukaryotic cells. They play defense, nutrition and movement roles for the cell. In eukaryotic cells of protists and animals they originate from centrioles that migrate towards the plasma membrane and differentiate into structures projected outside the cell. Each cilium or flagellum is made of nine peripheral pairs of microtubules and one central pair all covered by membrane. (In bacteria, flagella are made of a protein named flagellin and there can also be fimbria made of pilin.)
In the fixation base of each cilium or flagellum in the plasma membrane there are proteins that work as molecular motors providing movement for these structures with energy spending. Due to this energy spending ciliated or flagellated eukaryotic cells have a large number of mitochondria.
In humans ciliated cells can be found, for example, in the bronchial and tracheal epithelium. In these tissues the cilia have the defensive function of sweeping mucous and foreign substances that enter the airways. Sperm cells are a typical example of flagellated cells, their flagellum is the propulsion equipment for the movement towards the ovule.
Cytoskeleton and Cell Movement - Image Diversity: ciliated cell flagellate cell
6. How does the amoeboid movement occur? What are examples of beings and cells that use such movements for locomotion?
Amoeboid movements are created by cytoplasmic movements and plasma membrane projections called pseudopods. Their formation actively changes the external shape of some portions of the cell surface making it move along a substratum. Pseudopods appear from differences of viscosity among neighboring regions of cytoplasm near theplasma membrane and from the contractile action of microfilaments.
Amoeboid movements occur, for example, in amoebas (a protozoan), organisms that use their movement to find food. The leukocytes, cells of the immune system, when attracted by chemical substances (immune mediators) use amoeboid movements to get out from capillaries in regions of tissue damage to participate in the inflammatory process.
Cytoskeleton and Cell Movement - Image Diversity: pseudopods
7. What are some examples of movement created by the contraction of sarcomeres of the muscle cells?
The handling of a cup of coffee, the peristaltic movements of the bowels, the cardiac beats and even a smile are examples of movement created by contraction of the sarcomeres of the muscle cells. This contraction is a type of cell movement.
8. What is cyclosis?
Cyclosis is a type of internal cell movement in which an oriented flow of circulating material is created and maintained in the cytoplasm by the action of microfilaments. Cyclosis is more easily observed in plant cells.
Cytoskeleton and Cell Movement - Image Diversity: cyclosis
Q&A Review - Biology Knowledge
1. What is meant by cellular secretion?
Cell secretion is the elimination to the exterior of substances produced by the cell (for example, hormones, mucus, sweat, etc.)
2. Which cell organelles are well-developed in secretory cells?
In secretory cells, like the secretory cells of endocrine glands, organelles related to production, processing and “exportation” of substances are widely present and well-developed. These organelles are the rough endoplasmic reticulum and the Golgi apparatus.
The nuclear membrane of the secretory cells generally has more pores to allow the intense traffic of molecules related to protein synthesis between the cytoplasm and the nucleus.
rough endoplasmic reticulum Golgi apparatus
3. How do the rough endoplasmic reticulum and the Golgi apparatus act in the production and releasing of proteins?
The rough endoplasmic reticulum has in its outer membrane numerous ribosomes, structures where translation of messenger RNA and protein synthesis occur. These proteins are stored inthe rough endoplasmic reticulum and later they go to the Golgi apparatus. Within the Golgi apparatus proteins are chemically transformed and when ready they are put inside vesicles that detach from the organelle. These vesicles fuse with the plasma membrane (exocytosis) in the right place and its content is liberated outside the cell.
4. What are some examples of secretory cells?
Endocrine and exocrine pancreatic cells, thyroid and parathyroid endocrine cells, adenohypophysis, adrenal and pineal endocrine cells, the many types of gastric exocrine and endocrine cells, the mucus secretory cells of the lungs and of the bowels, the salivary gland cells, the lacrimal gland cells, the sebaceous gland cells, the secretory cells of the ovaries and testicles, etc., are all examples of secretory cells.
Cell Digestion Review
1. What is extracellular digestion?
Extracellular digestion is that in which food breaking into utile molecules that can be internalized by the cell is done in the extracellular space, i.e., outside the cell. In extracellular digestion, the cells secret substances that break big molecules into smaller ones in the external environment. Later the cell can benefit from these products of digestion.
2. What is intracellular digestion?
Intracellular digestion, or cellular digestion, is the breaking in the interior of the cell of big molecules coming from outside or even from its own cell metabolism into smaller molecules. Products and residues of the intracellular digestion are used by the cell or excreted.
Intracellular digestion is classified into two types: heterophagic intracellular digestion and autophagic intracellular digestion.
3. What is the main cell organelle involved in cell digestion? What are the properties of that organelle that enable it to do the task?
The organelles responsible for intracellular digestion are the lysosomes. Lysosomes are vesicles that contain digestive enzymes capable of breaking big molecules into smaller ones. These vesicles fuse with others that carry the material to be digested and then digestion takes place.
Cell Digestion Review - Image Diversity: lysosomes
4. What is heterophagic intracellular digestion? How is this process accomplished?
Heterophagic intracellular digestion is the breaking into smaller substances of external substances engulfed in the cell by pinocytosis or phagocytosis. Phagosomes or pinosomes fuse with lysosomes making the digestive vacuoles. Within the digestive vacuoles the molecules to be digested are hydrolyzed and the products of the digestion cross through the membrane and reach the cytoplasm or they are kept inside the vacuoles. The vacuole with residues from digestion is called residual body and by exocytosis it fuses with the plasma membrane and liberates its “waste” in the exterior space.
5. What is autophagic intracellular digestion? Why is this type of intracellular digestion intensified in an organism undergoing starvation?
Autophagic intracellular digestion is the cellular internal digestion of waste and residual materials. In general it is done by lysosomes.
Autophagic intracellular digestion is intensified in situations of starvation because in such condition the cell tries to obtain from its own constituent materials the nutrients necessary to stay alive.
6. What are some biological examples in which lysosomic enzymes play a fundamental role?
The remodelation of the osseous tissue, the function of acrosomes in sperm cells and the elimination of the tadpole tail are examples of biological processes in which lysosomic enzymes are key factors.
The bone is a tissue made of osteoblast-containing matrix (osteoblasts are the secretory cells of the osseous matrix), osteocytes (mature bone cells) and osteoclasts (the remodeling cells). Osteoclasts are responsible for the continual renovation of the osseous tissue since their lysosomic enzymes digest the osseous matrix.
The sperm acrosome, for carrying digestive enzymes within, is responsible for the perfuration of the egg cell membrane in the fertilization process. The acrosome, located in the anterior end of the sperm cell, is a specialized region of the Golgi apparatus that accumulates a great amount of digestive enzymes.
In tadpoles the tail regresses while the organism develops into an adult frog. This tissue destruction is a digestion of the tail's own cells and extracellular materials and it is made bylysosomes and their enzymes. The complete digestion of a cell by its own mechanisms is called autolysis, a type of apoptosis (cell suicide).
Cell Digestion Review - Image Diversity: sperm cell acrosome osteoclasts tadpole
Cell Nucleus - Questions and Answers
1. What are cells with a delimited nucleus called ? What are the main elements of the nucleus?
Cells with delimited nucleus are called eukaryotic cells. Organisms composed of one or more eukaryotic cells are called eukaryotes.
The mains elements of the nucleus are the chromatin (made of DNA molecules), the nucleolus, the karyolymph, or nucleoplasm, and the nuclear membrane (or karyotheca).
2. Do all eukaryotic cells have nucleus and only one nucleus?
There are eukaryotic cells without a nucleus and others with more than one nucleus. Osteoclasts, the cells responsible for resorption of the osseous matrix, for example, are multinucleate cells; striated muscle fibers are multinucleate too. Red blood cells are an example of enucleated specialized cells.
Cell Nucleus Review - Image Diversity: cell nucleus miltinucleate cells enucleated cells
3. Of which substances is chromatin made?
Chromatin is made of DNA molecules associated to proteins called histones.
Cell Nucleus Review - Image Diversity: chromatin
4. What are heterochromatin and euchromatin?
Chromatin is uncondensed nuclear DNA, the typical DNA morphology in interphase (the phase of the cell cycle in which the cells is not dividing itself). In this phase of the cell cycle chromatin can be found as heterochromatin, more condensed and dark (in electronic microscopy) portions of DNA molecules, and as euchromatin, less condensed and lighter portions of DNA molecules.
Since it is uncondensed the euchromatin is the biologically active portion of the DNA, i.e., the region that has active genes to be transcripted into RNA. The heterochromatin represents the inactive portions of the DNA molecule.
Cell Nucleus Review - Image Diversity: heterochromatin euchromatin
5. What is the relation between the concepts of chromatin and chromosome? Are euchromatin and heterochromatin part of chromosomes?
Every filament of chromatin is a complete DNA molecule (a complete double helix), i.e., a complete chromosome. A DNA molecule may form euchromatin and heterochromatin portions thus both are part of chromosomes.
Cell Nucleus Review - Image Diversity: chromosome structure
6. In the phase when the cell is not dividing (interphase) is there activity within the cell nucleus?
In the interphase there is intense metabolic activity in the cell nucleus: DNA is duplicating, euchromatin is being transcripted and RNA is produced.
7. How are the concepts of chromosome, chromatin and chromatids related? In which phase of the cell cycle does DNA duplicate?
Chromatin is a set of filamentous DNA molecules dispersed in the karyoplasm forming euchromatin and heterochromatin portions. Each chromatin filament is a complete chromosome (aDNA molecule , or double helix). The chromatin of the human somatic cell is formed by 46 DNA molecules (22 homologous chromosomes and 1 pair of sex chromosomes).
In interphase the cell prepares itself for division and duplication of DNA molecules occurs. The duplication of every DNA molecule forms two identical DNA double helix bound by a structure called centromere. In this phase each identical chromosome of these pairs is called chromatid. It is also during theinterphase that the chromatids begin to condensate assuming the thicker and shorter shape typical of chromosome illustrations. So the phase of the cell cycle in which DNA duplicates is theinterphase.
Some Biology textbooks call the chromosome a unique filament of chromatin as well as the condensed structure made of two identical chromatids after the DNA duplication. Rigorously the pair of identical chromatids bound in the centromere are two copies of the same chromosome and therefore they are two identical chromosomes (and not only one).
Cell Nucleus Review - Image Diversity: chromatids
8. What is the structure that maintains identical chromatids bound?
The structure that maintains identical chromatids bound is the centromere.
Cell Nucleus Review - Image Diversity: centromere
9. How is the chromosome region where the centromere is located called? How are chromosomes classified in relation to the position of their centromere?
The chromosome region where the centromere is located is called primary constriction. In microscopic view this region is narrower (a stricture) than most part of the chromosome.
According to the position of the primary constriction the chromosomes are classified as telocentric, acrocentric, submetacentric or metacentric.
10. What are the primary and the secondary constrictions of a chromosome? What is the other name given to the secondary constriction?
Primary constriction is the narrower region of a condensed chromosome where the centromere, the structure that unites identical chromatids, is located. Secondary constriction is a region similar to the primary constriction, narrower than the normal thickness of the chromosome too, and in general it is related to genes that coordinate the formation of the nucleolus and control the ribosomic RNA (rRNA) synthesis. For this reason the secondary contrictions (that can be one or more in chromosome) is called nucleolus organizer region (NOR).
11. What are homologous chromosomes? Which are the human cells that do not have homologous chromosomes?
Chromosomes contain genes (genetic information in the form of nucleotide sequences) that command the protein synthesis thus regulating and controlling the activities of the cell. In the nucleus of somatic cells of diploid beings every chromosome has its correspondent homologous chromosome, both containing alleles of the same genes related to same functions. This occurs because one chromosome of one pair comes from the father and the other comes from the mother of the individual. The chromosomes that form a pair with alleles of the same genes are called homologous chromosomes. In humans, there are 22 pairs of homologous chromosomes plus the pair of sex chromosomes (the sex chromosomes are partially homologous).
The only human cells that do not have homologous chromosomes are the gametes since during meiosis the homologous chromosomes are separated.
12. What is the difference between the concepts of karyotype and genome?
Genome is the set of DNA molecules that characterizes each living being or each species. The concept then includes the specific nucleotide sequence of the DNA molecules of each individual or species. Karyotype is the set of chromosomes of individuals of a given individual or species concerning morphology and number of each chromosome or pair of homologous.
Cell Nucleus Review - Image Diversity: karyotype
13. Can two normal individuals of the same species with sexual reproduction have identical genomes and identical karyotypes? How is the human karyotype usually represented?
Except for clones (individuals created from nucleus transplantation, like the Dolly sheep) and monozygotic twins, it is very improbable the genomes of two individuals of the same species and generated by sexual reproduction to be identical. Nevertheless the karyotypes of two normal individuals of the same species and of the same sex are always identical. The human normal karyotype is represented by the formula 44+XX for women and 44+XY for men.
14. What is the other name given to sex chromosomes? What is the function of sex chromosomes?
Sex chromosomes are also called allosomes (the other chromosomes that are not sex chromosomes are called autosomes).
Sex chromosomes get such name because they have genes that determine the sex (male or female) of an individual. Sex chromosomes also have genes related to other biological functions.
15. How many chromosomes does a human normal haploid cell have? How many chromosomes does a human normal diploid cell have? How many are the sex chromosomes within each of them?
The human haploid cell is the gamete (egg cell and sperm cell). The human gamete has 22 autosomes and 1 allosome, i.e., 23 chromosomes. The diploid cell is the somatic cell and it has 44 autosomes and 2 allosomes, i.e., 46 chromosomes.
Gametes have one sex chromosome and somatic cells have two sex chromosomes.
16. Do phylogenetically proximal species have cells with proximal chromosome counts?
The number of chromosomes typical of each species is proximal for phylogenetically proximal species (for example, orangutan, gorilla, chimpanzee and human). But it is not impossible that evolutionary distant species, like rat and oat, bears similar karyotypes and the same total number of chromosomes.
Even presenting equal number of chromosomes evolutionary distant species have radically different characteristics since the quantity and the sequence of nucleotides that compose their respective DNA molecules are quite different.
17. What is the nucleolus?
The nucleolus is a small and optically dense region in the interior of the cell nucleus. It is made of ribosomic RNA (rRNA) and proteins. One nucleus can have one or more nucleolus.
Cell Nucleus Review - Image Diversity: nucleolus
18. Of which structures is the nuclear membrane composed?
Eukaryotic cells have nucleus delimited by two juxtaposed membranes that continue with the membrane of the endoplasmic reticulum. The nuclear membrane, or karyotheca, presents pores through which substances pass. There are also ribosomes adhered to its external surface.
Cell Nucleus Review - Image Diversity: karyotheca
Cell Division Questions
1. What is mitosis? What is the importance of mitosis?
Mitosis is the process in which one eukaryotic cell divides into two cells identical to the parent cell (generally identical, since alterations in genetic material can occur, more or less organelles may be distributed between the daughter cells, etc.)
Mitosis is fundamental for asexual reproduction of eukaryotes, for the embryonic development, for the growth of pluricellular beings and for tissue renewal.
2. Why in some cases is mitosis a synonym of reproduction?
In some living beings asexual reproduction occurs by many means: binary division, schizogony, budding, grafting, etc. In asexual reproduction of eukaryotesmitosis is the mechanism by which the constituent cells of the new beings are made.
The term mitosis does not apply to prokaryotes since it involves nuclear division and eukaryotic structures.
3. What is the importance of mitosis for the embryonic development?
Every embryo grows from a single cell that suffers mitosis and generates other cells that also divide themselves by mitosis forming tissues and complete organs. The perfect regulation and control of each of those cell divisions are fundamental for the creation of a normal individual. Withoutmitosis the embryonic development would be impossible.
4. What are some examples of organs and tissues where mitosis is more frequent, less frequent or practically absent?
Generally in vertebrates mitosis is more frequent in tissues that require intense renewing due to their functions, like epithelial tissues and the bone marrow. In plants the meristem tissue has numerous cells undergoingmitosis.
Mitosis take place with low frequency in tissues of slow renovation, like the bones in adults and the connective tissues.
In some adult tissues mitosis is almost absent, like the nervous tissue and the striated muscle tissue (skeletal and cardiac). The nervous tissue develops from stimulus by development of new electrical networks between cells and the striated muscle tissue grows by cellular hypertrophy.
5. How does mitosis participate in the growth of pluricellular organisms?
All pluricellular beings grow with the increase in quantity of their cells. This increase is produced by mitosis (although some types of growth occur by cellular hypertrophy or by deposition of substances in interstitial spaces).
6. What is the uncontrolled mitotic process that occurs as disease in pluricellular beings called?
Uncontrolled mitotic cell division is called neoplasia. Neoplasia (the formation of new strange tissues) occurs when a cell suffers mutation in itsgenetic material, loses the ability to control its own division and the failure is transmitted to its descendants.
Cancers are malignant neoplasias. The term malignant means that neoplastic cells can disseminate to distant sites invading other organs and tissues. Neoplasias whose cells cannot disseminate to distant sites are called benign neoplasias.
7. Is the internal epithelium of the bowel the same as it was one month ago?
The internal epithelial covering of the intestine acts as protective barrier and also as means of nutrient absorption. The traffic of ingested material inside the intestinal lumen is very intense and the consequent tissue damage requires incessant epithelialrenovation through cell division. The tissue renovation is completed in two to three days and is made by mitosis.
8. What is cellular regeneration? How is mitosis related to this process?
Some tissues are able to regenerate when injured. The liver, for example, regenerates when small pieces of hepatic tissue are removed, bones make new tissues in fracture regions, etc. Some animals, like planarias, are capable of regenerating their bodies when sectioned. In tissue regeneration cellular proliferation happens bymitosis.
9. What is cell cycle?
Cell cycle, or mitotic cycle, is the time period that begins when the cell is created and finishes when it is divided by mitosis creating two daughter cells. The cell cycle is divided into interphase and the mitotic phase.
Cell Division Review - Image Diversity: cell cycle
10. Is cell division happening during the entire cell cycle? What is interphase?
Cell division properly occurs during the mitotic phase of the cell cycle. During interphase processes that are a preparation to cell division take place, like the duplication of DNA and centrioles. Interphase is the preceding phase and the mitotic is the following phase.
Cell Division Review - Image Diversity: interphase
11. What are the three periods into which interphase is divided?
Interphase is the preceding phase to the mitotic division. It is divided into three periods, G1, S and G2 (the letter G comes from “gap”, meaning interval or breach, and the letter S comes from “synthesis”, indicating the period in which DNA replicates).
In fact, “gap” is not totally appropriate for the periods immediately before and after the DNA synthesis. The idea of “growth” would be more adequate since in those periods (G1 and G2) the cell is growing to divide later inmitosis.
12. In general which phase of the cell cycle has longer duration?
The interphase comprises approximately 4/5 of the cell cycle and the mitotic phase has quite a shorter length.
13. What are the events that mark the beginning and the end of the first interphase period? What happens within the cell in this period?
The first interphase period is the G1. It begins with the end of the preceding cell division, i.e., with the formation of the new cell and it ends with the beginning of DNA replication. In the G1 period the cell is growing.
14. What are the events that mark the beginning and the end of the second interphase period? What happens in the cell in this period?
The second interphase period is the S. It starts with the beginning of DNA replication and finishes with the end of that process. The main event in this period is the synthesis of new polynucleotide chains, each bound to each DNA chain that served as a template, i.e., the duplication of the original set of DNA molecules.
15. What are the events that mark the beginning and the end of the third interphase period? What happens in the cell in this period?
The third interphase period is the G2. It begins with the end of DNA replication and ends with the beginning of the first period of the mitotic phase. On G2 the cell is growing too and the duplication of centrioles occurs (only in cells that have these structures).
16. Does mitosis properly occur before or after the interphase? Is it a mere “point of view” issue?
Mitosis must be considered a succeeding phase after interphase since this is a preparation step to mitosis. Thus it is not merely a point of view issue.
17. Into which periods is mitosis divided?
Mitosis is divided into four periods: prophase, metaphase, anaphase and telophase.
18. What are centrioles? In which type of cell are they present?
Centrioles are tiny cylindrical structures made of nine microtubule triplets. They appear in pairs in the cell. Centrioles participate in the making of cytoskeleton and of cilia and flagella. In cell division they play a role in the formation of the aster fibers.
Centrioles are structures present in animal cells, in most protists and in some primitive fungi. There are no centrioles in cells of superior plants and in general it is considered that plant cells do not have centrioles (although this is not entirely correct since some plants have centriole-containing cells).
The region where the centrioles are located is called the centrosome of the cell.
19. What are the main events of the first mitotic period?
The first mitotic period is prophase. During prophase the following events occur: migration of each centriole pair (centrioles were duplicated in interphase) to opposite cell poles; aster formation around the centriole pairs; formation of the spindle fibers between the two centriole pairs; end of chromosome condensation; disintegration of the nucleolus; breaking of the karyotheca; dispersion of condensed chromosomes in the cytoplasm; binding of chromosomes to the spindle fibers.
Cell Division Review - Image Diversity: mitosis prophase
20. What is the mitotic apparatus?
Mitotic apparatus is the set of aster fibers, radial structures around each centriole pair, plus the spindle fibers, fibers that extend across the cell between the two centriole pairs located in opposite cell poles. The mitotic apparatus appears in prophase and has important role in the orientation and gripping of chromosomes and other cellular elements causing them to separate and migrate to opposite cell poles.
Substances that disallow the formation of the mitotic apparatus, like colchicine, a molecule that binds to tubulin molecules and prevents the synthesis of microtubules, interrupt cell division. Colchicine is used to study chromosomes since it paralyzes mitosis when chromosomes are condensed and so are more easily viewed under the microscope.
21. What are the main events of the second mitotic period?
The second mitotic period is metaphase. In metaphase the following events occur: condensed chromosomes bind (in their centromere region) to the spindle fibers and get concentrated in the middle of the cell; the formation of the mitotic apparatus is completed. Metaphase ends with the breaking of the binding of identical chromatids and then anaphase begins.
Cell Division Review - Image Diversity: mitosis metaphase
22. What are the main events of the third mitotic period?
The third mitotic period is anaphase. In anaphase the following events occur: duplication and breaking of centromeres with separation of identical chromatids; traction (by the spindle fibers) of identical chromatids each to opposite cell poles; beginning of chromosome decondensation.
Cell Division Review - Image Diversity: mitosis anaphase
23. During mitotic anaphase is there separation of homologous chromosomes or separation of identical chromatids?
In the anaphase of mitosis the identical chromatids separate and complete pairs of homologous chromosomes continue to exist in each daughter cell. The separation of the homologous chromosomes occurs in the anaphase of the cell division by meiosis.
24. What are the main events of the final mitotic period?
The final mitotic phase is telophase. In telophase the following events occur: decondensation of chromosomes, each set located in opposite cell poles; karyotecha formation around each set of chromosomes forming two nuclei; destruction of the mitotic apparatus; reappearing of the nucleoli; beginning of cytokinesis (the division of cytoplasm to ultimately separate the new cells).
Cell Division Review - Image Diversity: mitosis telophase
25. What is the name of the cytoplasm division in the end of mitosis? What are the differences in this process between animal and plant cells?
Cytoplasm division occurs after telophase and it is called cytokinesis. In animal cells an invagination of the plasma membrane toward the cell center appears in the equator of the parent cell and then the cell is strangulated in that region and divided into two daughter cells. This type of division is called centripetal cytokinesis (from outside).
In plant cells the cytokinesis is not centripetal since the division happens from the inside. Membranous sacs full of pectin concentrate in the internal central region of the cell and propagate to the periphery toward the plasma membrane. The pectin-containing sacs fuse themselves and form a central structure called phragmoplast. On the phragmoplast cellulose deposition occurs and a true cell wall is created to separate the daughter cells. Plant cells thus present centrifugal cytokinesis.
The phragmoplast has “failures”, or pores, to permit cytoplasmic communications between the daughter cells. These openings are called plasmodesms.
Cell Division Review - Image Diversity: cytokinesis
26. Why is it important for chromosomes to be condensed during mitosis and decondensed during interphase?
During mitosis the main problem to be solved is the correct separation of chromosome sets between daughter cells. If chromosomes were decondensed long tiny fibers of DNA would be dispersed in cytoplasm after the karyotheca breaking and chromosomes could not be easily organized and pulled by the spindle fibers.
During interphase the function of chromosomes, i.e, of DNA molecules, is the synthesis of RNA and thus of proteins. For this task it is necessary for functional molecular regions to be decondensed (these regions form the euchromatin). During interphase in addition DNA replication occurs as a preparatory step for cell division. In this process it is fundamental for the exposition of DNA molecules to serve as templates to new DNA chains under production.
27. How does the quantity of genetic material vary within the cell during the sequential phases of the cell cycle?
The first period of the first phase (interphase) of the cell cycle is the G1, followed by S and G2 and then by the mitotic phase.
In G1 the ploidy (the quantity of DNA molecules in the cell) can be represented by the formula 2n (n is the number of DNA molecules in a gamete cell of a given species). In S DNA duplicates and the quantity of genetic material increases from 2n to 4n. In G2 that quantity is constant: 4n. After the mitotic phase the quantity of genetic material decreases to 2n in each daughter cell.
28. What are the differences between astral and anastral mitosis?
Astral mitosis is that in which there is formation of the aster, a structure made by the centrioles. Anastral mitosis is that in which there is no formation of the aster; it occurs in cells without centrioles, like plant cells (superior plants).
29. Can mitosis occur in haploid (n) cells? And in triploid cells?
The mitotic cell division can occur in haploid (n) cells, diploid (2n) cells, triploid (3n) cells, etc. Mitosis is a copying process that does not interfere with cell ploidy.
30. Concerning their final products (daughter cells and their ploidies) what are the differences between mitosis and meiosis?
In mitosis one cell, for example, with 2n chromosomes, duplicates its chromosomal set and divides generating two other cells, each with 2n chromosomes too. In meiosis, one diploid cell (2n) duplicates its chromosomes too, but four cells with n chromosomes are generated.
31. Concerning their biological function what is the difference between mitosis and meiosis?
The main biological function of mitosis is cellular multiplication, a fundamental process for the growth and development of multicellular organisms, tissue renewing, asexual reproduction, etc. The biological function of meiosis is gamete formation (in gametic meiosis) or spore formation (in sporic meiosis), i.e., the production of cells qualified for sexual reproduction with half the quantity of chromosomes compared to the original cell.
There is a special type of meiosis that happens in zygotes of some algae, protozoans and fungi. This meiosis, called zygotic meiosis, has the function of reducing to a half the number of chromosomes of adult individuals that will be formed from the zygote. In species with zygotic meiosis the adult individuals are haploid and they form gametes by mitosis. These gametes fuse in pairs with others and generate a diploid zygote that, then, undergoes meiosis to restitute the normal ploidy of adult individuals.
32. For the biological diversity is mitosis or meiosis the more important process?
Meiosis is the cell division process that allows the formation of gametes to sexual reproduction, with aleatory separation of each chromosome of the individual homologous pairs. These gametes can fecundate gametes from other individuals promoting combination of homologous chromosomes from different individuals. In that manner the chromosomal recombination provided by meiosis and sexual reproduction creates individuals with dissimilar genetic patrimony from their fathers and thus promotes biological diversity.
Some fungi species and plants, for example, present sporic meiosis, i.e., a structure where half of the chromosomes of the species is generated from meiosis. This structure, by mitosis, forms gametes. Even in this case diversity comes from meiosis. Meiosis then is the cell division process that in conjunction with genetic mutations is responsible for the biological diversity.
Even in species having zygotic meiosis the aleatory separation of homologous chromosomes in meiosis creates biological diversity.
33. What are the respective ploidies of gamete, zygote and somatic cells in a species with gametic meiosis?
Adopting as pattern an “x” quantity of chromosomes for gametes, zygotes will have 2x chromosomes and somatic cells will have 2x too.
34. Why is meiosis important for the maintenance of the normal quantity of chromosomes of a species with sexual reproduction?
A reduction to a half of the maximum normal quantity of chromosomes is mandatory in some phase of the life cycle of a species that reproduces sexually. If that could not happen in each generation, whenever a zygote is formed by fusion of gametes there would be duplication in the quantity of chromosomes in a geometric progression.
35. What is the difference between sexual spores and gametes? Do humans present sexual spores or gametes?
Sexual spores are structures generated from meiosis with ploidy (number of chromosomes) reduction to a half compared to the spore mother cell. Spores germinate and give existence to gametophytes, individuals that by mitosis form gametes. The meiosis that generates sexual spores is called sporic meiosis; it is, for example, the type of meiosis that occurs in plants.
Gametes are also cells with half the number of chromosomes of the normal cell of the species, but they are specialized in fecundation, the fusion with another gamete that generates the zygote, a cell with double the number of chromosomes than gametic cells. Gametes can appear from gametic meiosis or by mitosis in gametophytes originated from sexual spores.
In humans as well in most animals the meiosis is gametic. There are no spores nor alternation of generations. The male gamete is the sperm cell, and the female gamete is the egg cell.
36. Is the interphase of meiosis different from the interphase of mitosis?
The interphase that precedes meiosis is similar to the interphase that precedes mitosis. In them the main event is DNA replication (chromosome duplication).
37. What are the two divisions of meiosis? What are the main events that occur in those divisions?
Meiosis is divided into first meiotic division, or meiosis I, and second meiotic division, or meiosis II. During meiosis I the separation of homologous chromosomes occurs, with formation of two haploid cells. In meiosis II there is separation of identical chromatids of each of the two haploid cells created in meiosis I, giving birth to four haploid cells.
Meiosis II is a process identical to mitosis.
38. In which meiotic division does the separation of the homologous occur? What are the ploidies of the generated cells after the end of that process?
The separation of the homologous chromosomes occurs in the first division of meiosis, or meiosis I. After the end of this cell division two haploid cells are made, each having different chromosomes (with no set of homologous). Note that in the cells generated after meiosis I each chromosome is still duplicated since the homologous chromosomes and not the identical chromatids were separated.
Cell Division Review - Image Diversity: meiosis I
39. In which meiotic division does the separation of identical chromatids occur? After the end of this process what are the ploidies of the new cells?
The separation of identical chromatids occurs in the second meiotic division, or meiosis II. After this cell division (similar to mitosis and that does not alter ploidy) the cells are still haploid (they have become haploid after meiosis I).
Cell Division Review - Image Diversity: meiosis II
40. How many cells are made after meiosis I and meiosis II?
After meiosis I two cells with already separated homologous are created. After meiosis II four cells are created.
41. What are the periods of the first meiotic division?
Meiosis I is divided into prophase I, metaphase I, anaphase I and telophase I.
42. In which period of meiosis does the pairing of homologous chromosomes occur?
The pairing of homologous chromosomes is a vital step for meiosis because the rightness of the homologous separation depends on the process. This event occurs in prophase I of the cell division.
43. What is crossing over? In which period of meiosis does this event occur?
Crossing over is the eventual exchange of chromosomal fragments between homologous chromosomes. The phenomenon occurs in prophase I when homologous chromosomes are paired. Crossing over is of great importance for evolution and biodiversity since it provides recombination of alleles (of different genes) linked in the same chromosome during cell divison by meiosis.
Cell Division Review - Image Diversity: crossing over
44. What are the “chiasms” of homologous chromosomes seen in prophase I?
Chiasms are intersections of two tracts in the form of X.
The chiasms seen in prophase I are chromosome arms crossing over same arms of their homologous. In fact when chiasms are seen under the microscope chromatids are exchanging chromosomal segments with other chromatids of its homologous.
45. Is there interphase again between meiosis I and meiosis II?
There is no interphase nor DNA duplication between the divisions of meiosis. Only a short interval called diakinesis occurs.
46. What are the periods of the second meiotic division?
Meiosis II is divided into prophase II, metaphase II, anaphase II and telofase II.
47. What are the respective functions of the separation of homologous chromosomes and of the separation of identical chromatids in meiosis?
The separation of homologous chromosomes in meiosis I has two main functions: to reduce to a half the total number of chromosomes, generating haploid daughter cells at the end of the process, and to make possible genetic recombination since the separation is aleatory, i.e., each pair of daughter cells can be different from the other pair relating chromosomal combination from paternal and maternal origins. (And if crossing over is considered each of the four resulting cells can be different from the others.)
The separation of identical chromatids in meiosis II has the same function it has in mitosis: to separate the chromosomes already duplicated to the daughter cells.
48. During which meiosis division does ploidy reduction occur? Does ploidy reduction occur in mitosis?
In the cell division by meiosis ploidy reduction occurs in meiosis II. Initially, taking as example a 2n somatic cell, ploidy increases to 4n (duplication of DNA) during interphase. During meiosis I, since homologous chromosomes are separated, ploidy falls to 2n (the original number) and then during meiosis II ploidy finally falls to n in the resulting daughter cells.
Ploidy reduction does not occur in mitosis. This fact shows that, although in meiosis ploidy is decreased from its original number, in meiosis II, a process similar to mitosis, the cause of that reduction is what happens in meiosis I, i.e., the separation of the homologous chromosomes.
The Photosynthesis Process - Q&A Review
1. What is the primary source of energy for living beings on earth?
The sun, center of our planetary system and star of the milky way galaxy (our galaxy), is the source of the energy that is processed and consumed by living beings. Intense nuclear reactions in the sun liberate light and other energetic radiations into the surrounding space. Some of this energy reaches our planet.
2. How is light from the sun transformed into chemical energy to be used by the living beings on earth?
Light from the sun is transformed into chemical energy contained in organic material by the photosynthesis process. In photosynthesis light, water and carbon dioxide react and highly energetic glucose molecules and molecular oxygen are made.
3. What is the chemical equation of photosynthesis?
The chemical equation of photosynthesis is the following:
6 CO2 + 6 H2O + light --> C6H12O6 + 6 O2
4. Which are the living beings that carry out photosynthesis? Which is the cell organelle responsible for the absorption of light for the photosynthesis process in plants and algae?
There are many beings (including all animals) that do not carry out photosynthesis. There are also autotrophic beings that do not perform photosynthesis but they perform chemosynthesis. Plants, algae and cyanobacteria are photosynthetic beings.
In plants and algae, light is absorbed by chlorophyll, a molecule present in cytoplasmic organelles called chloroplasts.
Photosynthesis Process - Image Diversity: chloroplast structure
5. Are there chloroplasts in cyanobacteria?
In cyanobacteria there are no chloroplasts and the chlorophyll layers are dispersed in cytosol.
6. Which chemical element is central in the chlorophyll molecule?
The chemical element that is central in the chlorophyll molecule is magnesium. One atom of magnesium is present in the center of an amalgam of eight nitrogen-containing carbon rings.
Photosynthesis Process - Image Diversity: chlorophyll molecule
7. How do chloroplasts multiply?
Like mitochondria chloroplasts have their own DNA, RNA and ribosomes and they self-replicate through binary division.
8. How can the hypothesis that asserts that chloroplasts as well as mitochondria were primitive prokaryotes that associated in mutualism with primitive anaerobic eukaryotic cells be corroborated?
The described hypothesis is known as the endosymbiotic hypothesis about the evolutionary origin of mitochondria and choloroplasts.
Mutualism is explained as: mitochondria and chloroplasts can offer energy and nutrients to the cell in exchange for protection. The hypothesis is strengthened since those organelles have their own DNA, RNA and protein synthesis machinery and they divide themselves through binary division like bacteria do.
9. What are the main structures of chloroplasts?
Chloroplasts are involved by two membrane layers, the outer and the inner membranes. Inside the organelle the formative unit is called the granum, a coin-shaped structure that, piled with others grana, forms several structures called thylakoids. The thylakoids fill the chloroplast and an intergrana membrane permeates the interior of the organelle.
10. In which chloroplast structure are chlorophyll molecules found?
Chlorophyll molecules are placed in an organized manner in order to enhance the exposure to light on the thylakoid surfaces.
11. What do ATP and ADP mean? What are the roles of these molecules for the cellular energetic metabolism?
ATP is an abbreviation of adenosine triphosphate, a molecule made of adenosine bound to three inorganic phosphates. ADP is an abbreviation of adenosine diphosphate, two molecules of phosphate bound to adenosine. ATP is a molecule that stores energy for the cell. When ATP hydrolyzes and becomes ADP energy is liberated and then consumed by several metabolic reactions of the organism.
Photosynthesis Process - Image Diversity: ATP molecule
12. What is ADP phosphorylation? What respectively are photophosphorylation and oxidative phosphorylation?
ADP phosphorylation is the addition of one inorganic phosphate in the molecule of adenosine diphosphate thus creating ATP (adenosine triphosphate) and incorporating energy. The phosphorylation is oxidative when the energy incorporated comes from the breaking of organic molecules having oxygen as reagent, as in aerobic cellular respiration. The reaction is called photophosphorylation when the energy source is light, as in photosynthesis.
The energy incorporated into ATP is disposable (liberated) to other cellular reactions when ATP hydrolyzes and ADP is formed again.
13. What are the stages into which photosynthesis is divided?
Photosynthesis is divided into the photochemical stage, or light reactions, and the chemical stage.
Photosynthesis Process - Image Diversity: photosynthesis reaction
14. What are the processes of the photochemical stage of the photosynthesis process?
Photolysis of water, with liberation of molecular oxygen, and photophosphorylation of ADP, with production of ATP and NADPH, are the processes that occur during the photochemical stage of photosynthesis.
15. How is the photic energy absorbed by chlorophyll transfered to ATP molecules in photophosphorylation? How will be the resulting ATP used?
Light excites chlorophyll and energizes electrons that jump off the molecule. The energy liberated when these electrons escape is used in the phosphorylation of ADP, forming ATP. The enzyme that catalyzes the reaction is the ATP synthase.
The resulting ATP is then consumed in the next chemical stage of photosynthesis to energetically enrich carbon dioxide for the formation ofglucose.
16. Is it correct to consider water decomposition by the action of light the basis of the photosynthesis process?
Besides ADP photophosphorylation, photic energy is also responsible for the breaking of water molecules during photosynthesis in a process known as waterphotolysis. In this reaction water molecules are exposed to photic energy and liberate protons (hydrogen ions), highly energetic electrons and molecular oxygen (O2). Later the hydrogen atoms will be incorporated into carbon dioxide molecules to formglucose. Since water is the hydrogen donor for photosynthesis it is correct to say that the water photolysis is the basis of the process.
17. What are the chemical substances produced by water photolysis? What is the destination of each of those substances?
Free electrons, hydrogen ions and molecular oxygen are liberated, after the water photolysis.
The electrons will replace those electrons lost by chlorophyll molecules in photophosphorylation. The hydrogen ions will be incorporated into hydrogen acceptor molecules (NADP) and later will be used in the synthesis ofglucose during the chemical stage. Molecular oxygen is liberated to the atmosphere.
18. In sulfur photosynthetic bacteria what is the molecule that donates hydrogen for photosynthesis?
In sulfur photosynthetic bacteria the substance that donates hydrogen is hydrogen sulfide (H2S) and not water. Therefore there is no liberation of molecular oxygen but there is production of molecular sulfur (S2). (Oxygen and sulfur have same number of valence electrons.)
19. Why is it said that during photosynthesis carbon dioxide is enriched to form glucose?
During photosynthesis carbon dioxide is energetically enriched with hydrogen from water. Water broken by photolysis is the hydrogen donor of the reaction. Glucose is made of carbon and oxygen atoms obtained from carbon dioxide and of hydrogen atoms obtained from water.
20. What is the complete chemical equation of photosynthesis?
The complete chemical equation of photosynthesis is the following:
6 CO2 + 12 H2O + light --> C6H12O6 + 6 H2O + 6 O2
21. What is an example of a lab experiment that shows the variation of the photosynthesis efficiency in relation to different photic energy frequencies to which the reaction is exposed? Was it expected that green light frequency favored the reaction?
The experiment: Plants of same species and ages are placed each under (respecting their photoperiods) light sources emitting only one of the colors of the light spectrum (violet, anil, blue, green, yellow and red). The experiment is executed with each of the colors and after days each plant's development is compared. Those plants whose development was normal performed satisfactory photosynthesis while those with abnormal development underused the offered light.
Chlorophyll is green because it reflects the green light frequency, i.e., it does not “use” the green range of the electromagnetic spectrum. Thus green light does not favor photosynthesis (curiously green is the light that plants “dislike”).
22. What are the divisions of white light according to the electromagnetic spectrum? Which are the two most efficient colors for photosynthesis?
The color divisions of the electromagnetic spectrum in decreasing order of frequency are: red, orange, yellow, green, blue, anil and violet. When mixed together these colors generate white.
Experimentally it is verified that the most useful colors for photosynthesis are blue and red.
Photosynthesis Process - Image Diversity: electromagnetic spectrum
23. What is NADP and NADPH?
NADP is the abbreviation of the nicotinamide adenine dinucleotide phosphate cation, a hydrogen acceptor. NADPH is made when NADP binds to one hydrogen atom and it is the form that actually transports hydrogen.
24. Photosynthesis is the most important producer of molecular oxygen (O2) on our planet. From which molecule do oxygen atoms liberated by photosynthesis come? From which other molecule could one suspect they have come? What are the destinations of those oxygen atoms?
The oxygen atoms liberated as molecular oxygen by the photosynthesis process come from water.
One indeed could suspect that those oxygen atoms would have come from carbon dioxide. Oxygen atoms from carbon dioxide however are incorporated into glucose molecules and into water molecules liberated in the chemical stage of photosynthesis.
25. Where do the photochemical and the chemical stages of photosynthesis occur?
The photochemical stage of the photosynthesis process occurs mainly on the thylakoids (the green part) and the chemical stage occurs in the stroma (the colorless framework) of the chloroplasts.
26. Which are the subproducts of the photochemical stage that are essential for the chemical stage of photosynthesis?
The chemical stage of photosynthesis depends on NADPH and ATP produced in the “light reactions” (photochemical stage).
27. What are the roles of NADPH and ATP in the chemical stage of photosynthesis?
NADPH acts as reductant of carbon dioxide, it delivers highly energetic hydrogens to precursor molecules during the glucose formation process. ATP is an energy source for the reactions of chemical stage.
28. Why is the nickname “dark reactions” not entirely correct for the chemical stage of photosynthesis?
“Dark reactions” is not a correct name for the chemical stage of photosynthesis since the reactions of the chemical stage also occur in the presence of light.
29. What is the general chemical equation of photosynthesis? Why doesn't that equation clearly show the real origin of the molecular oxygen liberated?
The general equation of photosynthesis is:
6 CO2 + 6 H2O + light --> C6H12O6 + 6 O2.
Water molecules are also produced in the chemical stage of photosynthesis as the following complete equation reveals:
6 CO2 + 12 H2O + light --> C6H12O6 + 6 H2O + 6 O2
Water molecules are present in the reagent side as well in the product side of the equation. Pure mathematical simplification of stoichiometric coefficients however leads to elimination of water from the product side and it then seems that 6 molecules of molecular oxygen (O2), i.e., 12 atoms of oxygen, are made for each 6 molecules of water, i.e., 6 oxygen atoms, in the reagent side. Thus a false impression that 6 other oxygens come from carbon dioxide is created.
30. What are the three main limiting factors of photosynthesis?
The three main limiting factors of photosynthesis process are light intensity, carbon dioxide concentration and temperature.
31. Photosynthesis rate varies according to the photic energy intensity. Does the same occur in aerobic respiration? What happens to the glucose balance as a result of these variations?
In a photosynthetic being the aerobic respiration rate can be superior, inferior or equal to the photosynthesis rate. Respiration rate depends on the energetic needs of the plant while the photosynthesis rate varies, as other conditions are maintained, with the variation of light energy.
In a situation in which the respiration rate is greater than the photosynthesis rate glucose consumption is higher than glucose production. In a situation in which the respiration rate is lower than photosynthesis rate there is accumulation of glucose (positive balance). In a situation in which the rates are equal all molecular oxygen produced by the photosynthesis process is used in respiration and all carbon dioxide liberated by respiration is consumed in photosynthesis and so there is no positive balance of glucose nor depletion of carbohydrate stores.
32. What is the compensation point? What is the implication of the compensation point for the plant growth?
The (photic) compensation point is the photic energy intensity under which aerobic respiration rate equals photosynthesis rate. In this situation all produced glucose is consumed and there is no incorporation of material into the plant and thus the plant growth discontinues.
33. Why is the carbon dioxide concentration a limiting factor of the photosynthesis process? When the carbon dioxide concentration is increased indefinitely is photosynthesis also increased indefinitely?
The availability of carbon dioxide is a limiting factor for the photosynthesis process because this gas is a reagent of the reaction.
Since enzymes catalyze the building of organic molecules with carbon atoms from carbon dioxide photosynthesis stops as soon as these enzymes become saturated, i.e., when all their activation centers are bound to their substrates. In that situation an increase of the carbon dioxide concentration will not increase the photosynthesis rate.
34. Why do some trees lose their green color in the autumn?
In autumn days become shorter and nights longer thus there is a reduction of the photosynthesis rate and some plants prepare themselves for the winter making nutrient stores. In this process, nutrients from the leaves travel towards storage sites: limbs, trunk and roots. With less chlorophyll produced in leaves the typical green color of the plant fades.
Cell Respiration Explained
1. How do cells obtain energy for their functioning?
Cells obtain energy for their metabolic reactions from the breaking of organic molecules with high energetic content. This energy is mostly stored as ATP molecules.
The process of obtaining energy in order to produce ATP molecules is named cellular respiration.
2. What is the compound that is phosphorylated for ATP formation? What is the resulting compound when ATP liberates energy?
ATP, or adenosine triphosphate, is formed after the binding of one phosphate (phosphorylation) to one ADP (adenosine diphosphate) molecule. This is a process that stores energy into the produced ATP molecule.
When ATP gives energy to the cellular metabolism it loses one of its phosphates and ADP reappears.
ADP can also lose more phosphates and generate AMP (adenosine monophosphate) or even non-phosphorylated adenosine. Adenosine production from ATP is a solution used in tissues that need urgent oxygen supply, for example, in the heart during myocardial infarction (heart attack), since adenosine has a local vasodilator effect thus providing faster vasodilation than other physiological methods.
Cell Respiration Review - Image Diversity: ATP phosphorylation
3. What are the types of cell respiration?
There are two types of cell respiration: aerobic cell respiration, a reaction with participation of molecular oxygen (O2), and anaerobic cell respiration, without participation of molecular oxygen but with other inorganic molecules as oxidant. There are several varieties of anaerobic cell respiration, the main one is fermentation.
4. Under which conditions do aerobic cells carry out fermentation?
Some cells that usually obtain energy from aerobic cellular respiration can carry out fermentation when oxygen is not available.
There are bacteria and fungi that under absence of oxygen use their anaerobic metabolic capability for energetic supply. Muscle cells carry out fermentation too when oxygen is scarce.
5. What is the difference between aerobic and anaerobic beings?
Aerobic organisms are those whose cells do not survive without oxygen since they depend on aerobic cell respiration to obtain energy for ATP production. Anaerobic organisms are those that live or can live under oxygen-lacking environments.
6. What is the difference between facultative anaerobic beings and obligate anaerobic beings?
Facultative anaerobic beings, like the fungi Saccharomyces cerevisiae, a brewing yeast, can survive under oxygen-poor environments carrying out fermentation. However when oxygen is available these beings carry out aerobic respiration.
Obligate anaerobic beings are those that cannot survive when oxygen is present. Some fungi, some bacteria (like the agent of botulism Clostridium botulinum, and the agent of tetanus, Clostridium tetani) and some protozoans are examples of obligate anaerobes.
7. What are the two types of fermentation? What are their chemical equations?
The two main types of fermentation are alcoholic fermentation and lactic fermentation.
In alcoholic fermentation pyruvic acid, an intermediate molecule, is converted into ethanol with liberation of carbon dioxide. The alcoholic fermentation equation is as follows:
C6H12O6 + 2 ADP + P --> 2 C2H5OH + 2 CO2 + 2 ATP
In lactic fermentation pyruvic acid is transformed into lactic acid and there is no production of carbon dioxide. The lactic fermentation equation is:
C6H12O6 + 2 ADP + P --> 2 C3H5OOH + 2 ATP
8. In general what are the reagents and products of fermentation?
In fermentation glucose (sugar) is degraded into pyruvic acid (each glucose molecule forms two pyruvic acid molecules). In this process two molecules of ATP are produced.
According to the type of fermentation, pyruvic acid can produce ethanol and carbon dioxide (in alcoholic fermentation) or lactic acid (in lactic fermentation). There are other varieties of fermentation in which pyruvic acid can generate acetic acid (acetic fermentation), propionic acid, isopropanol (an alcohol too), etc. The type of fermentation depends on the species of the involved organisms.
9. Why in cake and bread manufacture are alcoholic fermenting organisms used and not lactic fermenting organisms?
Fermentation has the function of making cakes and breads grow. This is accomplished by liberation of carbon dioxide in alcoholic fermentation as the gas passes through the dough and makes it grow. In lactic fermentation there is no liberation of carbon dioxide and the desired result would not be obtained.
10. To what substance is the acidic flavor of fermented milk due?
Some bacteria ferment milk lactose by lactic fermentation producing lactic acid. This product is responsible for the acidic flavor of yogurts, curd and milk.
11. How can the knowledge about fermentation explain the origin of muscle cramps and pains after intense physical exertion?
A typical fermentation process due to oxygen scarcity happens in the muscle tissue. Under intense use muscles demand too much energy (ATP) and consume much more oxygen to produce that energy. High consumption leads to oxygen scarcity and the muscle cells begin to make lactic fermentation trying to satisfy their energetic needs. In this situation muscle pain, cramps and fatigue are due to the lactic acid released by fermentation.
12. How many ATP molecules are produced for each glucose molecule used in fermentation? How many ATP molecules are produced for each glucose molecule used in aerobic respiration?
In fermentation from one glucose molecule two ATP molecules are produced. In aerobic respiration, a much more productive process, from one glucose molecule 36 ATP molecules are made.
13. Which is the cell organelle that is specialized in aerobic respiration?
The cell organelles that are specialized in aerobic respiration are the mitochondria.
Cell Respiration Review - Image Diversity: mitocondria
14. Of which main compounds is the mitochondrion structure made?
Mitochondria are organelles delimited by two lipid membranes. The inner membrane invaginates to the interior of the organelle forming cristae and delimiting an internal space known as the mitochondrial matrix.
Cell Respiration Review - Image Diversity: mitochondria structure
15. What are the three phases into which the cell respiration is divided?
The three phases of aerobic cell respiration are glycolysis, Krebs cycle and respiratory chain (also known as the electron transport chain).
16. What is glycolysis? What are the products of this process?
Glycolysis, the first stage of the aerobic cell respiration, is a process in which glucose is degraded (broken) to form two pyruvic acid molecules along with the formation of two ATP and two NADH.
Glycolysis is a complex reaction implying the formation of several intermediate molecules until pyruvic acid molecules are made. Although two ATP molecules are consumed in the reaction, there is also production of four molecules of ATP, thus a positive balance of two ATP molecules is obtained. Two NADH molecules are also produced. In glycolysis the 6-carbon structure of glucose is broken and two organic chains of three carbons each are made; these chains give birth to two pyruvic acid molecules.
Cell Respiration Review - Image Diversity: glycolysis
17. Does glycolysis occur within the mitochondria?
Glycolysis happens in the cytosol and not within the mitochondria. Pyruvic acid molecules later enter mitochondria to participate in the next phase of the aerobic cell respiration.
18. How many ATP molecules are made after glycolysis?
Glycolysis is a process similar to glucose degradation in fermentation. It produces (final balance) two molecules of ATP for each broken glucose.
19. What is NAD? What is the role of the NAD molecule in glycolysis?
NAD (nicotinamide adenine dinucleotide) is a hydrogen acceptor necessary as reductant (to receive hydrogen) in some reactions in which it is reduced and converted into NADH2. During glycolysis two NAD molecules retrieve hydrogens liberated after an intermediate reaction thus forming NADH2.
20. What happens during aerobic respiration to the pyruvic acid molecules made by glycolysis? What is the sequence of reactions that then follows?
The pyruvic acid molecules made in cytosol by glycolysis enter into the mitochondria.
Within the mitochondria each pyruvic acid molecule is converted into one molecule of acetyl-CoA (acetyl coenzyme A) with liberation of one carbon dioxide. The Krebs cycle (also known as citric acid cycle), the second stage of aerobic respiration, then begins.
21. What is the official name of pyruvic acid?
Pyruvic acid is 2-oxopropanoic acid. It is thus a molecule made of three linearly bound carbons with one extremity forming the organic acid function (COOH) and the middle carbon binding to an oxygen atom by double bond.
22. Why can it be said that each glucose molecule runs the Krebs cycle twice?
Each glucose molecule “cycles” the Krebs cycle twice because after glycolysis each used glucose has generated two pyruvic acid molecules and each pyruvic acid is converted in a 1:1 proportion into acetyl CoA. Each acetyl CoA then cycles the Krebs cycle once.
Cell Respiration Review - Image Diversity: Krebs cycle
23. Why is the Krebs cycle also called the final common pathway of the degradation of organic compounds?
The Krebs cycle is called the final common pathway of the degradation of organic compounds because it is also possible to generate acetyl CoA from the degradation of lipids and proteins. Since acetyl CoA is the substrate that triggers the Krebs cycle, this process is called the final common pathway for being activated by other organic molecules (lipids and proteins) and not only by glucose.
The organism uses energetic reserves of fat and proteins to cycle the Krebs cycle when undergoing malnutrition or when there is no glucose available for the cells.
24. What are the final energetic products of each round of the Krebs cycle? Where is most part of the utile energy at the end of Krebs cycle found?
After each round of the Krebs cycle two carbon dioxide molecules, eight protons (hydrogen ions) captured by NAD and FAD (a hydrogen acceptor too) and one ATP molecule are produced.
During the Krebs cycle acetyl CoA is degraded. At the end the utile energy is incorporated into hydrogens transported by FADH2 and NADH2 molecules.
25. How many carbon dioxide molecules are liberated after each cycle of the Krebs cycle? For a single glucose how many carbon dioxide molecules were already liberated by the aerobic respiration at that point?
Each round of the Krebs cycle liberates two carbon dioxide molecules.
At the end of the cycle all carbon atoms from the original glucose molecule degraded in glycolysis are already liberated incorporated into carbon dioxide molecules. That occurs because for each glucose two pyruvic acid molecules were made by glycolysis. Each of these two pyruvic acids then is converted into acetyl CoA with liberation of one carbon dioxide molecule (two in total). Since each of the two produced acetyl CoA cycles the Krebs cycle once, from the initial glucose two rounds of the Krebs cycle is generated and so four other carbon dioxide molecules are made.
All of the six carbons of the glucose molecule are then incorporated into six carbon dioxide molecules (two made during acetyl CoA formation and four during the two cycles of the Krebs cycle).
26. Where in mitochondria does the process called respiratory chain occur? Which are the products of the Krebs cycle used in that final phase of the aerobic respiration?
Respiratory chain, or the electron transport chain, is performed by protein systems located in the inner membrane of the mitochondria. Energized electrons of hydrogen atoms transported by NADH2 and FADH2 are the products of the preceding phases used in the respiratory chain.
Cell Respiration Review - Image Diversity: respiratory chain
27. What are cytochromes?
Cytochromes are proteins of the internal mitochondrial membrane that are specialized in electron transfer and participate in the respiratory chain. Energized electrons liberated by the hydrogen donors NADH2 and FADH2 (then reconverted into NAD and FAD) pass through a sequence of cytochromes losing energy in each passage. The energy is then used in the synthesis of ATP.
28. How in the respiratory chain do electrons from FADH2 and NADH2 passing through cytochromes liberate energy for the ATP synthesis? What is this ATP synthesis called?
FADH2 and NADH2 oxidate into FAD and NAD and liberate hydrogen ions and highly energized electrons in the beginning of the respiratory chain.
The energy lost by electrons that pass through the cytochromes is used to pump protons (hydrogen ions) out of the inner mitochondrial membrane (to the region between the inner and the outer membranes of the mitochondrion). Hydrogen concentration gradient between the inner and the outer spaces delimited by the inner membrane forces protons (hydrogen ions) to return to the mitochondrial matrix (the region inside the inner membrane) however that return is only possible if hydrogen ions pass through an enzymatic complex called ATP synthetase embedded in the inner membrane. In that passage the ATP synthetase phosphorylates ADP and then ATP molecules are produced.
Hydrogen liberated in the mitochondrion then combines with oxygen to form water. As a reaction that depends on oxygen this type of ATP synthesis is called oxidative phosphorylation.
29. Until the Krebs cycle, aerobic respiration can be described without mentioning oxygen, the chemical element after which the reaction gets its name. Where in the process does this chemical element take part? What is its importance?
Oxygen enters the aerobic respiration in its final phase, the respiratory chain. It is of fundamental importance because it is responsible for the maintenance of the hydrogen concentration gradient between the spaces separated by the inner mitochondrial membrane. This gradient promotes the functioning of the ATP synthetase and thus the phosphorylation of ADP to form ATP. In the space inside the inner membrane oxygen binds to free hydrogens to form water and this hydrogen consumption keeps the hydrogen gradient and the proton traffic through the ATP synthetase.
The entire aerobic respiration process has the intent to make the ATP synthetase work. Aerobic beings, for example, we humans, need to breathe oxygen to maintain that hydrogen concentration gradient and keep the ATP synthetase working.
Cell Respiration Review - Image Diversity: ATP synthetase
30. How does the poison cyanide act upon the aerobic respiration?
Cyanide is a poison that inhibits the last cytochrome of the respiratory chain, interrupting the ATP formation and thus leading the cell to death.
31. What is anoxia?
Anoxia is a situation in which there is no available oxygen in the cell. Whitout oxygen the respiratory chain stops, there is no ATP production, the cell does not obtain energy and dies.
Anoxia can be caused, for example, by pulmonary insufficiency (drowning, extensive pulmonary injuries, etc.), by obstructions, halts and deficiencies in tissue circulation (atherosclerosis of the coronary arteries that irrigate the myocardium, tourniquets, heart arrest), by hemolysis (lysis of red blood cell) or hemoglobin diseases (anemias, fetal erythroblastosis), etc.
32. How many ATP molecules are made after the aerobic respiration and what is the net energetic gain of the process?
After aerobic respiration 38 ATP molecules are made with the consumption of one glucose molecule (but two of these ATP are consumed by glycolysis). The net gain of the process is then 36 ATP molecules per glucose molecule.
33. What is the general equation of the aerobic respiration (also representing ADP and phosphate)?
The general equation of the aerobic respiration is:
C6H12O6 + 6 O2 + 36 ADP + 36 P --> 6 CO2 + 6 H2O + 36 ATP
34. Why can the consumption of molecular oxygen indicate the metabolic rate of aerobic organisms?
Molecular oxygen (O2) consumption has direct relation to the cell metabolic rate in aerobic cells and so to the metabolic rate of the organisms. Cells having higher metabolic activity demand more energy and such energy comes from ATP molecules. As there is need for ATP production, the intensity of aerobic cell respiration is also higher and then more oxygen is consumed.
Protein Synthesis Knowledge
1. What is the genetic code?
Genetic code is the key for the conversion of DNA nucleotide sequences (and thus RNA nucleotide sequences) into amino acids sequences that will compose proteins.
Protein Synthesis - Image Diversity: genetic code
2. Which is the biological molecule that contains the genetic information that is transmitted hereditarily and controls the cellular functioning?
The hereditary molecule that controls the cellular functioning is the DNA (deoxyribonucleic acid). The DNA contains information forprotein synthesis in cells.
3. How are the concepts of DNA, gene, proteins and characteristics of living beings related?
Characteristics of organisms depend on chemical reactions that occur in them. These reactions are catalyzed by enzymes, highly specific proteins. Every protein of an organism is made from information contained in RNA molecules that are made according to a template sequence of nucleotides of a DNA chain.
A gene is a DNA polynucleotide sequence that contains information for the production of a protein.
4. What is the role of messenger RNA and ribosomes for the protein synthesis?
The mRNA is produced within the cellular nucleus and migrates to the cytoplasm where associated to ribosomes it guides the building of amino acid sequences that will compose proteins. Ribosomes are sites for the meeting and binding of mRNA and transfer RNA (tRNA), they are the structures where amino acids transported by tRNA are united by peptide bonds forming polypeptide chains (proteins).
Protein Synthesis - Image Diversity: messenger RNA
5. Of what subunits are ribosomes are made?
Ribosomes are made of two subunits, the small subunit and the large subunit. These subunits are made of ribosomic RNA (rRNA) and proteins.Ribosomes have three binding sites, one for mRNA and two for tRNA.
Protein Synthesis - Image Diversity: ribosome structure
6. How different are the location of ribosomes in eukaryotic and in prokaryotic cells?
In prokaryotes ribosomes are found free in cytoplasm. In eukaryotic cells they can also be found free in cytoplasm and mainly adhered to the external membrane of the karyotheca and of the rough endoplasmic reticulum.
7. How is the finding of ribosomes inside mitochondria and chloroplasts explained?
It is a strong hypothesis that mitochondria and chloroplasts were prokaryotes that associated to primitive eukaryotic cells under mutualism (gaining protection and offering energy). This explains why within those organelles there are DNA andprotein synthesis machinery, including ribosomes. This hypothesis is known as the endosymbiotic hypothesis on the origin of mitochondria and chloroplasts.
8. What are some examples of human cells that produce proteins for exportation? Which cytoplasmic organelle is expected to be well-developed and abundant in those cells?
Specialized cells of the glands, like the Langerhans cells of the pancreas (that produce insulin) or the saliva-producing cells, are examples of secretory cells. In cells specialized in secretion, the endoplasmic reticulum and the Golgi apparatus are well-developed since they participate in the storage and processing of proteins forexportation.
Protein Synthesis - Image Diversity: secretory cells
9. Which are the more abundant ribosomes in secretory cells - the free cytoplasmic ribosomes or those associated with the rough endoplasmic reticulum?
Free cytoplasmic ribosomes are more related to protein production for internal cellular consumption while those adhered to the rough endoplasmic reticulum are more important inprotein synthesis for exportation. Proteins made by adhered ribosomes enter the rough endoplasmic reticulum and are later transferred to the Golgi apparatus. So in secretory cells ribosomes adhered to the endoplasmic reticulum are more notable.
10. Where in eukaryotic cells does mRNA synthesis occur? To where do these molecules migrate?
Messenger RNA molecules are synthesized within the nucleus, pass through pores of the nuclear membrane and gain the cytoplasm to reach theribosomes where protein synthesis occurs.
11. After the fact that it is based on information from mRNA what is the process of protein synthesis called?
Protein synthesis is called translation (of genetic information into proteins).
Protein Synthesis - Image Diversity: protein translation
12. What is the difference between transcription and translation?
Transcription is the name given to the formation of DNA molecules from an open DNA chain used as a template. Translation is the making of polypeptides (amino acids bound in sequence) and thus of proteins based on information encoded in the mRNA molecule.
In eukaryotic cells transcription occurs in the nucleus and translation occurs in ribosomes. Transcription precedes translation.
13. How do nucleotides of mRNA chains encode information for the formation of the amino acids sequences of a protein?
There are only four types of nitrogen-containing bases that can compose RNA nucleotides: adenine (A), uracil (U), guanine (G) and cytosine (C).Amino acids however are 20 different ones. Considering only one nucleotide (a 1:1 coding) it would be impossible to codify all amino acids.
Considering two nucleotides there would be an arrangement of 4 elements, 2 x 2, resulting in a total of only 16 possible codifier units (4 x 4). Nature may know combinatory analysis since it makes a genetic code by arrangement of the 4 RNA bases, 3 x 3, providing 64 different triplets (4 x 4 x 4).
So each triplet of nitrogen-containing bases of RNA codifies one amino acid of a protein. As these triplets appear in sequence in the RNA molecule, sequentialamino acids codified by them are bound together to make polypeptide chains. For example, a UUU sequence codifies the amino acid phenylalanine, as well the UUC sequence; the ACU, ACC, ACA and ACG sequences codify the amino acid threonine; and so on for all possible triplet sequences and all otheramino acids.
14. What is the name of an RNA sequence that codifies one amino acid?
Each sequence of three nitrogen-containing bases of RNA that codifies one amino acid is called a codon. The codon is the codifier unit of the genetic code.
15. Since among the 64 codons of mRNA 61 codify amino acids that form polypeptide chains what are the functions of the three remaining codons?
Since there are 20 amino acids and 64 possibilities of mRNA codons, it is expected some amino acids to be codified by more than one codon. And that really happens.
Not all 64 codons however codify amino acids. Three of them, UAA, UGA and UAG, work on information that the last amino acid of a polypeptide chain under productions was already bound, i.e., they signal the end of the polypeptide synthesis. These codons are called terminal codons. The codon AUG codifies the amino acid methionine and at the same time it signals the beginning of the synthesis of a polypeptide chain (it is an initialization codon).
In prokaryotic cells there is a sequence called Shine-Dalgarno sequence (in general AGGAGG) in the position that antecedes the initialization codon AUG. The function of this sequence is distinctness between the initialization AUG and other AUG codons of the RNA.
16. What is the cellular structure to which mRNA molecules bind to start the protein synthesis?
To make proteins mRNA molecules necessarily associate to ribosomes. Ribosomes have two sites for the binding of two neighboring mRNA codons and where anticodons of tRNA bind by hydrogen bond. Thus ribosomes are the structure responsible for the positioning and exposure of mRNA codons to be translated. In ribosomes the peptide bond between two amino acids brought by tRNA molecules also occurs. The peptide bond happens when tRNAs carrying amino acids are bound to exposed mRNA codons.
17. How are amino acids brought to the cellular site where translation takes place? What is an anticodon?
Amino acids are brought to ribosomes by RNA molecules known as transfer RNA, or tRNA. One tRNA associated to its specific amino acid binds by a special sequence of three nucleotides to a mRNA codon exposed in the ribosome. This sequence in the tRNA is known as anticodon. The tRNA anticodon must be complementary to the mRNA codon to which it binds, according to the rule A-U, CG. The ribosome then slides along the mRNA molecule (a process called translocation) to expose the following codon to the binding of other tRNA. Whenamino acids corresponding to neighboring codons bind by peptide bond the first tRNa is liberated.
Protein Synthesis - Image Diversity: transfer RNA
18. Why is the proximity between ribosomes and amino acids important for the protein formation? What is the enzyme that catalyzes that reaction?
The proximity between ribosomes and amino acids is important because the enzyme that catalyzes the peptide bond resides in ribosomes. As substrates of these enzymes, amino acids need to bind to the enzyme activation centers.
The enzyme that catalyzes the peptide bond is the peptidyl transferase.
19. Why do ribosomes move along mRNA during translation?
During translation the ribosome always exposes two mRNA codons to be translated by moving along the mRNA. When a peptide bond is made the ribosome moves to expose the next codon. This moving is called ribosomal translocation. (In the rough endoplasmic reticulum ribosomes are attached outside the membrane and mRNA molecules rather moving through them).
Protein Synthesis - Image Diversity: ribosomal translocation
20. How many of the same proteins are made at the same time by each ribosome in the translation of one mRNA molecule? How does consecutive protein production occur in translation?
Ribosomes do not make several different proteins simultaneously. They make them one after another.
Along one single mRNA molecule however many ribosomes may move in a real mass manufacturing of the same protein. The unit made of many ribosomes working upon the same mRNA molecule is called polysome.
Protein Synthesis - Image Diversity: polysome
21. An mRNA molecule codifies only one type of protein?
Eukaryotic cells have monocistronic mRNA, i.e., each mRNA codifies only one polypeptide chain. Prokaryotes can present polycistronic mRNA.
At the end of the assembling of amino acids into a polypeptide chain, the mRNA, by one of its terminal codons, signals to the ribosome that the polypeptide is complete. The ribosome then liberates the produced protein. In prokaryotes after this conclusion the information for the beginning of the synthesis of another different protein may follow in the same mRNA.
22. If a tRNA anticodon is CAA what is its corresponding mRNA codon? For the genetic code which amino acid does this codon codify?
According to the A-U , C-G rule, the corresponding codon to the CAA anticodon is GUU.
The genetic code table for translation is related to codons and not to anticodons. The amino acid codified by GUU, according to the genetic code, is valine.
23. If a fragment of nucleic acid has a nucleotide sequence TAC can one assert that it is a codon or an anticodon?
A nucleic acid having a TAC sequence surely is not tRNA, it is DNA since RNA does not present the nitrogen-containing base thymine. Since it is not RNA it cannot be a codon or an anticodon.
24. Why can the genetic code be qualified as a “degenerate code”?
The genetic code is a degenerate code because there are amino acids codified by more than one type of codon. It is not a system in which each element is codified by only one codifying unit.
For example, the amino acid arginine is codified by six codons: CGU, CGC, CGA, CGG, AGA and AGG.
25. What is the concept of universality of the genetic code? What are the exceptions to this universality?
The genetic code is universal because the rules of protein codification based on mRNA codons are practically the same for all known living beings. For example, the genetic code is the same for humans, for bacteria and for invertebrates.
The protein synthesis in mitochondria, chloroplasts and some protozoans however are accomplished by different genetic codification.
26. How does the universality of the genetic code make the recombinant DNA technology possible?
The universality of the genetic code refers to the fact that all living beings have their protein synthesis machinery functioning according to the same principles of storage, transmission and recognition of information, including translation of mRNA codons. This fact makes possible the exchanging of genes or gene fragments between different organisms and secures that these genes continue to command protein synthesis.
This universality, for example, makes feasible the insertion of a fragment of human DNA containing a gene for the production of a given protein into the genetic material of bacteria. Since the bacterial transcription and translation systems work in the same manner as the correspondent human systems do, the bacteria will begin to synthesize the human protein related to the inserted DNA fragment. There are industries that produce human insulin (for use by diabetic patients) in this way, synthesized by bacteria with modified DNA. If the genetic code was not universal this kind of genetic manipulation would be impossible or very difficult to accomplish without new technological progresses.
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