ANSWERS TO REVIEW QUESTIONS – CHAPTER 06 1. What is glycolysis and where does it occur in a cell? How much ATP is produced in glycolysis compared with oxidative respiration? (pp. 117–120) Glycolysis is the anaerobic catabolism of glucose to two molecules of pyruvate, with the associated production of two molecules of ATP. It occurs in the cytoplasm and has two main stages. In the first, two molecules of ATP are used to phosphorylate and change glucose into two 3-carbon molecules. This proceeds in a series of five sequential reactions. In the second stage, these 3-carbon molecules are oxidised to pyruvate through a further series of five reactions. Four ATP molecules are produced, giving an overall net production of two ATP molecules after discounting the two used in the first stage. Additionally, four electrons and two protons (H +) are accepted by NAD+, producing two molecules of NADH. The fate of pyruvate is oxygen dependent and varies between animal, plant and prokaryotic cells. Under anaerobic conditions, pyruvate is reduced to produce lactate (animal cells) or ethanol (plant cells) or both (prokaryotic cells), using the NADH produced in the second stage. This, in turn, regenerates NAD+ that is used to sustain glycolysis. Under aerobic conditions in eukaryotic cells the pyruvate enters a mitochondrion for oxidative respiration. In aerobic prokaryotic cells oxidative respiration occurs in the cytoplasm. The yield of ATP from glucose via glycolysis is low compared to that of oxidative respiration, producing an overall gain of only two ATP molecules compared to the 36 to 38 produced in oxidative respiration. 2. Explain how NADH and FADH2 are used by the inner membrane of mitochondria to drive ATP synthesis. Of what significance are the cristae of mitochondria? (pp. 120–121) An overview of glucose catabolism shows four main stages. The first is glycolysis (see question 1), which occurs anaerobically in the cytoplasm. In eukaryotic cells, all further stages occur in the mitochondria. In the second stage, the pyruvate produced in glycolysis is oxidised to form acetyl CoA that is passed into the third stage, the citric acid cycle, which is a cycle of eight reactions. Ultimately, each molecule of glucose generates four ATP molecules directly and 12 reduced electron carriers (10 of which are NADH molecules and two are FADH2 molecules). These carriers participate in the electron transport chain to generate far more ATP. The electron transport chain is the fourth and final stage. It is a series of three membrane-associated protein complexes, which pass the electrons delivered by NADH and FADH 2 from one to the other along the chain. Associated with this transfer of electrons is the translocation of protons out of the mitochondrial matrix and into the inter-membrane space. Thus the proton concentration in the intermembrane space rises above that of the matrix that, in turn, promotes transfer of the protons back into the matrix through special channels (see Chapter 5 for a discussion of channels and movement across membranes). This re-entry of the protons is coupled to the production of ATP, producing the bulk of the ATP generated during oxidative respiration. Mitochondria are surrounded by a double membrane. Whereas the outer membrane is smooth, the inner membrane is thrown into folds, called cristae, which greatly increase the surface area of this membrane. The inner membrane contains the proteins of the electron transport chain and the enzyme ATPase, which is responsible for ATP generation. The cristae thus provide a large surface area for ATP synthesis. It is not surprising to find that tissues that have a high demand for ATP, such as heart and skeletal muscle, contain mitochondria with many cristae. 3. Summarise the different biochemical pathways by which energy is extracted from carbohydrates and lipids. (pp. 117–120) These pathways are summarised clearly in Figure 6.2. Complex carbohydrates are broken down into glucose, which in turn enters glycolysis in the cytoplasm. In eukaryotic cells the pyruvate produced by glycolysis enters a mitochondrion where it is converted to acetyl CoA, which enters the citric acid cycle for oxidative respiration. In contrast, lipids are first hydrolysed into free fatty acids and glycerol. The fatty acids are broken down in -oxidation to produce acetyl CoA, which enters into the citric acid cycle. -oxidation occurs in the mitochondria of animal and plant cells. The text makes no mention of the cellular respiration of protein. In this situation, proteins are broken down into their constituent amino acids, the nitrogen-containing amino group is removed from each amino acid (deamination) and the remaining carbon chain is then converted into a molecule that can enter the citric acid cycle. 4. What is the significance of fermentation reactions in cells? How does fermentation differ in a yeast cell and a muscle cell? (p. 122) Fermentation is the anaerobic production of ethanol (plant and yeast cells), lactic acid (animal cells, such as skeletal muscle cells) or both (bacteria) as a result of glycolysis in the absence of oxygen. It is far less efficient than aerobic respiration but, from an evolutionary perspective, was the sole means of catabolism by the earliest living organisms under the anaerobic conditions that then prevailed. Today, fermentation is significant in industrial processes such as beer and wine making. Anaerobic fermentation is also significant for both plants and animals at times of low oxygen availability. These include conditions of waterlogging around the roots of plants and the so-called ‘diving response’ of airbreathing vertebrates. 5. Make a table showing the major differences between energy-harvesting pathways (glycolysis, oxidative respiration and photosynthesis) in prokaryotic and eukaryotic cells. Indicate where the different reactions occur. (pp. 117–131) Type of cell Glycolysis Oxidative respiration Photosynthesis Prokaryote Occurs in the cytoplasm. The products are ethanol and/or lactic acid and 2 ATP. Occurs in the cytoplasm of aerobic prokaryotes. The products are carbon dioxide, water and 38 ATP. Primitive photosynthetic bacteria have chlorophyll on the plasma membrane. Cyanobacteria have chlorophyll on the thylakoid membranes within the cytoplasm. Primitive bacteria cannot use water as a source of electrons (i.e. they lack PSII and do not produce oxygen). Cyanobacteria use both PSI and PSII and produce oxygen. Carbon dioxide is always the source of carbon. Eukaryote (plant) Occurs in the cytoplasm. The products are ethanol and 2 ATP. Occurs in the mitochondria and produces carbon dioxide, water and 38 ATP. Occurs in the chloroplasts. Both PSI and PSII are involved. Carbon dioxide is the source of carbon and oxygen is produced from water. Eukaryote (animal) Occurs in the cytoplasm. The products are lactic acid and 2 ATP. Occurs in the mitochondria and produces carbon dioxide, water and 36 or 38 ATP (depending on the tissue). Does not occur. 6. How do pigment molecules in the light-harvesting complexes trap and transfer solar energy? What is the role of the reaction centres in the photosystems? (pp. 125–129) When a chlorophyll molecule absorbs a photon, one of the electrons of the magnesium atom, centrally located within the porphyrin ring of that chlorophyll (Figure 6.13), is excited to a higher energy level. The excited chlorophyll molecule then interacts with its neighbour, transferring energy and exciting it in turn. The arrangement of pigment molecules within a light-harvesting complex is such that the absorbed energy is channelled to a particular pair of chlorophyll molecules, known as the reaction centre (see Figure 6.16). These chlorophyll molecules (called P680 in PSII and P700 in PSI) differ from others in that they lose an electron upon excitation. An electron acceptor on the stromal side of the thylakoid membrane takes up the electron, thus becoming reduced. 7. What organisms use water in the light-dependent reactions of photosynthesis? What is the role of water? (pp. 127–129) All photosynthetic organisms, except green and purple bacteria, use water in the light-dependent reactions of photosynthesis. The water is used as a source of electrons, which pass from PSII to PSI ultimately generating ATP and NADPH for the light-independent reactions of photosynthesis. The oxygen atoms from the water form oxygen gas as a by-product. 8. If ATP and NADPH were supplied externally to leaves, leaf cells or isolated intact chloroplasts, would you expect photosynthetic carbon fixation to continue in the dark? Explain your answer. (pp. 127–131) The capture of atmospheric CO2 (carbon fixation) and its incorporation into carbohydrates is lightindependent using the energy (ATP and NADPH) captured in the light-dependent reactions. The process of carbohydrate synthesis can thus function in darkness as well as light, provided that there is an adequate supply of ATP and NADPH. However, since ATP and NADPH are impermeable to cellular membranes, an external supply of them cannot be used to sustain the reactions of the Calvin-Benson cycle, which only occur within the stroma of the chloroplasts. Rather, the ATP and NADPH need to be generated within the chloroplast to sustain the carbon-fixation reactions. 9. Why is the enzyme ribulose bisphosphate carboxylase-oxygenase so-named? (p. 129–130) This enzyme, often known by its abbreviation Rubisco, has two enzymatic activities. If CO 2 binds to its active site, it catalyses the key carbon-fixing reaction of photosynthesis in which CO2 is bound to the 5carbon sugar ribulose 1,5-bisphosphate (RuBP). Alternatively, if O2 binds to the active site of Rubisco, its second activity leads to the oxygenation of RuBP. This latter process, together with the subsequent production of CO2 from the product of oxygenation, is called photorespiration. Photorespiration reduces the efficiency of photosynthesis, because it releases some CO2, resulting in a lower rate of carbohydrate synthesis. 10. How does C4 photosynthesis differ from CAM? What are the advantages of these two pathways for plants living in hot or dry environments? (pp. 133–135) In C4 plants, the enzymes of the Calvin-Benson cycle are located in special bundle-sheath cells that are relatively impermeable to CO2. The CO2 in these cells is generated enzymatically from malate leading to a high concentration of CO2, which thus restricts photorespiration. The chemical reactions comprising the C4 pathway use more ATP than the C3 pathway, but this is balanced by the reduction in photorespiration. Because they can concentrate CO2, C4 plants can also narrow their stomatal openings. This reduces water loss, which is a significant advantage in hot, dry climates. CAM plants adopt a temporal rather than a spatial solution to the problem. Their stomata open only at night and close during the day. This prevents water loss by transpiration during the day, while the build-up of CO2 in the leaves minimises photorespiration. The fixation of CO2 takes place at night when the stomata are open. Plants in hot, dry environments face problems of water loss and the increasing inefficiency of photosynthesis because of heightened photorespiration at higher temperatures. C4 and CAM plants use variations of the carbon-fixation process as solutions to these problems.