Chapter 23

1. The Calvin cycle is the primary means of converting gaseous CO₂ into organic matter—that is, biomolecules. Essentially, every carbon atom in your body passed through rubisco and the Calvin cycle at some time in the past.

2. Autotrophs can use the energy of sunlight, carbon dioxide, and water to synthesize carbohydrates, which can subsequently be used for catabolic or anabolic purposes. Heterotrophs require chemical fuels and are thus ultimately dependent on autotrophs.

3. Nothing grim or secret about these reactions. They are sometimes called the dark reactions because they do not directly depend on light.

4. Stage 1 is the fixation of CO₂ with ribulose 1,5-bisphosphate and the subsequent formation of 3-phosphoglycerate. Stage 2 is the conversion of some of the 3-phosphoglycerate into hexose. Stage 3 is the regeneration of ribulose 1,5-bisphosphate.

5. Complete the interactive matching exercise to see answers.

6. Rubisco catalyzes a crucial reaction, but it is very slow. Consequently, it is required in large amounts to overcome its slow catalysis.

7. Because the carbamate forms only in the presence of CO₂, this property would prevent rubisco from catalyzing the oxygenase (or the carboxylase) reaction when CO₂ is absent.

8. ATP is required to form phosphoenolpyruvate (PEP) from pyruvate. PEP combines with CO₂ to form oxaloacetate and, subsequently, malate. Two ATP molecules are required because a second ATP molecule is required to phosphorylate AMP to ADP.

9. Because NADPH is generated in the chloroplasts by the light reactions

10. The stroma will accumulate Mg²⁺ and become alkaline as a consequence of the movement of protons from the stroma to the thylakoid space. Thus, rubisco is primed for activity when the light reactions are providing ATP and NADPH required for carbon fixation and glucose synthesis.

11. The light reactions lead to an increase in the stromal concentrations of NADPH, reduced ferredoxin, and Mg²⁺, as well as an increase in pH.

12. ATP is converted into AMP. To convert this AMP back into ATP, two molecules of ATP are required: one to form ADP and another to form ATP from the ADP.

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13. The oxygenase activity of rubisco increases with temperature. Crabgrass is a C₄ plant, whereas most grasses lack this capability. Consequently, the crabgrass will thrive at the hottest part of the summer because the C₄ pathway provides an ample supply of CO₂.

14. Photorespiration is the consumption of oxygen by plants with the production of CO₂, but it does not generate energy. Photorespiration is due to the oxygenase activity of rubisco. It is wasteful because, instead of fixing CO₂ for conversion into hexoses, rubisco generates CO₂.

15. High concentrations of CO₂ prevent O₂ from entering the active site of rubisco.

16. As global warming progresses, C₄ plants will invade the higher latitudes, whereas C₃ plants will retreat to cooler regions.

17. C₄ metabolism allows rubisco to function efficiently even when the temperatures are high, which favor oxygenase activity. Moreover, C₄ metabolism allows desert plants to accumulate CO₂ at night when the temperatures are cooler and water evaporation is not a problem.

18. 1. 

    2. Despite the fact that the specificity constant for CO₂ as a substrate is much greater than that of O₂, the concentration of O₂ in the atmosphere is higher than that of CO₂, allowing the oxygenation reaction to occur.

19. Calvin cycle	Krebs cycle
    Stroma	Matrix
    Carbon chemistry for photosynthesis	Carbon chemistry for oxidative phosphorylation
    Fixes CO2	Releases CO2
    Requires high-energy electrons (NADPH)	Generates high-energy eletrons (NADH)
    Regenerates starting compound (ribulose 1,5-bisphosphate)	Regenerates starting compound (oxaloacetate)
    Requires ATP	Generates ATP
    Complex stoichiometry	Simple stoichiometry

20. The reduction of each mole of CO₂ to the level of a hexose requires 2 mol of NADPH. The reduction of NADP⁺ is a two-electron process. Hence, the formation of 2 mol of NADPH requires the pumping of 4 mol of electrons by photosystem I. The electrons given up by photosystem I are replenished by photosystem II, which needs to absorb an equal number of photons. Hence, eight photons are needed to generate the required NADPH. The energy input of 8 mol of photons is 1594 kJ (381 kcal). Thus, the overall efficiency of photosynthesis under standard conditions is at least 477/1594, or 30%.

21. 1. The blue curve on the right in graph A was generated by the C₄ plant. Recall that the oxygenase activity of rubisco increases with temperature more rapidly than does the carboxylase activity. Consequently, at higher temperatures, the C₃ plants fix less CO₂. Because C₄ plants can maintain a higher CO₂ concentration, the rise in temperature is less deleterious.

    2. The oxygenase activity predominates. Additionally, when the temperature rise is very high, the evaporation of water might become a problem. The higher temperatures can begin to damage protein structures as well.

    3. The C₄ pathway is a very effective active-transport system for concentrating CO₂, even when environmental concentrations are very low.

    4. With the assumption that the plants have approximately the same capability to fix CO₂, the C₄ pathway is apparently the rate-limiting step in C₄ plants.

22. (a) 3-Phosphoglycerate; (b) the other members of the Calvin cycle

23. The concentration of 3-phosphoglycerate will increase, whereas that of ribulose 1,5-bisphosphate will decrease.

24. The concentration of 3-phosphoglycerate will decrease, whereas that of ribulose 1,5-bisphosphate will increase.

25. (a) CABP resembles the addition compound formed in the reaction of CO₂ and ribulose 1,5-bisphosphate. (b) CABP will be a potent inhibitor of rubisco.