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.

5. (a) 3-Phosphoglycerate. (b) The other members of the Calvin cycle.

6. 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.

7. It catalyzes a crucial reaction, but it is highly inefficient. Consequently, it is required in large amounts to overcome its slow catalysis.

8. Because carbamate forms only in the presence of CO₂, this property prevents rubisco from catalyzing the oxygenase reaction exclusively when CO₂ is absent.

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

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

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

12. Aspartate + glyoxylate → oxaloacetate + glycine

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. The C₄ pathway allows the CO₂ concentration to increase at the site of carbon fixation. High concentrations of CO₂ inhibit the oxygenase reaction of rubisco. This inhibition is important for tropical plants because the oxygenase activity increases more rapidly with temperature than does the carboxylase activity.

15. ATP is required to form phosphoenolpyruvate (PEP) from pyruvate. The 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.

16. 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 is generating CO₂.

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

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

19. (a) 5; (b) 1; (c) 7; (d) 2; (e) 10; (f) 3; (g) 6; (h) 4; (i) 8; (j) 9.

20. The enzymes catalyze the transformation of the five-carbon sugar formed by the oxidative phase of the pentose phosphate pathway into fructose 6-phosphate and glyceraldehyde 3-phosphate, intermediates in glycolysis (and gluconeogenesis).

21. (a) C; (b) B and F; (c) G; (d) F; (e) E; (f) H; (g) I; (h) D; (i) A; (j) F; (k) B.

22. The label emerges at C-5 of ribulose 5-phosphate.

23. Oxidative decarboxylation of isocitrate to α-ketoglutarate. A β-ketoacid intermediate is formed in both reactions.

24. (a) 5 Glucose 6-phosphate + ATP → 6 ribose 5-phosphate + ADP + H⁺.

(b) Glucose 6-phosphate + 12 NADP⁺ + 7H₂O → 6 CO₂ + 12 NADPH + 12 H⁺ + P_i.

25. The nonoxidative phase of the pentose phosphate pathway can be used to convert three molecules of ribose 5-phosphate into two molecules of fructose 6-phosphate and one molecule of glyceraldehyde 3-phosphate. These molecules are components of the glycolytic pathway.

26. The conversion of fructose 6-phosphate into fructose 1,6-bisphosphate by phosphofructokinase requires ATP.

27. When much NADPH is required. The oxidative phase of the pentose phosphate pathway is followed by the nonoxidative phase. The resulting fructose 6-phosphate and glyceraldehyde 3-phosphate are used to generate glucose 6-phosphate through gluconeogenesis, and the cycle is repeated until the equivalent of one glucose molecule is oxidized to CO₂.

28. Fava beans contain vicine, a purine glycoside that can lead to the generation of peroxides—reactive oxygen species that can damage membranes as well as other biomolecules. Glutathione is used to detoxify the ROS. The regeneration of glutathione depends on an adequate supply of NADPH, which is synthesized by the oxidative phase of the pentose phosphate pathway. People with low levels of the dehydrogenase are especially susceptible to vicine toxicity.

29. Because red blood cells do not have mitochondria and the only means to obtain NADPH is through the pentose phosphate pathway. There are biochemical means to convert mitochondrial NADH into cytoplasmic NADPH.

30. Reactive peroxides are a type of reactive oxygen species. The enzyme glutathione peroxidase uses reduced glutathione to neutralize peroxides by converting them into alcohols while generating oxidized glutathione. Reduced glutathione is regenerated by glutathione reductase with the use of NADPH, the product of the oxidative phase of the pentose phosphate pathway.

A27

31. for the reduction of glutathione by NADPH is +0.09 V. Hence, ΔG°′ is −17.4 kJ mol⁻¹ (−4.2 kcal mol⁻¹), which corresponds to an equilibrium constant of 1126. The required [NADPH]/[NADP⁺] ratio is 8.9 × 10⁻⁵.

32.

33.

34. Incubate an aliquot of a tissue homogenate with glucose labeled with ¹⁴C at C-1, and incubate another with glucose labeled with ¹⁴C at C-6. Compare the radioactivity of the CO₂ produced by the two samples. The rationale of this experiment is that only C-1 is decarboxylated by the pentose phosphate pathway, whereas C-1 and C-6 are decarboxylated equally when glucose is metabolized by the glycolytic pathway, the pyruvate dehydrogenase complex, and the citric acid cycle. The reason for the equivalence of C-1 and C-6 in the latter set of reactions is that glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are rapidly interconverted by triose phosphate isomerase.

35. Lacking mitochondria, red blood cells metabolize glucose to lactate to obtain energy in the form of ATP. The CO₂ results from extensive use of the pentose phosphate pathway coupled with gluconeogenesis. This coupling allows the generation of much NADPH with the complete oxidation of glucose by the oxidative branch of the pentose phosphate pathway.

36. (a)

(b) 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.

37. The reduction of each mole of CO₂ to the level of a hexose requires two moles of NADPH. The reduction of NADP⁺ is a two-electron process. Hence, the formation of two moles of NADPH requires the pumping of four moles 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 eight moles of photons is +1594 kJ (+381 kcal). Thus, the overall efficiency of photosynthesis under standard conditions is at least 477/1594, or 30%.

38. It is neither a violation nor a miracle. The equation in Section 20.1 requires not only 18 ATP, but also 12 NADPH. These electrons, if transferred to NAD⁺ and used in the electron-transport chain, would yield 30 ATP. Thus, the synthesis of glucose requires the equivalent of 48 ATP.

39. (a) The 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 would fix less carbon. Because C₄ plants can maintain a higher CO₂ concentration, the rise in temperature is less deleterious.

(b) The oxygenase activity will predominate. 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.

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

(d) 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.