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| 1. PSI operates at the most negative potential. PSI generates the strongest reducing agent. Both photosystems must absorb four photons during each round. 2. C 4 plants. 3. Reduced ferredoxin is the strongest reducing agent, O 2 (or Tyr z + ) is the strongest oxidizing agent, O 2 (or Tyr z + ) has the greatest affinity for electrons, reduced ferredoxin has the most energetic electrons. 4. #s 2, 4, and 5. 5. 236 mV 6. When NADPH levels are high. 7. Isolated chloroplasts raise the pH of the medium, whereas isolated mitochondria lower it. 8. In chloroplasts, the internal compartment of the thylakoid can be maintained at a very low pH because there are virtually no enzymes in that compartment. In contrast, the mitochondria eject protons into the medium, which would seriously affect enzymes of the cytosol. 9. PSII is directly responsible for the translocation of protons and electrons, whereas PSI is responsible only for the movement of electrons, which are transferred to NAD + and do not contribute to the electrochemical gradient. 10. No. The C 4 plant has to use additional ATP to convert pyruvate to PEP. 11. P680 and P700; P700 and NADPH 12. 18 ATPs (6 to regenerate RuBP) and 12 NADPHs are presumably required for each hexose produced from CO 2 . According to the numbers shown in Figure 6.16, noncyclic electron transport should be able to generate approximately 3 molecules of ATP for each pair of NADPHs, which is the ratio required for CO 2 assimilation. 13. Tyr z is the primary donor of PSII, and plastocyanin is the primary donor of PSI. 14. Manganese is part of the oxygen-evolving complex and acts to store electrons removed from H 2 O; Magnesium is part of all the chlorophyll molecules and is involved in light absorption; Iron is part of cytochrome b 6 f and the varous iron-sulfur proteins and undergoes oxidation and reduction during electron transport. 15. It would have a greater effect on C 3 plants, which are more sensitive to CO 2 levels, because their only CO 2 fixing enzyme has relatively low affinity for CO 2 . 16. Yes, because C 3 plants have to keep their stomata open to maintain a high internal CO 2 concentration, would lead to less water loss in a humid environment than a dry environment. 17. Greater CO 2 levels would be expected to lead to an increased ratio of photosynthesis over photorespiration and, consequently, to increased plant life, which would increase O 2 levels. In fact, it is estimated that at current CO 2 levels of 380 ppm, O 2 levels might rise over a long period of time to up to 28% of atmospheric gases. 18. It would greatly favor C 3 plants, because the CO 2 concentration mechanism of C 4 plants would be of little value and its extra energy costs would place C 4 plants at a disadvantage. 19. The products of photorespiration, e.g., glycolate, glycine, serine. 20. If the proton-motive force should drop, as can occur if a cell is deprived of O 2 , the inhibitor will block the enzyme from catalyzing the reverse reaction in which ATP is hydrolyzed. |