1. Ultimately, all of the carbon atoms of which we are made, not just carbohydrates, enter the biosphere through the process of photosynthesis. Moreover, the oxygen that we require is produced by photosynthesis.

2. 2 NADP⁺ + 3 ADP³⁻ + 3 P_i²⁻ + H⁺ → O₂ + 2 NADPH + 3 ATP⁴⁻ + H₂O

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

4. Photosystem II, in conjunction with the oxygen-generating complex, powers oxygen release. The reaction center of photosystem II absorbs light maximally at 680 nm.

5. Oxygen consumption will be maximal when photosystems I and II are operating cooperatively. Oxygen will be efficiently generated when electrons from photosystem II fill the electron holes in photosystem I, which were generated when the reaction center of photosystem I was illuminated by light of 700 nm.

6. Photosystem I generates ferredoxin, which reduces NADP⁺ to NADPH, a biosynthetic reducing power. Photosystem II activates the water-oxidizing complex, generating electrons for photosynthesis, and generating protons to form a proton gradient and to reduce ferredoxin and O₂.

7. The light reactions take place on thylakoid membranes. Increasing the membrane surface increases the number of ATP- and NADPH-generating sites.

8. These complexes absorb more light than can a reaction center alone. The light-harvesting complexes funnel light to the reaction centers.

9. NADP⁺ is the acceptor. H₂O is the donor. Light energy.

10. The charge gradient, a component of the proton-motive force in mitochondria, is neutralized by the influx of Mg²⁺ into the lumen of the thylakoid membranes.

11. Chlorophyll is readily inserted into the hydrophobic interior of the thylakoid membranes.

12. Protons released by the oxidation of water; protons pumped into the lumen by the cytochrome bf complex; protons removed from the stroma by the reduction of NADP⁺ and plastoquinone.

13. 700-nm photons have an energy content of +172 kJ mol⁻¹. The absorption of light by photosystem I results in a of −1.0 V. Recall that , where F = 96.48 kJ mol⁻¹. V⁻¹ Under standard conditions, the energy change for the electrons is 96.5 kJ. Thus, the efficiency is 96.5/172 = 5 56%.

14. The electron flow from photosystem II to photosystem I is uphill, or exergonic. For this uphill flow, ATP would need to be consumed, defeating the purpose of photosynthesis.

15. .

16. (a) All ecosystems require an energy source from outside the system, because the chemical energy sources will ultimately be limited. The photosynthetic conversion of sunlight is one example of such a conversion.

(b) Not at all. Spock would point out that chemicals other than water can donate electrons and protons.

17. DCMU inhibits electron transfer in the link between photosystems II and I. O₂ can evolve in the presence of DCMU if an artificial electron acceptor such as ferricyanide can accept electrons from Q.

18. DCMU will have no effect, because it blocks photosystem II, and cyclic photophosphorylation uses photosystem I and the cytochrome bf complex.

19. (a) +120 kJ einstein⁻¹ (+28.7 kcal einstein⁻¹)

(b) 1.24 V

(c) One 1000-nm photon has the free energy content of 2.4 molecules of ATP. A minimum of 0.42 photon is needed to drive the synthesis of a molecule of ATP.

20. At this distance, the expected rate is one electron per second.

21. The distance doubles, and so the rate should decrease by a factor of 64 to 640 ps.

22. The cristae.

23. In eukaryotes, both processes take place in specialized organelles. Both depend on high-energy electrons to generate ATP. In oxidative phosphorylation, the high-energy electrons originate in fuels and are extracted as reducing power in the form of NADH. In photosynthesis, the high-energy electrons are generated by light and are captured as reducing power in the form of NADPH. Both processes use redox reactions to generate a proton gradient, and the enzymes that convert the proton gradient into ATP are very similar in both processes. In both systems, electron transport takes place in membranes inside organelles.

24. We need to factor in the NADPH because it is an energy-rich molecule. Recall from Chapter 18 that NADH is worth 2.5 ATP if oxidized by the electron-transport chain.12 NADPH = 30 ATP. Eighteen molecules of ATP are used directly, and so the equivalent of 48 molecules of ATP is required for the synthesis of glucose.

25. Both photosynthesis and cellular respiration are powered by high-energy electrons flowing toward a more-stable state. In cellular respiration, the high-energy electrons are derived from the oxidation of carbon fuels as NADH and FADH₂. They release their energy as they reduce oxygen. In photosynthesis, high-energy electrons are generated by absorbing light energy, and they find stability in photosystem I and ferridoxin.

A26

26. The electrons flow through photosystem II directly to ferricyanide. No other steps are required.

27. (a) Thioredoxin

(b) The control enzyme is unaffected, but the mitochondrial enzyme with part of the chloroplast γ subunit increases activity as the concentration of DTT increases.

(c) The increase was even larger when thioredoxin was present. Thioredoxin is the natural reductant for the chloroplast enzyme, and so it presumably operates more efficiently than would DTT, which probably functions to keep the thioredoxin reduced.

(d) They seem to have done so.

(e) The enzyme is susceptible to control by the redox state. In plant cells, reduced thioredoxin is generated by photosystem I. Thus, the enzyme is active when photosynthesis is taking place.

(f) Cysteine.

(g) Group-specific modification or site-specific mutagenesis.