A crucial prereq. Human beings do not produce energy by photosynthesis, yet this process is critical to our survival. Explain.

The accounting. What is the overall reaction for the light reactions of photosynthesis?

Like a fife and drum. Match each term with its description.

A single wavelength. Photosynthesis can be measured by measuring the rate of oxygen production. When plants are exposed to light of wavelength 680 nm, more oxygen is evolved than if the plants are exposed to light of 700 nm. Explain.

Combining wavelengths. If plants described in problem 4 are illuminated by a combination of light of 680 nm and 700 nm, the oxygen production exceeds that of either wavelength alone. Explain.

Complementary powers. Photosystem I produces a powerful reductant, whereas photosystem II produces a powerful oxidant. Identify the reductant and oxidant and describe their roles.

If a little is good. What is the advantage of having an extensive set of thylakoid membranes in the chloroplasts?

Cooperation. Explain how light-harvesting complexes enhance the efficiency of photosynthesis.

One thing leads to another. What is the ultimate electron acceptor in photosynthesis? The ultimate electron donor? What powers the electron flow between the donor and the acceptor?

Neutralization compensation. In chloroplasts, a greater pH gradient across the thylakoid membrane is required to power the synthesis of ATP than is required across the mitochondrial inner membrane. Explain this difference.

Environmentally appropriate. Chlorophyll is a hydrophobic molecule. Why is this property crucial for the function of chlorophyll?

Proton origins. What are the various sources of protons that contribute to the generation of a proton gradient in chloroplasts?

Efficiency matters. What fraction of the energy of 700-nm light absorbed by photosystem I is trapped as high-energy electrons?

That’s not right. Explain the defect or defects in the hypothetical scheme for the light reactions of photosynthesis depicted here.

Electron transfer. Calculate the and ΔG°′ for the reduction of NADP⁺ by ferredoxin. Use data given in Table 18.1.

To boldly go. (a) It can be argued that, if life were to exist elsewhere in the universe, it would require some process like photosynthesis. Why is this argument reasonable? (b) If the starship Enterprise were to land on a distant planet and find no measurable oxygen in the atmosphere, could the crew conclude that photosynthesis is not taking place?

Weed killer 1. Dichlorophenyldimethylurea (DCMU), a herbicide, interferes with photophosphorylation and O₂ evolution. However, it does not block O₂ evolution in the presence of an artificial electron acceptor such as ferricyanide. Propose a site for the inhibitory action of DCMU.

Weed killer 2. Predict the effect of the herbicide dichlorophenyldimethylurea (DCMU) on a plant’s ability to perform cyclic photophosphorylation.

Infrared harvest. Consider the relation between the energy of a photon and its wavelength.

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Missing acceptors. Suppose that a bacterial reaction center containing only the special pair and the quinones has been prepared. Given the separation of 22 Å between the special pair and the closest quinone, estimate the rate of electron transfer between the excited special pair and this quinone.

Close approach. Suppose that energy transfer between two chlorophyll a molecules separated by 10 Å takes place in 10 picoseconds. Suppose that this distance is increased to 20 Å with all other factors remaining the same. How long would energy transfer take?

Functional equivalents. What structural feature of mitochondria corresponds to the thylakoid membranes?

Compare and contrast. Compare and contrast oxidative phosphorylation and photosynthesis.

Energy accounts. In Section 20.1, the balance sheet for the cost of the photosynthetically powered synthesis of glucose is presented. Eighteen molecules of ATP are required. Yet, when glucose undergoes combustion in cellular respiration, 30 molecules of ATP are produced. Account for the difference.

Looking for a place to rest. Albert Szent-Györgyi, Nobel Prize–winning biochemist, once said, “Life is nothing but an electron looking for a place to rest.” Explain how this pithy statement applies to photosynthesis and cellular respiration.

Hill reaction. In 1939, Robert Hill discovered that chloroplasts evolve O₂ when they are illuminated in the presence of an artificial electron acceptor such as ferricyanide [Fe³⁺(CN)₆]³⁻. Ferricyanide is reduced to ferrocyanide [Fe²⁺(CN)₆]⁴⁻ in this process. No NADPH or reduced plastocyanin is produced. Propose a mechanism for the Hill reaction.

The same, but different. The α₃β₃γ complex of mitochondrial or chloroplast ATP synthase functions as an ATPase in vitro. The chloroplast enzyme (both synthase and ATPase activity) is sensitive to redox control, whereas the mitochondrial enzyme is not. To determine where the enzymes differ, a segment of the mitochondrial γ subunit was removed and replaced with the equivalent segment from the chloroplast γ subunit. The ATPase activity of the modified enzyme was then measured as a function of redox conditions. Dithiothreitol (DTT) is a reducing agent.