Chapter 21

(Integrates with Chapter \(20 .)\) Write a balanced equation for the \(Q\) cycle as catalyzed by the cytochrome \(b_{6} f\) complex of chloroplasts.

If noncyclic photosynthetic electron transport leads to the translocation of \(3 \mathrm{H}^{+} / e^{-}\) and cyclic photosynthetic electron transport leads to the translocation of \(2 \mathrm{H}^{+} / e^{-},\) what is the relative photosynthetic efficiency of ATP synthesis (expressed as the number of photons absorbed per ATP synthesized) for noncyclic versus cyclic photophosphorylation? (Assume that the \(\mathrm{CF}_{1} \mathrm{CF}_{0}-\mathrm{ATP}\) synthase yields \(3 \mathrm{ATP} / 14 \mathrm{H}^{+}\).)

(Integrates with Chapter \(20 .\)) In mitochondria, the membrane potential \((\Delta \psi)\) contributes relatively more to \(\Delta p\) (proton-motive force) than does the pH gradient \((\Delta \mathrm{pH})\). The reverse is true in chloroplasts. Why do you suppose that the proton-motive force in chloroplasts can depend more on \(\Delta\) pH than mitochondria can? Why is \((\Delta \psi)\) less in chloroplasts than in mitochondria?

Predict the consequences of a \(\mathrm{Y} 161 \mathrm{F}\) mutation in the amino acid sequence of the D1 subunit of PSII.

Write a balanced equation for the synthesis of a glucose molecule from ribulose-1,5-bisphosphate and \(\mathrm{CO}_{2}\) that involves the first three reactions of the Calvin cycle and subsequent conversion of the two glyceraldehyde-3-P molecules into glucose.

The photosynthetic \(\mathrm{CO}_{2}\) fixation pathway is regulated in response to specific effects induced in chloroplasts by light. What is the nature of these effects, and how do they regulate this metabolic pathway?

Write a balanced equation for the conversion of phosphoglycolate to glycerate-3-P by the reactions of photorespiration. Does this balanced equation demonstrate that photorespiration is a wasteful process?

The overall equation for photosynthetic \(\mathrm{CO}_{2}\) fixation is \\[6 \mathrm{CO}_{2}+6 \mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}+6 \mathrm{O}_{2}\\] \(A l l\) the \(\mathrm{O}\) atoms evolved as \(\mathrm{O}_{2}\) come from water; none comes from carbon dioxide. But \(12 \mathrm{O}\) atoms are evolved as \(6 \mathrm{O}_{2}\), and only \(6 \mathrm{O}\) atoms appear as \(6 \mathrm{H}_{2} \mathrm{O}\) in the equation. Also, \(6 \mathrm{CO}_{2}\) have \(12 \mathrm{O}\) atoms, yet there are only \(6 \mathrm{O}\) atoms in \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6} .\) How can you account for these discrepancies? (Hint: Consider the partial reactions of photosynthesis: ATP synthesis, NADP' reduction, photolysis of water, and the overall reaction for hexose synthesis in the Calvin-Benson cycle.)

Plastoquinone oxidation by cytochrome \(b c_{1}\) and cytochrome \(b_{i} f\) complexes apparently leads to the translocation of \(4^{+} / 2 e^{-} .\) If \(\mathscr{E}_{0}^{\prime}\) for cytochrome \(f=0.365 \mathrm{V} \text { (Table } 20.1)\) and \(\mathrm{E}_{\mathrm{o}}^{\prime}\) for \(\mathrm{PQ} / \mathrm{PQH}_{2}=0.07 \mathrm{V},\) calculate \(\Delta G\) for the coupled reaction: \\[2 h v+4 \mathrm{H}^{+}_{\mathrm{in}} \longrightarrow 4 \mathrm{H}_{\mathrm{out}}^{+}\\] (Assume a value of \(23 \mathrm{kJ} / \mathrm{mol}\) for the free energy change \((\Delta G)\) associated with moving protons from inside to outside.