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Chapter 21: Problem 4

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

Short Answer

Expert verified
The balanced equation for the Q cycle as catalyzed by the cytochrome b6f complex of chloroplasts is \(2 PQH_2 + 2 PC + 2 H^+_n\) → \(2 PQ + 2 PC\(red\) + 4 H^+_p\)

Step by step solution

01

- Understand the process

The Q cycle is a series of electron transfers within the cytochrome b6f complex. The initial reactants include two molecules of plastoquinol, which are reduced forms of plastoquinone.
02

- Formulate the reactions

Two cycles are needed for the process, including both the electron transfer and proton translocation. The reactions are as follows:- 2 Plastoquinol (PQH2) each donate an electron to different entities: 1. An electron from the first PQH2 is passed on to Cytochrome b6, then to Cytochrome f, and finally reducing plastocyanin (PC). 2. An electron from the second PQH2 is passed on to a semiubiquinone radical (PQ-), therefore forming another PQH2.Both Plastoquinols (PQH2) are then oxidized to Plastoquinones (PQ).
03

- Write Balanced Equation for Q Cycle

The overall balanced equation, combining both reactions from Step 2:- \(2 PQH_2 + 2 PC + 2 H^+_n\) → \(2 PQ + 2 PC\(red\) + 4 H^+_p\)where \(H^+_n\) indicates the protons in the n-side (inside) of the membrane and \(H^+_p\) in the p-side (outside) of the membrane, and PC(red) represents the reduced plastocyanin.

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Most popular questions from this chapter

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?

(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?

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 \(3 \mathrm{ATP} / 14 \mathrm{H}^{+}\).)

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