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Chapter 21: Problem 5

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

Short Answer

Expert verified
In noncyclic photophosphorylation, about 9.34 photons are absorbed per ATP and in cyclic photophosphorylation, 14 photons are absorbed per ATP.

Step by step solution

01

Phosphorylation Calculation

Calculate the photons absorbed per ATP in noncyclic phosphorylation. The noncyclic photosynthetic electron transport leads to the translocation of \(3 \mathrm{H}^{+} / e^{-}\), and each ATP synthase yields 3 ATP for 14 hydrogen ions translocated. So we have to find how many electrons are needed for 14 \( \mathrm{H}^{+}\)
02

Hydrogen Ion Calculation

To find the number of electrons, divide 14 by the number of hydrogen ions translocated per electron in noncyclic phosphorylation. In this case, it is 3. \( \frac{14}{3} \approx 4.67 \) electrons.
03

Photon Calculation for Noncyclic Photophosphorylation

Each electron flow in the photosynthetic transport requires 2 photons. So multiply the number of electrons by 2 to find the number of photons per ATP in noncyclic phosphorylation. In this case, \(2 \times 4.67 = 9.34 \) photons per ATP.
04

Phosphorylation Calculation for Cyclic Photophosphorylation

Calculate the photons absorbed per ATP in cyclic phosphorylation by repeating the same steps as above. Cyclic photosynthetic electron transport leads to the translocation of \(2 \mathrm{H}^{+} / e^{-}\). So, we have, \( \frac{14}{2} \approx 7 \) electrons.
05

Photon Calculation for Cyclic Photophosphorylation

Each electron flow requires 2 photons. So, multiply the number of electrons by 2 to find the number of photons per ATP in cyclic phosphorylation. In this case, \(2 \times 7 = 14 \) photons per ATP.

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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.

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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.)
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