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Chapter 21: Problem 12

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?

Short Answer

Expert verified
The effects of light in chloroplasts induce the production of ATP and NADPH in light-dependent reactions, which consequently regulate the photosynthetic CO2 fixation pathway, also known as the Calvin Cycle, by providing the necessary energy for the conversion of atmospheric CO2 into sugars.

Step by step solution

01

Understanding the light-dependent phase of photosynthesis

Photosynthesis is the process where chloroplasts within plant cells convert sunlight into chemical energy. It is divided into light-dependent reactions and light-independent reactions. The light-dependent reactions take place within the thylakoid membrane of the chloroplast, where absorbed light energy is converted into ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).
02

Role of light effects in chloroplasts

Sunlight influences the transfer of electrons in chlorophyll, the pigment in chloroplasts that absorbs light. This electron transfer leads to the production of ATP and NADPH, and the release of oxygen. Moreover, light modulation controls opening and closing of stomata, impacting CO2 availability. The energy from ATP and NADPH allows plants to convert CO2 absorbed from the atmosphere into more complex organic molecules in a process known as the Calvin cycle.
03

Regulation of CO2 fixation pathway

The light-inducible effects in chloroplasts regulate the CO2 fixation pathway predominantly through the production of ATP and NADPH. The CO2 fixation pathway, also known as the Calvin cycle, uses ATP and NADPH from the light-dependent reactions to convert atmospheric CO2 into sugars. This regulation via availability of ATP and NADPH ensures that the energy intensive synthesis of sugars mainly happens when sunlight and thus energy is readily available.

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Most popular questions from this chapter

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

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

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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