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Chapter 19: Problem 41

Function of Cyclic Photophosphorylation When the \([\mathrm{NADPH}] /\left[\mathrm{NADP}^{+}\right]\) ratio in chloroplasts is high, photophosphorylation is predominantly cyclic (see Fig. \(19-58\) ). Is \(\mathrm{O}_{2}\) evolved during cyclic photophosphorylation? Is NADPH produced? Explain. What is the main function of cyclic photophosphorylation?

Short Answer

Expert verified
No O鈧� is evolved; no NADPH is produced. It primarily generates ATP.

Step by step solution

01

Understand Cyclic Photophosphorylation

Cyclic photophosphorylation involves the cycling of electrons within photosystem I, where electrons are transferred from ferredoxin back to the plastoquinone pool and eventually return to PSI without passing through photosystem II.
02

Analyze O鈧� Production

This process does not involve the oxidation of water (H鈧侽), and thus oxygen (O鈧�) is not produced. The main site for O鈧� production is photosystem II, which is not involved in cyclic photophosphorylation.
03

NADPH Formation

In cyclic photophosphorylation, no electrons are transferred to NADP鈦� to form NADPH, because the electrons are cycled back to PSI, not reducing NADP鈦� to NADPH.
04

Identify the Main Function

The main function of cyclic photophosphorylation is to produce ATP, which is essential for the energy requirements of the Calvin cycle and other cellular processes, without producing NADPH or O鈧�.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

ATP production
Cyclic photophosphorylation is primarily aimed at generating ATP in the chloroplasts. This process is essential as ATP acts as the energy currency of the cell, fueling numerous biological activities, especially during the Calvin cycle. During cyclic photophosphorylation, electrons are recycled around Photosystem I (PSI) rather than proceeding to Photosystem II (PSII). This cycle allows the energy from the sun to promote the synthesis of ATP through photophosphorylation.
  • ATP generated is used for the Calvin cycle, where carbon dioxide is fixed into glucose.
  • It also powers other essential cellular processes that require energy input.
Without cyclic photophosphorylation, the ATP supply would be insufficient to sustain the plant's metabolic needs.
Photosystem I
Photosystem I is a crucial component of the cyclic photophosphorylation pathway. It plays an essential role in capturing light energy and using it to energize electrons, which are then transported through an electron transport chain.
During cyclic photophosphorylation, PSI receives electrons from its surroundings and excites them with energy absorbed from light.
  • These high-energy electrons are eventually returned to PSI, making the process a cycle.
  • PSI does not involve water splitting; hence no oxygen evolution occurs here.
This recycling of electrons enables PSI to continuously contribute to ATP production without the involvement of PSII.
Electron transport chain
The electron transport chain in cyclic photophosphorylation is the pathway followed by energized electrons departing from PSI. This chain includes various proteins and coenzymes that facilitate electron transfer.
  • As electrons move through the chain, they release energy utilized to pump protons into the thylakoid lumen.
  • This build-up of protons across the thylakoid membrane creates a proton gradient, essential for ATP synthesis.
While moving through the chain, the electrons do not proceed to NADP鈦�; instead, they loop back to PSI, contributing to a continuous electron cycle that sustains ATP production.
NADPH formation
Cyclic photophosphorylation is unique because it does not produce NADPH. NADPH is commonly formed when electrons reduce NADP鈦� during non-cyclic photophosphorylation.
In the cyclic process, however, electrons are not used for NADP鈦� reduction, as they return to PSI after passing through the electron transport chain.
  • The primary focus is on ATP production rather than reducing power.
  • When NADPH levels are sufficient, cyclic photophosphorylation ensures the continuous generation of ATP.
Without NADPH formation, this process effectively balances the energy budget of the chloroplast.
Oxygen evolution
Oxygen evolution is notably absent in cyclic photophosphorylation. This is because the process bypasses Photosystem II, where the splitting of water molecules occurs, releasing oxygen as a byproduct.
Cyclic photophosphorylation involves only Photosystem I, where there is no photolysis of water. Therefore, no oxygen is generated.
  • This pathway allows the plant to meet its ATP requirements without producing extra oxygen or NADPH.
  • It's particularly beneficial under conditions where NADPH is abundant.
Understanding this characteristic helps clarify the distinct roles of the two types of photophosphorylation in plant physiology.

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Most popular questions from this chapter

27\. Balance Sheet for Photosynthesis In 1804 Theodore de Saussure observed that the total weight of oxygen and dry organic matter produced by plants is greater than the weight of carbon dioxide consumed during photosynthesis. Where does the extra weight come from\(ff\).

Limited ATP Synthesis in the Dark In a laboratory experiment, spinach chloroplasts are illuminated in the absence of ADP and \(P_{i}\), then the light is turned off and ADP and \(P_{i}\) are added. ATP is synthesized for a short time in the dark. Explain this finding.

The Pasteur Effect When \(\mathrm{O}_{2}\) is added to an anaerobic suspension of cells consuming glucose at a high rate, the rate of glucose consumption declines greatly as the \(\mathrm{O}_{2}\) is used up, and accumulation of lactate ceases. This effect, first observed by Louis Pasteur in the 1860 s, is characteristic of most cells capable of both aerobic and anaerobic glucose catabolism. (a) Why does the accumulation of lactate cease after \(\mathrm{O}_{2}\) is added? (b) Why does the presence of \(\mathrm{O}_{2}\) decrease the rate of glucose consumption? (c) How does the onset of \(\mathrm{O}_{2}\) consumption slow down the rate of glucose consumption? Explain in terms of specific enzymes.

Effect of Venturicidin on Oxygen Evolution Venturicidin is a powerful inhibitor of the chloroplast ATP synthase, interacting with the CF ont of the enzyme and blocking proton passage through the \(\mathrm{CF}_{\circ} \mathrm{CF}_{1}\) complex. How would venturicidin affect oxygen evolution in a suspension of well-illuminated chloroplasts? Would your answer change if the experiment were done in the presence of an uncoupling reagent such as 2,4 -dinitrophenol (DNP)? Explain.

Photochemical Efficiency of Light at Different Wavelengths The rate of photosynthesis, measured by \(\mathrm{O}_{2}\) production, is higher when a green plant is illuminated with light of wavelength 680 nm than with light of 700 nm. However, illumination by a combination of light of \(680 \mathrm{nm}\) and \(700 \mathrm{nm}\) gives a higher rate of photosynthesis than light of either wavelength alone. Explain.
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