An investigator exposes chloroplasts to 700-nm photons and observes low O2 production, but high ATP production. Which of the following best explains this observation? a. The electrons from water are directly transferred to NADP+, which is used to generate ATP. b. Photosystem II is not splitting water, and the ATP is being produced by cycling electrons via photosystem I. c. The O2 is being converted to water as a terminal electron acceptor in the production of ATP. d. Electron transport has stopped and ATP is being produced by the Calvin cycle.

Verified step by step guidance

1. Understand the role of light in photosynthesis: Photosynthesis in plants involves two main processes, the light reactions and the Calvin cycle. The light reactions use light energy to produce ATP and NADPH, while the Calvin cycle uses these molecules to produce glucose.
2. Identify the role of different wavelengths: Photons with a wavelength of 700 nm are primarily absorbed by Photosystem I (PSI). PSI is involved in the cyclic electron flow, which primarily produces ATP without the production of NADPH or oxygen.
3. Analyze the options: Option b suggests that Photosystem II (PSII) is not active, meaning water is not being split into oxygen, protons, and electrons. This would result in low oxygen production. The electrons are cycled back through PSI, leading to the production of ATP.
4. Evaluate other options: Option a is incorrect because it suggests direct transfer of electrons to NADP+, which contradicts the observation of high ATP and low oxygen. Option c is incorrect as oxygen is not converted back to water in photosynthesis. Option d is incorrect because the Calvin cycle does not produce ATP directly.
5. Conclude with the correct answer: Based on the analysis, the best explanation for the observation of low oxygen but high ATP production when chloroplasts are exposed to 700-nm photons is that PSII is not active, and ATP is being produced by cyclic electron flow through PSI. Therefore, the correct answer is b.

Key Concepts

Chloroplast Function

Chloroplasts are organelles in plant cells responsible for photosynthesis, converting light energy into chemical energy. They contain thylakoids where light-dependent reactions occur, producing ATP and NADPH, and the stroma where the Calvin cycle synthesizes glucose. Understanding chloroplast function is crucial for interpreting how light energy is transformed and how this relates to oxygen and ATP production.

Photosystems and Electron Transport Chain

Photosystems I and II are protein complexes in the thylakoid membranes that capture light energy and drive the electron transport chain. Photosystem II splits water molecules to release oxygen and provide electrons, while Photosystem I helps in the reduction of NADP+ to NADPH. The cycling of electrons through these systems is essential for ATP production, especially under varying light conditions.

ATP Production Mechanisms

ATP can be produced through photophosphorylation during the light-dependent reactions of photosynthesis, primarily via chemiosmosis. In this process, a proton gradient generated by the electron transport chain drives ATP synthase to convert ADP and inorganic phosphate into ATP. Understanding the conditions under which ATP is produced, such as the absence of water splitting, is key to explaining the observed high ATP production despite low oxygen output.