Which of the following is cycled in the cyclic variation of the light reactions?

• A. ATP
• B. NADPH
• C. Ribulose bisphosphate
• D. Protons
• E. Electrons

Answer: (E)

Verified step by step guidance

1. Understand the context: The light reactions of photosynthesis occur in the thylakoid membranes of chloroplasts and involve the conversion of light energy into chemical energy.
2. Identify the cyclic electron flow: In the cyclic variation of the light reactions, electrons are cycled back to the photosystem rather than being used to reduce NADP+ to NADPH.
3. Recognize the role of electrons: Electrons are excited by light energy and transferred through a series of proteins in the electron transport chain, eventually returning to the photosystem in cyclic electron flow.
4. Differentiate between cyclic and non-cyclic flow: In non-cyclic electron flow, electrons are used to produce both ATP and NADPH, while in cyclic flow, only ATP is produced, and electrons return to the photosystem.
5. Conclude that electrons are the component cycled in the cyclic variation of the light reactions, as they are repeatedly excited and returned to the photosystem.