Where do the light-dependent reactions of photosynthesis occur?

Where do the light-dependent reactions of photosynthesis occur?

• A. Thylakoid membranes
• B. Stroma
• C. Cytoplasm
• D. Nucleus

Answer: (A)

Light-dependent reactions occur in the thylakoid membranes of the chloroplast. This is where the photosystems and the electron transport chain are located, and where light energy drives electron flow, splits water, and creates a proton gradient across the membrane. The flow of protons powers ATP synthase to make ATP, and electrons are ultimately used to reduce NADP+ to NADPH on the stroma side. The stroma, not the site of these reactions, hosts the Calvin cycle, which uses ATP and NADPH to convert carbon dioxide into sugars. Cytoplasm and nucleus are not involved in hosting the light-dependent reactions.

Light-dependent reactions occur in the thylakoid membranes of the chloroplast. This is where the photosystems and the electron transport chain are located, and where light energy drives electron flow, splits water, and creates a proton gradient across the membrane. The flow of protons powers ATP synthase to make ATP, and electrons are ultimately used to reduce NADP+ to NADPH on the stroma side. The stroma, not the site of these reactions, hosts the Calvin cycle, which uses ATP and NADPH to convert carbon dioxide into sugars. Cytoplasm and nucleus are not involved in hosting the light-dependent reactions.