Which statement correctly identifies photosystems?

Which statement correctly identifies photosystems?

• A. They absorb sunlight and generate high-energy electrons
• B. They directly synthesize sugars from CO2
• C. They pump protons across the membrane to create a gradient
• D. They are the main site of glycolysis

Answer: (A)

Photosystems are the light-collecting units in the thylakoid membranes that capture photons and transfer that energy to electrons, raising them to a higher energy level. This initial step is what starts the light-dependent reactions, setting electrons in motion through the electron transport chain to drive the production of ATP and NADPH. That direct role—absorbing sunlight and generating high-energy electrons—is why the statement is the best descriptor of what photosystems do. They don’t directly synthesize sugars from CO2; the Calvin cycle in the stroma uses ATP and NADPH from the light reactions to fix carbon into sugars. They also don’t themselves pump protons across the membrane; proton pumping is performed by the electron transport chain complexes as electrons pass along, with ATP synthase using the resulting gradient. And glycolysis occurs in the cytoplasm, not in chloroplasts, so photosystems aren’t the site of glycolysis.

Photosystems are the light-collecting units in the thylakoid membranes that capture photons and transfer that energy to electrons, raising them to a higher energy level. This initial step is what starts the light-dependent reactions, setting electrons in motion through the electron transport chain to drive the production of ATP and NADPH. That direct role—absorbing sunlight and generating high-energy electrons—is why the statement is the best descriptor of what photosystems do. They don’t directly synthesize sugars from CO2; the Calvin cycle in the stroma uses ATP and NADPH from the light reactions to fix carbon into sugars. They also don’t themselves pump protons across the membrane; proton pumping is performed by the electron transport chain complexes as electrons pass along, with ATP synthase using the resulting gradient. And glycolysis occurs in the cytoplasm, not in chloroplasts, so photosystems aren’t the site of glycolysis.