Which part of the chloroplast hosts the light-dependent reactions?

• A. Stroma
• B. Thylakoid membrane
• C. Matrix
• D. Chlorophyll

Answer: (B)

Light-dependent reactions happen in the thylakoid membrane of the chloroplast. This internal membrane houses the photosystems, the electron transport chain, and ATP synthase. When light excites chlorophyll in the photosystems, electrons flow through the chain and protons are pumped into the thylakoid lumen, creating a gradient that powers ATP synthesis. Water splitting at photosystem II releases O2, and electrons eventually reduce NADP+ to NADPH. The ATP and NADPH produced here are then used in the stroma for the Calvin cycle to fix carbon into sugars. The stroma is where the light-independent reactions occur, the mitochondrial matrix belongs to mitochondria, and chlorophyll is a pigment—not the site of the light-dependent processes.