Where do the light-dependent reactions occur in the chloroplast?

Where do the light-dependent reactions occur in the chloroplast?

• A. Outer membrane
• B. Stroma
• C. Thylakoid lumen
• D. Thylakoid membranes

Answer: (D)

Light-dependent reactions happen at the thylakoid membranes because the photosystems and the electron transport chain are embedded in those membranes. When light excites chlorophyll in photosystems II and I, electrons are driven through the chain while protons are pumped into the thylakoid lumen, creating a proton gradient. ATP synthase in the membrane uses that gradient to produce ATP, and electrons are transferred to NADP+, forming NADPH on the stromal side. The Calvin cycle then occurs in the stroma, using the ATP and NADPH just produced. The thylakoid lumen is the space inside the membranes where protons accumulate, but the actual reactions happen at the membrane itself.

Light-dependent reactions happen at the thylakoid membranes because the photosystems and the electron transport chain are embedded in those membranes. When light excites chlorophyll in photosystems II and I, electrons are driven through the chain while protons are pumped into the thylakoid lumen, creating a proton gradient. ATP synthase in the membrane uses that gradient to produce ATP, and electrons are transferred to NADP+, forming NADPH on the stromal side. The Calvin cycle then occurs in the stroma, using the ATP and NADPH just produced. The thylakoid lumen is the space inside the membranes where protons accumulate, but the actual reactions happen at the membrane itself.