Where do the light-dependent reactions take place in chloroplasts?

The light-dependent reactions of photosynthesis take place in the thylakoid membrane of chloroplasts. This internal membrane contains the photosystems, electron carriers, and ATP synthase needed to convert light energy into chemical energy.

When chlorophyll and other pigments absorb photons, energized electrons move through an electron-transport chain embedded in the membrane. Water is split to replace electrons lost by photosystem II, releasing oxygen as a by-product. As electrons move through the chain, protons accumulate inside the thylakoid lumen, creating a concentration gradient.

ATP synthase uses that proton gradient to make ATP, while photosystem I helps generate NADPH. ATP and NADPH then travel into the stroma, where they power the Calvin cycle and help convert carbon dioxide into carbohydrate.

The thylakoid membrane is often arranged in stacks called grana, connected by unstacked regions called stroma lamellae. A frequent mix-up is choosing the stroma: the stroma is where the Calvin cycle occurs, while the light reactions occur on the membrane surrounding the thylakoid lumen. The chloroplast’s outer and inner envelope membranes are not the main site of these reactions.