Where do the light reactions of photosynthesis take place?

The light reactions of photosynthesis occur in the thylakoid membrane of the chloroplasts. Within the thylakoid membrane, chlorophyll and other pigments capture light energy, which is then used to drive the synthesis of ATP and NADPH. These energy-rich molecules are essential for the subsequent sugar-producing reactions that occur in the stroma.

The thylakoid membrane contains a series of protein complexes involved in the electron transport chain, where the light energy captured by chlorophyll is used to split water molecules, releasing oxygen as a byproduct. This process also generates a proton gradient across the thylakoid membrane, which is utilized by ATP synthase to produce ATP.

In contrast, the stroma, where the Calvin cycle takes place, is the fluid-filled space surrounding the thylakoids. The mitochondria are the sites of cellular respiration, and the cytosol is the liquid portion of the cytoplasm outside of organelles, neither of which are involved in the light-dependent reactions of photosynthesis. Therefore, the thylakoid membrane is specifically equipped with the necessary components for these light-driven processes.