Advanced Placement (AP) Biology Practice Exam

Where do the light-dependent reactions occur within chloroplasts?

1. Stroma

2. Thylakoid Membrane

3. Outer Membrane

4. Granum

The correct answer is: Thylakoid Membrane

The light-dependent reactions of photosynthesis occur in the thylakoid membrane of chloroplasts. This membrane contains chlorophyll and other pigments that capture light energy from the sun. When light is absorbed by these pigments, it energizes electrons, which leads to the production of ATP and NADPH, energy-rich compounds vital for the subsequent light-independent reactions (Calvin cycle) that take place in the stroma. The thylakoid membranes are organized into stacks known as granum, which maximize the surface area available for light absorption. While the granum is an important structure within the chloroplasts, it is specifically the thylakoid membrane that directly facilitates the processes of the light-dependent reactions. The stroma is the fluid-filled space surrounding the thylakoid membranes, where the light-independent reactions occur, but it is not the location for light-dependent reactions. The outer membrane of the chloroplast serves primarily as a protective barrier and does not participate in the photosynthetic process. Therefore, the thylakoid membrane is the precise site where the conversion of solar energy into chemical energy occurs, confirming it as the correct answer.