A Level Biology Practice Exam 2025 – 400 Free Practice Questions to Pass the Exam

The light reactions of photosynthesis occur in the thylakoid membranes of the chloroplasts. These membranes house the pigments and proteins necessary for capturing light energy and converting it into chemical energy in the form of ATP and NADPH. Chlorophyll, the primary pigment involved in these reactions, is imbedded within the thylakoid membranes and plays a vital role in absorbing the light energy required to drive the photosynthetic process.

The granum refers to a stack of thylakoids and is a structural component that enhances the efficiency of light absorption by increasing the surface area available for light capture. Therefore, both the granum (as a collection of thylakoids) and the thylakoid membranes themselves are crucial to the light reactions.

This inclusion of both the granum and the thylakoid membranes highlights the interconnectedness of different components in the photosynthetic process, leading to the conclusion that the correct answer encompasses both structures.