Advanced Placement (AP) Biology Practice Exam

Which condition is necessary for the light-dependent reactions to occur?

1. Availability of water

2. Presence of glucose

3. Dark environment

4. Low temperatures

The correct answer is: Availability of water

The light-dependent reactions of photosynthesis require the availability of water, as this is essential for the process of photolysis, where water molecules are split into oxygen, protons, and electrons. This reaction takes place in the thylakoid membranes of chloroplasts when light is absorbed by chlorophyll pigments. The electrons released from the water molecules are then transferred through the electron transport chain, leading to the production of ATP and NADPH, which are crucial energy carriers used in the subsequent light-independent reactions (Calvin cycle). In contrast, the presence of glucose is not necessary for the light-dependent reactions; rather, glucose is a product of the light-independent reactions that occur later in the photosynthetic process. A dark environment would inhibit the light-dependent reactions since they rely on light energy; without light, there would be no energy to drive the capture of electrons and production of energy-rich molecules. While low temperatures may affect enzyme activity in photosynthesis, they do not specifically negate the requirement for light, making water essential for the initiation of these light-dependent processes.