Advanced Placement (AP) Biology Practice Exam

Which photosystem initiates the electron transport chain in photosynthesis?

1. Photosystem I

2. Photosystem II

3. Photosystem III

4. Light-Harvesting Complex

The correct answer is: Photosystem II

The electron transport chain in photosynthesis is initiated by Photosystem II. This photosystem absorbs light energy, primarily in the blue and red wavelengths, and uses that energy to oxidize water molecules. This process leads to the release of oxygen as a by-product while generating high-energy electrons. These electrons are then transferred through the electron transport chain, a series of proteins embedded in the thylakoid membrane of chloroplasts. As the electrons move through the chain, they lose energy, which is harnessed to pump protons into the thylakoid lumen, creating a proton gradient. This gradient is essential for the synthesis of ATP, as protons flow back across the membrane through ATP synthase. Photosystem II is thus crucial in the light-dependent reactions of photosynthesis, setting in motion the production of energy-rich molecules that fuel subsequent stages of the process.