In the light reactions of photosynthesis, which molecule directly absorbs the energy of a photon to initiate excitation of an electron in a photosystem?

• A. Chlorophyll (e.g., the reaction-center chlorophyll in photosystem II)
• B. Ribulose-1,5-bisphosphate (RuBP)
• C. ATP synthase
• D. NADP+

Answer: (A)

Verified step by step guidance

1. Understand that the light reactions of photosynthesis begin when a pigment molecule absorbs light energy (photons).
2. Identify the primary pigment responsible for absorbing light energy in the photosystems, which is chlorophyll, specifically the reaction-center chlorophyll molecules in photosystem II and photosystem I.
3. Recognize that ribulose-1,5-bisphosphate (RuBP) is involved in the Calvin cycle (dark reactions), not in the initial light absorption step.
4. Know that ATP synthase is an enzyme that synthesizes ATP using a proton gradient, but it does not absorb light energy directly.
5. Understand that NADP+ acts as an electron carrier, accepting electrons at the end of the light reactions, but it does not absorb photons to initiate electron excitation.