Final answer:
The ultimate acceptor of photon energy in the reaction center is a special pair of chlorophyll a molecules, which transfer the excited electron to a primary electron acceptor, initiating the process of converting light energy into chemical energy in photosynthesis.
Step-by-step explanation:
The pigment molecule that serves as the ultimate acceptor of the photons' energy in the reaction center during photosynthesis is a special pair of chlorophyll a molecules. When these chlorophyll a molecules at the reaction center absorb light energy, one of their electrons becomes excited to a higher energy level and is subsequently transferred to the primary electron acceptor. This step is crucial as it initiates the conversion of light energy into chemical energy, which is used to power the synthesis of ATP and NADPH in the process of photosynthesis. This primary electron acceptor is not itself a pigment but rather an organic molecule or pigment protein complex that is specifically designed to receive the excited electron and pass it on to the electron transport chain.