The reaction center in the light-harvesting complex is formed by a single chlorophyll a molecule. This chlorophyll a molecule is responsible for capturing and transferring the energy from photons of light during the process of photosynthesis.

The light-harvesting complex is a group of pigment molecules, including chlorophylls and accessory pigments, that are embedded in the thylakoid membrane of chloroplasts. These pigments absorb light energy at specific wavelengths and transfer it to the reaction center where it is converted into chemical energy.

The reaction center is the site where the primary photochemical reactions of photosynthesis occur. It consists of a special pair of chlorophyll a molecules, known as P680 in photosystem II and P700 in photosystem I. These chlorophyll molecules have a unique structural arrangement that allows them to efficiently capture and transfer the energy from photons.

The reaction center chlorophyll a molecule is surrounded by other pigments in the light-harvesting complex, which act as antenna pigments. These antenna pigments absorb light energy and transfer it to the reaction center chlorophyll a molecule through a process called resonance energy transfer. This transfer of energy occurs via excitons, which are energy packets that move between pigments.

Once the reaction center chlorophyll a molecule captures the energy from a photon, it becomes excited and enters an excited state. This excitation triggers a series of electron transfer reactions, leading to the generation of chemical energy in the form of ATP and NADPH.

In summary, the reaction center in the light-harvesting complex is formed by a single chlorophyll a molecule. This molecule plays a crucial role in capturing and transferring the energy from photons of light, initiating the process of photosynthesis.