When photons of sunlight fall into chloroplasts, chlorophyll molecules are absorbed and activated. Active chlorophyll
An energized electron from the double bond at the head of the molecule moves from the lower energy ring of the atom to the higher energy ring. The primary electron acceptor accepts electrons from the high energy ring and photosynthesis begins.
There are three types of transitions of electrons in the high energy ring. These are-
1. Electrons from the higher energy band lose energy and return to the lower energy band. In this case the absorbed energy is released as heat or emitted as fluorescence. This energy is not used in photosynthesis.
2. The absorbed energy gives an electron to a neighboring pigment and the electron from the higher energy ring returns to the lower energy ring. In this case energy is transferred but electrons are not transferred. Thus the absorbed energy of the antenna complex is transferred to the reaction center chlorophyll-a.
3. The high energy electron from the high energy ring is accepted by the primary acceptor. In this case the high energy electron is transferred and the process of photosynthesis starts.