However, energy absorbed by chlorophyll a is not the only energy that can be used in photosynthesis. Chlorophyll b is very similar in structure to chlorophyll a , but it absorbs mainly blue and orange light, reflecting yellow-green. It then transfers the collected energy to chlorophyll a , which actually utilizes it in the light reactions. The absorption spectra of the two types of chlorophyll are shown on the graph to the right, obtained from this site . Notice that the graph of chlorophyll absorption efficiency correlates closely with the graph of overall photosynthetic efficiency shown above, indicating that chlorophyll is the main photosynthetic pigment.
Some carotenoids act like chlorophyll b in passing their energy on to chlorophyll a .
Others help to diffuse excess light energy, which could denature the chlorophyll. When a pigment absorbs a photon of light energy, one of its electrons gains energy. In photosynthesis, however, excited electrons are donated to a primary electron acceptor before they revert to their former state. In chloroplasts, pigments are clustered in systems of 200-300 molecules. Each has one main chlorophyll a molecule that donates excited electrons to primary electron acceptors.