Summary for Plant Pigments and Photosynthesis

Light energy in the form of photons is harvested by systems of conjugated double bonds in pigment molecules. Because each pigment can only absorb photons with an energy that exactly matches the amount to excite an electron to an excited state, each pigment has a characteristic color. Both chlorophyll and carotenoid pigments are produced in biosynthetic pathways that also produce other important biomolecules. 

Summary Diagram: Overall illustration of the two photosystems in plants.

The chlorophyll and carotenoid molecules are organized in the chloroplast membranes into two photosystems. Most of the pigments in each photosystem act as antenna and harvest light. A few specialized pigments form the reaction centers and carry out the photochemical electron transfers. Absorption of light by the two photosystems excites electrons in two steps and transfers them from water to the NADPH; two photosystems are required because the energy difference between the electrons in water and NADPH is greater than the energy in a single photon.

Simultaneously, the electron transfer pumps protons across the photosynthetic membrane, providing the energy to synthesize ATP. With sufficient quantities of NADPH and ATP, the chloroplast is able to complete the process of photosynthesis by producing sugars from CO2.