How a Photosystem Converts Light Energy to Chemical Energy

Types

• Photosynthesis requires a complex of proteins and chlorophyll molecules called a photosystem. There are two types: photosystem I and photosystem II.
  
  

Function

• When electrons in chlorophyll molecules absorb light energy they become "excited" and jump to a higher energy level. The molecule transfers the energy it absorbed to an electron in a neighboring molecule -- "passing along the torch" -- until finally the energy reaches a special pair of chlorophyll molecules, the P680 pair in Photosystem II and the P700 pair in Photosystem I. When an electron in P680 or P700 becomes excited, the P680 or P700 loses it to a protein called the primary electron acceptor.
  
  

Effects

• P680/P700 must replace the electron it lost. In Photosystem II, it does so with the aid of an enzyme that breaks up water molecules. Meanwhile, the primary electron acceptor transfers the electron it gained to a series of proteins called the electron transport chain, which conveys the electron to P700 in Photosystem I. The electron transport chain pumps hydrogen ions across the membrane, creating a high concentration of hydrogen ions inside. Another protein called ATP synthase uses this high concentration as a source of energy, producing a compound called adenosine triphosphate, thereby storing what was once light energy as chemical energy.