Photosynthesis - The Light Reactions

The light reactions of photosynthesis convert radiant (sunlight) energy into the potential chemical energy found between the carbon, hydrogen, and oxygen bonds in sugar (glucose).


  1. Photosynthesis uses most of the energy in sunlight except green wavelengths (color that's reflected)


  1. The light reactions occur on thylakoid membranes in plants (similar membranes in protista or cyanobacteria)


  1. Chlorophyll molecules contain an atom of Mg (magnesium metal) which loses electrons and becomes oxidized by light. The electrons are accepted by an adjacent electron transport chain.


  1. There are 2 photosystems: photosystem I (cyclic) and photosystem II (noncyclic)


  1. Photosystem II (P680 - noncyclic)


After chlorophyll has lost electrons, enzyme X splits water (photolysis) releasing electrons to reduce chlorophyll. Two H+ ions made available by photolysis are pumped into the thylakoid lumen by PQ a mobile carrier in the electron transport chain embedded in the thylakoid membrane. Electrons do not stop until they pass through photosystem I and finally reduce NADP+ to NADPH2.


  1. Photosystem I (P700 - cyclic)


Photosystem I can act on its own in a cyclic manner sending electrons to FD (ferrodoxin) and back to P700 to pump H+ and make ATP.