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).
- Photosynthesis uses most of the energy in
sunlight except green wavelengths (color that's
- The light reactions occur on thylakoid
membranes in plants (similar membranes in protista or
- 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
- There are 2 photosystems: photosystem I
(cyclic) and photosystem II (noncyclic)
- 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.
- 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.