Photosystem I & II
photosystem II absorbs light, an electron excited to a higher energy level
in the reaction center chlorophyll (P680) is captured by the primary
electron acceptor. The oxidized
chlorophyll is now a very strong oxidizing agent; its electron “hole” must
enzyme extracts electrons from water and supplies them to P680, replacing
the electrons that the chlorophyll molecule lost when it absorbed light
energy. This reaction splits a water
molecule into two hydrogen ions and an oxygen atom, which immediately
combines with another oxygen atom to form O2. This splitting of water is responsible
for the release of O2 into the air.
photoexcited electron (energized by light) passes from the primary
electron acceptor in photosystem II to photosystem I
via an electron transport chain.
This electron transport chain is very similar to the one in
cellular respiration; however, the carrier proteins in the chloroplast ETC
are different from those in the mitochondrial ETC.
electron move down the chain, their exergonic “fall”to a lower energy
level is harnessed by the thylakoid membrane to produce ATP (by
chemiosmosis). The production of
ATP in the chloroplast is called photophosphorylation because the energy
harnessed in the process originally came from light. This process of ATP production is called
The ATP generated in this process will provide the energy for the
synthesis of glucose during the Calvin cycle (light independent
an electron reaches the “bottom” of the electron transport chain, it fills
an electron “hole” in the chlorophyll a molecule in the reaction center of
photosystem I (P700). The hole was
created when light energy drives an electron from P700 to the primary
electron acceptor of photosystem I.
primary electron acceptor of photosystem I passes the excited electrons to
a second electron transport chain which transmits them to an
iron-containing protein. An enzyme
reaction transfers the electrons from the protein to NADP+ that
forms NADPH (which has high chemical energy due to the energy of the
electrons). NADPH is the reducing
agent needed for the synthesis of glucose in the Calvin cycle.
Cyclic vs. Non-cyclic Electron Flow
Under certain conditions, the photoexcited electrons take an
alternative path called cyclic electron flow, which uses photosystem I (P700)
but not photosystem II (P680). This
process produces no NADPH and no O2, but it does make ATP.
This is called cyclic photophosphorylation.
The chloroplast shifts to this process when the ATP supply drops and the
level of NADPH rises. Often the amount
of ATP needed to drive the Calvin cycle exceeds what is produced in non-cyclic
photophosphorylation. Without sufficient
ATP, the Calvin cycle will slow or even stop.
The chloroplast will continue cyclic photophosphorylation until the ATP
supply has been replenished. ATP is
produced through chemiosmosis in both cyclic and non-cyclic