Photosynthesis; Light Dependent Reactions--Non-Cyclic Electron Flow

Well, there are five steps in the Z diagram involving water and a protein called the Z protein. At the end of the Z diagram, the products will be H+, electrons , and 2 2H2O molecules.

a. H20 + Z protein —————-> Z-OH + H+ + e-

b. H20 + Z-OH ——————–> Z-2OH + H+ + e-

c. H20 + Z-2OH ——————> Z-3OH + H+ + e-

d. H20 + Z-3OH ——————> Z-3OH + H+ + e-

e. Z-4OH —————————> Z protein + 2 H2O + O2

Water is found in the thylakoid space. The H+ from water is pumped into the medium inside the thylakoid membrane and are used in making ATP. The four electrons are pushed into the reaction center, the O2 is released as free oxygen, and the water is placed back into the Z diagram. Thus, if the Z-protein were mutated, these reactionary process generally would not occur.

Recall that in non-cyclic flow, the electrons do not return the source, but in cyclic flow, the electrons come back to the source. Most often, the non-cyclic process occurs to produce ATP AND NADH, which will be used by the Calvin cycle to produce carbohydrates, but sometimes there occurs a cyclic process to produce ATP to cope up with Calvin cycle as it requires more ATP than the NADH. In addition, cyclic electron flow could operate independent of photosystem II. The production of oxygen and NADPH takes place in non-cyclic electron flow and the system could switch to cyclic flow upon accumulation of oxygen and NADPH.

See for more detail:

http://www.life.illinois.edu/govindjee/ZSchemeG.html
http://library.thinkquest.org/27819/ch4_10.shtml

Thanks so much !

Anytime.