Photophosphorylation?

Noncyclic Photophosphorylation:

Photosystem II works with Photosystem I and two series of enzymes imbedded in the thylakoid membrane to transfer energy from the form of light to that stored in chemical bands and gradients which the plant can use in a process called noncyclic photophosphorylation. It not only transfers the electrons for PS I but also is responsible for ATP production.



The steps of N.C. P.P.

1)Photo II absorbs light and energy which causes the P680 molecule to excite its e- and pass it onto an enzyme called plastoquinone.

2) This creates an e- deficiency in Photo II. This deficiency if filled by a molecule called Z protein, a molecule containing Mn. This enzyme is somehow stimulated by the loss of e- in photo II to split two molecules of water. The e- from this reaction are then released to the waiting e- hungry Photosystem II. This step also releases H+ in to the thylakoid space helping to create a proton gradient. O2 is also released in this step.

3)The e- ejected from Photo II are accepted by a molecule called pheophyton.

4) Pheophyton is then reduced(has its e- taken by) plastoquinone which has a higher affinity for e- than Pheophyton.

5) Plastoquinone passes the e- to a cytochrome complex called b6-f complex which has a higher affinity for e- the plastoquinone. This complex passes protons from the stroma into the thylakoid space increasing the proton gradient even more.

Plastoquinone is found within the membrane - it has a long hydrophobic tail.
6) The e- is then passed to plastocyanin which then transfers the e- to an e- deficient Photo I.

Plastocyanin is a small protein that carries the electrons on copper.. Cu +2 oxidized and +1 is reduced state; electrons just move short distances as they are loosely associated.
7)Photo I accepts energy from light and then an e- from P700 is excited and passed on to an electron acceptor called FeS.

8) FeS then passes its e- to Ferrodoxin. Ferrodoxin donates its e- to NADP+ reductase.

9)NADP+ reductase donates the e- to a molecule of NADP+ and stabilizes it by adding a proton to form NADPH. This NADPH is then released into the stroma where it becomes part of the dark reactions of biosynthesis.

10) The proton gradient created by the Z protein and the enzymes associated with Photo II is used to create ATP. H+ in the thylakoid space can only diffuse down it gradient through an enzyme called ATPsynthase. ATPsynthase consists of two parts. One is a proton channel that allows the H+ to diffuse into the stroma. The other part couples this process to the phosphorylation of ADP to from ATP.



One important thing to realize about the series of e- transfers is this: Every transfer from one enzyme to another is an oxidation / reduction reaction. The enzyme that is losing an electron is oxidized by the enzyme next to it which must have a higher affinity for e-. Another way of saying this is that the 2nd enzyme is reduced by the 1st because it (the 2nd enzyme) 'wants' the e- more.

Photosystem I works independently of Photosystem II to produce ATP through a process called cyclic photophosphorylation.



In cyclic photophosphorylation

If too little ATP is produced relative to a NADP, an alternative route is to take the electron from ferrodoxin of system I and move it to the plastoquinone of PSII instead of being used to make more of NADPH.

An e- and its energy are accepted by P700 and are passed to a series of enzymes that couple oxidation-> reduction reaction (the passing of an e- from one molecule to another with a higher affinity for e-) to the transport of protons from the stroma into the thylakoid space. This creates a proton gradient.

The only place a proton can diffuse down its gradient into the stroma, is through a molecule called ATP synthase. The molecule of ATPsynthase catalyzes the phosphorylation of an ADP to an ATP as a proton diffuses down its gradient through the enzyme. The e- that is being passed from enzyme to enzyme losses its "excess" energy and is passed back to the photosystem by another enzyme (plastocyanin) to complete this cycle. It can then be excited all over again