Introduction to photosynthesis

Nature of light


Light reactions

Calvin cycle and C4

Photosystem I and II and the Light Reaction

Pigments form aggregates on the thylakoid membrane called photosystems. The purpose of these photo systems is to collect energy over a "broad" range of wavelengths and concentrate it to one molecule called a reaction center which uses the energy to pass one of its electrons on to a series of enzymes.

This aggregate of different proteins is called an antennae complex.

The photosystems work through resonance effects. It works like this: A pigment in the photosystem absorbs the appropriate energy level of light which boasts its e- to a higher energy level. For this pigment to drops its e- back into the stable lower energy state, the molecule must pass its excess energy on to another pigment molecule.

When this happens, an e- in the other pigment is excited and the same things has to happen. Eventually the energy gets passed onto the reaction center.

The reaction center is then able to get rid of the energy by passing the energy and its e- onto a series of enzymes. The reaction center is the only molecule which can relieve the photosystem of the excess energy. This means that all other pigments will pass the light energy through resonance until it reaches the reaction center.

There are two kinds of Photosystems in most photosynthetic eukaryotes. When working together, they absorb enough energy from the sun to split an molecule of water.

Photosystem I probably was the 1st to develop and can exist independently of Photosystem II to create energy for a plant. However, the enzymes it is associated with when it works independently are different then those it is associated with when it works with Photosystem II.

Photo I consists largely of chlorophyll a molecules and contains no or few chlorophyll b.

Its reaction center , a molecule called P700, absorbs light of 700 nm maximally.

Photosystem II contains both chla a and chl b

Its reaction center absorbs P680 maximally


In photosystem I energy is absorbed by a pair of P700 chl a molecules raising to an excited energy level. From there they pass onto FeS4, then onto ferrodoxin, and finally onto ferrodoxin-NADP reductase. After 2 electrons have reduced ferrodoxin-NADP reductase they are transferred to NADP+ reducing it to NADPH and a H+

Photosystem II is the second photosystem to develop in most higher autotrophs. It works together with Photosystem I to absorb enough energy to the separate the oxygen of a water molecule from its e-. Remember this is the first half of the photosynthesis half reaction : 2H2O -> O2 + 4e- + 4H+.

Photosystem II contains chlorophyll a, as well as up to 50% chlorophyll b. It probably evolved later as a supplement to Photo I. It is needed to capture enough energy to do the biosynthetic reactions of the dark reaction. Its reaction center is a molecule called P680 which absorbs light maximally at 680 nm.


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