Photosynthesis: Light Reactions

Light Reactions of Photosynthesis. 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.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. Read more: http://www2.mcdaniel.edu/Biology/botf99/photo/l4ightrx.html

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Channels: Biochemistry

Tags: Light Reactions

Uploaded by: ( Send Message ) on 24-11-2010.

Duration: 4m 7s