Photosynthesis

What is phtosynthesis

It is the process in which light energy is converted into chemical energy.

Generally we say that thisprocess occurs in green plants due to the presence of pigment cholorophyll.

But actually there are four pigments involved inThis process.These are chlorophyll a [bluegreen] holchlorophyll

b[yellow green]xantkophyll[yellow]carotenoids[yellow orange]

What is light reation

• Light reations are also known as photochamicalphase.• It include light absorption,water

splitting,oxygen release and the formation of highenergy chemical intermediates ATP and NADPH.

Light harvesting complexes• The pigments are organised into two photochemical light harvesting

complexes LHCwhithin the photosystem1[PS1 ]and photosystem11[PS11].• Each photosystem is made up of hundreds of pigment molecules bound

to proteins.each photosystem has a molecule of chlorophyll a which form the reaction centre and a large no. of molecules of other pigments which makes antennae these pigment helps to make photosynthesis more efficient by absorbing different wavelengths of light the reaction centre is different in both the photosystem in PS1 it has an absorption peak at 700nm so it is called P700 and in PS2 it has abortion at 680nm and is called P680.

• The light reaction involes two type of photophosphorylation.1. Non cyclic photophosphorylation.2. Cyclic photophosphorylation.

Synthesis of ATP from light reaction To explain this process chemiosmotic hypothesis has been given according to this hypothesis proton greadient helps in the synthesis of ATP the points of this hypothesis are as follows •Splitting of water produces protons which accumulate in the lumen of the thylakoids.•As electon move through the photosystems protons are tansported in the lumen.•The NADP reductase is located on the iner side of the membran so they take protons from the stroma.This causes proton greadint that is more proton in the lemun of thylakoid then stroma so there is movement of proton from thylokoid lemun to stroma & energy is released.This energy is converted into ATP by the enzyme ATPasc

Dark reaction (the calvin cycle) In this process the ATP produced in light reactions are utilized in the fixation of carbondioxide into starch molecules it occurs in three stages .Carboxilation :- this reaction is catalysed by RuBP carboxilase which result in the formation of two molecules of 3 PGA .Reduction:- In this step NADPH is reduced and its hydrogen is utilized convertion of one molecule of carbondioxide into glucose .Regeneration:- in this process RuBP is regenerated to continue the cycle