Photosynthesis

Green plants make their own food by photosynthesis. During this process carbon dioxide form the
atmosphere is taken in by leaves through stomata and used for making carbohydrates, principally glucose
and starch. Photosynthsis takes place only in the green parts of the plants, mainly the leaves. Within the
leaves, the mesophyll cells have a large number of chloroplasts that are responsible for CO2 fixation.
Within the chloroplasts, the membranes are sites for the light reaction, while the chemosynthetic pathway
occurs in the stroma. photosynthesis has two stages the light reaction and the carbon fixing reations. in
the light reaction the light energy is absorbed by the pigments present in the antenna and funnelled to
special chlorophyll a molecules called reaction centre chlorophylls there are two photosystems, PS I
and PS II. PS I has a 700 nm absorbing chlorophyll a P700 molecule at its reaction center, while PS II
has a P680 reaction centre that absorbs red loght at 680 nm After absorbing light, electrons are excited
and transferred through PS II and PS I and finally to NAD forming NADH. during this process a proton
gradient due to movement through the thylakoid The breakdown of the protons gradient due to movement
through the F0 part of the ATPase enzyme releases enough energy for synthesis of ATP splitting of water
molecles is associated with PS II resulting in the release of 02, protons and transfer of electrons to PS
II In the carbon fixation cyale, CO2 is added by the enzyme, RuBisCO, to a 5 carbon compound RuBP
that is converted to 2 molecules of 3- carbon PGA. this is then converted to sugar by the Calvin cycle,
and the RuBp is regenerated. During this process ATP and NADPH synthesised in the light reaction are
utilsed RuBisCO also catalyses a wasteful oxygenation reaction in C3 plants: Photorespiration Some
tropical plants show a special type of photosynthesis called C4 pathway in these plants the first product
of CO2 fixation that takes place in the mesophyll, is a 4-carbon compound. in the bundle sheath cells
the Calvin pathway is carried out for the synthesis of carbohydrates.

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