Question
Describe the basic characteristics of a photosystem and explain how photosystem I differs from photosystem II.
✓
Answer
- Photosystems are the discrete units of organisation of the pigments in the thylakoid (grana) membranes.
- Each photosystem has about 250-400 pigment molecules, which include chlorophyll a, that forms the reaction centre and other accessory pigments, chlorophyll b, carotene and xanthophyll, forming the antenna of light harvesting complex (LHC).
- Chlorophyll a that forms reaction centre has two absorption maxima, at 680 nm and 700 nm; these pigments are designated as $P_{680}$ and $P_{700}$.
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PS I
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PS II
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It is constituted by those chlorophyll $P_{700}$ and other pigment molecules which absorb light energy and pass on to $P_{700}$.
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It is constituted by those chlorophyll $P_{680}$ and accessory pigments that pass on light energy to $P_{680}$.
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The reaction centre is $P_{700}$
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The reaction centre is $P_{680}$
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The electrons emitted by this come back as de-energized electrons.
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The electrons emitted by this do not come back, but are replaced by electrons generated by photolysis of water.
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It is involved only in non-cyclic photophosphorylation.
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It is involved both in cyclic and non-cyclic photophosphorylation.
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