In the thylakoid membranes, pigment molecules are not scattered randomly. They are grouped with proteins into photosystems, each of which works like a funnel for light energy.A photosystem has two parts:
• a light-harvesting complex (antenna complex): hundreds of chlorophyll a, chlorophyll b and carotenoid molecules held by proteins. When any of them absorbs light, the energy is passed from pigment to pigment towards the centre.
• a reaction centre: a special pair of chlorophyll a molecules. When energy reaches it, an electron in the chlorophyll is excited to a higher energy level and leaves the molecule. It is taken up by an electron acceptor, and the chlorophyll is left oxidised.
This is where light energy is first converted into chemical energy, as a high-energy electron that can be passed along an electron transport chain.There are two types:
• Photosystem I (PSI): its reaction centre absorbs light best at 700 nm, so it is called P700. PSI is found mainly on the intergranal lamellae.
• Photosystem II (PSII): its reaction centre absorbs best at 680 nm (P680). PSII is found mainly in the grana.They are numbered in the order they were discovered, not the order in which they act. In non-cyclic photophosphorylation, light is absorbed by PSII first, then by PSI.Having many pigment molecules feed one reaction centre lets the photosystem absorb light efficiently over a wide range of wavelengths, even in dim light.
• a light-harvesting complex (antenna complex): hundreds of chlorophyll a, chlorophyll b and carotenoid molecules held by proteins. When any of them absorbs light, the energy is passed from pigment to pigment towards the centre.
• a reaction centre: a special pair of chlorophyll a molecules. When energy reaches it, an electron in the chlorophyll is excited to a higher energy level and leaves the molecule. It is taken up by an electron acceptor, and the chlorophyll is left oxidised.
This is where light energy is first converted into chemical energy, as a high-energy electron that can be passed along an electron transport chain.There are two types:• Photosystem I (PSI): its reaction centre absorbs light best at 700 nm, so it is called P700. PSI is found mainly on the intergranal lamellae.
• Photosystem II (PSII): its reaction centre absorbs best at 680 nm (P680). PSII is found mainly in the grana.They are numbered in the order they were discovered, not the order in which they act. In non-cyclic photophosphorylation, light is absorbed by PSII first, then by PSI.Having many pigment molecules feed one reaction centre lets the photosystem absorb light efficiently over a wide range of wavelengths, even in dim light.