Photosystems at Higher Level: An AHL statement: it opens the light-dependent reactions (3.3.9-3.3.14), which turn light into ATP and reduced NADP.
Practise this as you read
- Outline the structure and function of a photosystem.
- State where photosystems are.
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The big idea: A photosystem is an array of many pigment molecules, chlorophylls and accessory pigments, held by proteins in a membrane. Light energy absorbed anywhere in the array is passed to a reaction centre, which emits an excited electron.
The points to remember
- Photosystems are always in membranes: thylakoid membranes in chloroplasts, the cell membranes of cyanobacteria.
- Each is an array of chlorophyll and accessory pigments held by proteins.
- Absorbed light excites electrons; the energy is passed between pigments.
- The reaction centre (a pair of chlorophyll a) emits an excited electron.
- There are two kinds: photosystem II and photosystem I, each with its own reaction centre.
Remember it as: Many pigments catch, one centre throws.
Real example: cyanobacteria. Cyanobacteria have no chloroplasts, yet they photosynthesise: their photosystems sit in folded internal membranes called thylakoids, inside the cell.
Trap: It is the reaction centre that emits the excited electron, not every pigment in the array: the others pass on energy.
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How it is asked: Expect an Outline question on the structure, location and function of photosystems, or a multiple-choice statement about them.
IB-style questionOutline[3 marks]
Outline the relationship between the structure and the function of a photosystem.
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