Light-dependent reactionsEdexcel International A Level Biology: Revision notes

The light-dependent reactions take place on the thylakoid membranes of the chloroplast, which are stacked into grana. The membranes contain photosystems, which are groups of photosynthetic pigments (chlorophylls and accessory pigments) arranged around a reaction centre, together with electron carriers and ATP synthase.

There are two photosystems: photosystem I (PSI) and photosystem II (PSII). Pigments absorb light, mostly red and blue, and pass the energy to the chlorophyll in the reaction centre. This raises electrons in the chlorophyll to a higher energy level, so the electrons are excited and leave the chlorophyll molecule. The chlorophyll has been oxidised (it has lost electrons).

The excited electrons are passed along an electron transport chain, a series of electron carriers in the thylakoid membrane. At each step they lose some energy.

This energy is used to pump protons (H⁺) from the stroma into the thylakoid space, which creates a proton gradient. The protons then diffuse back into the stroma through ATP synthase, and the energy of the flow is used to make ATP from ADP and inorganic phosphate. This formation of ATP using light energy is photophosphorylation, and the idea that a proton gradient drives ATP synthesis is called chemiosmosis.

In non-cyclic photophosphorylation, both photosystems are used and the electrons do not return to where they began:

1. Light is absorbed by PSII, and electrons are excited and leave the chlorophyll.
2. The electrons pass along an electron transport chain to PSI, and the energy released is used to make ATP.
3. Light is absorbed by PSI, which excites electrons again.
4. The electrons are accepted by NADP, which is reduced (with H⁺ from the stroma) to form reduced NADP.

The products are ATP, reduced NADP and (from photolysis) oxygen. The ATP and reduced NADP are used in the light-independent reactions to reduce carbon dioxide to glucose.

The electrons lost from PSII must be replaced. They come from water, which is split by light energy in a reaction called photolysis, catalysed by an enzyme associated with PSII:

2H₂O → 4H⁺ + 4e⁻ + O₂

• The electrons replace those lost from chlorophyll in PSII.
• The protons (H⁺) help to reduce NADP, and add to the proton gradient.
• The oxygen is a by-product. It is used in respiration by the plant, or leaves through the stomata.

This is the source of all the oxygen released in photosynthesis.

In cyclic photophosphorylation, only PSI is involved:

1. Light excites electrons in PSI, and they leave the chlorophyll.
2. The electrons pass along an electron transport chain, and the energy is used to make ATP.
3. The electrons return to PSI, so they are used again.

There is no photolysis, no oxygen is released and no NADP is reduced. The only product is ATP.

Cyclic photophosphorylation is useful because the light-independent reactions need more ATP than reduced NADP. It supplies extra ATP, and it can continue when water is short or when NADP is not available to accept electrons.

• Photosystems: non-cyclic uses PSII and PSI; cyclic uses PSI only.
• Electrons: non-cyclic electrons end in reduced NADP and are replaced from water; cyclic electrons return to PSI.
• Products: non-cyclic gives ATP, reduced NADP and oxygen; cyclic gives ATP only.
• Photolysis: occurs in non-cyclic only.

For exam answers, state where the electrons come from, where they go and what is made.