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The light-dependent reactions of photosynthesis

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The light-dependent reactions of photosynthesis

This version refers to hydroxide ions as the source of electrons, as stated by some exam boards. A more recent version is available here.

The light-dependent reactions take place in the membranous sections of the chloroplast - thylakoids (granae/lamellae) - which offer a large surface area to absorb light energy.

The main function of these reactions is to provide a source of ATP and reduced NADP which are used to reduce CO2 in the light independent reactions.

Photolysis is the splitting of water by light the basis for non-cyclic photophosphorylation.

Water is always in equilibrium with (small quantities of) hydrogen ions (protons) and hydroxide ions:

4H2Oreversible reaction 4H+ + 4OH-

4OH- reversible reaction4e- + O2 + 2H2O

Electrons (e-) are taken up by chlorophyll and passed to a chain of other compounds (see below).

Hydrogen ions react with NADP (at the end of the chain).

Oxygen is released as a by-product.

Chlorophyll absorbs light energy.

This causes it to lose electrons (which makes it positively charged, oxidised).

These electrons are repaid by taking from OH- which came from water.

This effectively pulls the equilibrium to the right, causing more water to dissociate.

Hydrogen ions (H+, protons) are also produced from water.

Electrons leaving chlorophyll have been excited (raised to a higher energy level).

They pass to an electron acceptor and on to a chain of electron carriers.

ATP is produced (twice for each electron) as energy is extracted from the electrons - phosphorylation

Similarly, after electrons are given a second excitation by another chlorophyll they are passed to another set of electron acceptor/carriers.

NADP acts as an electron acceptor and a hydrogen ion acceptor, resulting in the production of reduced NADP.

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