When chlorophyll a absorbs light energy, an electron gains energy and is ‘excited’. The excited electron is transferred to another molecule (called a primary electron acceptor). The chlorophyll molecule is oxidized (loss of electron) and has a positive charge. Photoactivation of chlorophyll a results in the splitting of water molecules and the transfer of energy to ATP and reduced nicotinamide adenine dinucleotide phosphate (NADP).
The chemical reactions involved include:
• Condensation reactions – responsible for water molecules splitting out, including phosphorylation (the addition of a phosphate group to an organic compound)
• Oxidation/reduction (redox) reactions involving electron transfer
Photosynthesis is a two stage process.
The light dependent reactions, a light-dependent series of reactions which occur in the grana, and require the direct energy of light to make energy-carrier molecules that are used in the second process:
• Light energy is trapped by chlorophyll to make ATP (photophosphorylation)

• At the same time water is split into oxygen, hydrogen ions and free electrons:

2H2O 4H+ + O2 + 4e- (photolysis)

• The electrons then react with a carrier molecule nicotinamide adenine dinucleotide phosphate (NADP), changing it from its oxidised state (NADP+) to its reduced state (NADPH):

NADP+ + 2e- + 2H+ NADPH + H+

The light-independent reactions, a light-independent series of reactions which occur in the stroma of the chloroplasts, when the products of the light reaction, ATP and NADPH, are used to make carbohydrates from carbon dioxide (reduction); initially glyceraldehyde 3-phosphate (a 3-carbon atom molecule) is formed.