ATP is a molecule that can store small packets of energy. It is made by combining the adenosine diphoshate molecule (ADP) and a single phosphate molecule (P). The energy that is used to combine the Phosphate with the ADP is exactly the same as the amount released when the ATP molecule breaks down. This is approximately 30KJ of energy.
ADP + P + 30KJ ENERGY ------> ATP
The enzyme used to synthesise ATP is known as ATP synthase. ATP synthases use Proton Gradients to accelerate the bonding of ADP and P to form ATP.
During the process of photosynthesis light energy from the sun is absorbed by the Chlorophyll pigment. This "excites" an electron in the chlorophyll molecule i.e. an electron is promoted to an excited state.This promoted electron can then move to an acceptor molecule and the acceptor molecule is reduced. The acceptor molecule then binds to a hyrogen atom from a molecule of water and thus a hydroxide ion is generated inside the cell. The generation of the hydroxide ion inside the cell is equivalent to the generation of a proton outside of the cell. Therefore, a proton gradient develops across the membrane. Protons move from where they are in high concentration (outside the cell) to where they are in low concentration (inside the cell) by the process of Diffusion. They do this through the ATP synthase enzyme and thus generate ATP.
Author: Alexander Coward (document modification date: 16th May 2003)