The light-dependent reaction in such detail as to show that • light energy excites electrons in chlorophyll • energy from these excited electrons generates ATP and reduced NADP • the production of ATP involves electron transfer associated with the electron transfer chain in chloroplast membranes • photolysis of water produces protons, electrons and oxygen.

Inside a plant cell there are chlorophyll organelles and these contain grana, stacks of thylakoids.

The thylakoids have phospho-lipid membranes which have proteins studded throughout. There are intrinsic proteins which have the pigment chlorophyll attached and these are called photosystems (PS). There are two different types of photosystems because there are two different pigments of chlorophyll which absorb different wavelengths of light, they are PSII and PSI.

When light hits PSII the chlorophyll absorbs protons (light) which energise two electrons. These electrons have so much energy they leave the outer shell (highest energy level) of their atom and become free.

They move to a protein which can take them on- an electron carrier- but are then moved from this to another one and so forth in what is know as an electron transfer chain (ETC). (When every the electrons join a molecule we say the molecule is reduced and when it leaves the molecule is oxidised so you often hear 'the electrons move through the ETC in a series of oxidation reduction reactions).

Each time the electrons move they loose some energy to the proteins this means that 1) the electron will end up attached to PSI at a normal energy level and 2) the ETC will have gained some energy from them; it uses this to change a ADP molecule and a inorganic phosphate into ATP, which moves into the stroma.

The second photosystem absorbs protons and the energy excites the electrons again. The energy is also used to split water into hydrogen and oxygen (and electrons which go to PSII to replace the ones that were energised out). The oxygen is a waste product, but the hydrogen joins up with the excited electrons and a molecule of NADP to form NADPH (reduced NADP) which is carrys the two electrons and hydrogen out of the thylakoids and into the stroma.