The process of photosynthesis occurs in two phases.

The light dependent stage, or Hill reaction, traps light energy as chemical energy in ATP and splits water into hydrogen and oxygen. This stage occurs on the inner membranes or grana of the chloroplast.

The dark stage, light independent stage, or Calvin cycle then creates organic molecules. This phase occurs in the stroma, a liquid found in the chloroplast which contains the enzymes necessary for the reactions to occur. These reactions are anabolic, that is they build up large complex molecules from smaller simpler ones

The Light dependent stage or Hill reaction:

The chlorophyll molecules in the chloroplasts are grouped into photosystems, collections of a couple of hundred of chlorophyll molecules and associated molecules such as carotene. These are referred to as quantasomes. The quantasomes absorb the energy from the blue and red ends of the light spectrum and pass this energy to a central chlorophyll molecule that has its outer electrons excited. These excited electrons eventually are lost from the chlorophyll molecule and are caught by an electron carrier system. The electrons may be passed by the electron carriers back into the photosystem from which they came, which releases energy that can combine ADP and P’ to form ATP. This is known as cyclic photophosphorylation. Alternatively, the electrons may be passed into photosystem II that can boost the electrons to even higher levels of excitation so that when they are released they can combine with NADP to give it a negative electric charge so that it can combine with two hydrogen ions from disassociated water molecules. This will leave two OH^- groups without their H⁺partners and that would create increasingly alkaline conditions in the chloroplast. The two OH^- groups are combined to give H₂O₂ and 2 spare electrons that are passed by electron carriers back into photosystem I, from which 2 electrons were originally lost and as this happens the energy released is used to create more ATP. Because the electrons returned are not the ones that were originally lost we refer to this as non-cyclic photophosphorylation. The H₂O₂, hydrogen peroxide, is quite dangerous as an oxidiser, and is converted by a peroxidase enzyme called catalase into water and oxygen.

Calvin cycle:

In this stage the hydrogen is donated by the NADPH₂ to phosphoglyceric acid molecules to form phophoglyceraldehyde, a triose sugar phosphate. Five molecules of triose phosphate are used to create 3 molecules of 5C ribulose bisphosphate through a complex series of reactions called the pentose shunt. This requires a supply of ATP to drive the reactions. CO₂ is then added to the ribulose bisphosphates by a carboxylase enzyme to form unstable 6C acid molecules which spontaneously break down to form molecules of phosphoglyceric acid. Every time 5 triose phosphates go around the cycle we get 6 phosphoglyceric acid molecules out. The extra molecule can be converted to triose phosphate and as more and more of these build up they can be combined to form glucose phosphates and then starch. Alternatively, the extra phosphoglyceric acids can be combined together to produce amino acids or fats.