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Photosynthesis is the process by which plants convert carbon dioxide, water, and sunlight into sugar and oxygen. The formula for photosynthesis is

Carbon dioxide + water + light energy -> carbohydrate (glucose) + oxygen + water

Water appears on both sides of the equation because it is produced as well as consumed; but only half as much water exits the process as enters it.

Photosynthesis takes place in chloroplasts; these are found in all green parts of plants but the process is mostly carried out in the leaves. Carbon dioxode enters the leaves and oxygen exits through stomata, pores in the leaf surface. Inside the leaves are chloroplasts, containing chlorophyll, which gives plants their green colour; they occur mostly in mesophyll cells in the middle of the leaf. There are 30-40 chloroplasts per mesophyll cell; they contain thylakoid membranes folded into stacks called grana. The chlorophyll molecules are embedded in the surface of the thylakoid membrane. The two types of chlorophyll molecules - a and b - are structurally very similar, consisting of a porphyrin ring and hydrocarbon tail.

Photosynthesis occurs in two parts: the light reactions, and the Calvin cycle (dark reactions or light-independent reactions). The first can only take place during daylight hours, the second at any time. In the light reactions, light molecules (photons) entering the leaf are collected by the light-harvesting complex. Chlorophyll a and chlorophyll b absorb light at slightly different wavelengths, mostly in the blue and red areas of the spectrum. Photons absorbed by chlorophyll b are transferred to chlorophyll a. A photon striking chlorophyll can excite an electron - boost it to a higher energy level. The energy bounces from molecule to molecule until it reaches the chlorophyll a molecule at the reaction centre, whic includes a primary electron acceptor. This traps the high-energy electron produced by chlorophyll a before it can drop back to its ground state.

The reaction centres and their surrounding chlorophyll molecules form photosystems. There are two photosystems, I and II, which differ in their reaction centres. Their reaction centre chlorophylls are P700 and P680 respectively; the numbers designate their wavelength absorbtion peaks. The chlorophylls differ only in their associated proteins.

Light is absorbed by PSII and the electron captured by the primary electron acceptor. The chlorophyll then accepts an electron from water as a replacement. The water molecule donating the

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