Plants use the simple inorganic molecules carbon dioxide and water, in the presence of chlorophyll and light, to make glucose and oxygen. This process is called photosynthesis.
The role of the green pigment, chlorophyll, is to absorb the light energy needed for the reaction to take place. The products of the reaction contain more energy than the carbon dioxide and water.
This simple equation is, in fact, a summary of a complex series of chemical reactions which take place in two stages.
Firstly, light energy absorbed by the chlorophyll is used to split water molecules into hydrogen and oxygen. The waste product, oxygen, is given off as a gas. Because this process needs light, it is known as the light reaction.
Next, the hydrogen from the water is used to reduce the carbon dioxide to glucose. In this stage the carbo dioxide, by joining with the hydrogen, is fixed, or converted into an organic compound. This stage does not need light, and is called the dark reaction.
Both these stages take place within the chloroplasts. The term dark reaction can be a little confusing. The point is that the dark reaction does not need light, even though it takes place in daylight.