Photosynthesis usually takes place in the leaves. Leaf cells contain many chloroplasts (up to 50 per cell). These organelles contain a green pigment, chlorophyll, capable of capturing light energy. This energy is used to make sugar molecules from water drawn from the soil by the roots and carbon dioxide from the atmosphere, captured by the leaves. The sugars produced subsequently distributed in the different parts of the plant.
Here is the reaction of photosynthesis:
6 CO2 + 6 H2O + light energy ==> C6H12O6 (glucose) + 6 O2
But this balance is in fact broken down into two successive stages:
photochemical reactions (clear phase):
12 H2O + light ==> 6 O2 + chemical energy;
the Calvin cycle (dark phase):
6 CO2 + chemical energy ==> C6H12O6 + 6 H2O.
The first phase of photosynthesis, called clear phase, requires the illumination of the leaf. Chlorophyll, which is embedded in the thylakoid membrane, absorbs light energy. This energy absorption is accompanied by the degradation of water molecules and the release of oxygen.
In a second phase independent of light, called the dark phase, the energy absorbed by chlorophyll is used for the transformation of carbon dioxide molecules into sugar molecules.
The sugars made by photosynthesis are stored in the cell as starch, transformed into other organic molecules, such as proteins, or degraded in the mitochondria during respiration to meet the energy needs of the plant.