Green plants have the capacity to prepare their own food. The process of synthesis of organic food (carbohydrate) from carbon di oxide and water in the presence of sunlight by plants is called photosynthesis.it is the most important synthetic process on earth because it is the only source of carbohydrates in which the solar energy is fixed in the form of chemical energy. Plants have the ability of capturing energy coming from the sun in the form of light and converting it into chemical energy and stored in carbohydrate molecules (organic food molecules) .this chemical energy is used for the growth and sustenance of our biosphere. All animals including human beings depend on plants for their food.
Actually photosynthesis occurs in two steps, photochemical and biochemical.
Photochemical phase-The reaction of this phase are driven by light energy. They are of two types-photolysis of water and formation of assimilatory power.
Biochemical phase-It is the light independent reaction which can occur both in light and in dark. It requires the energy and reducing power contained in assimilatory power of light reactions.
The chemical equation for photosynthesis is
6CO2 + 12H20 --> C6H12O6 + 6O2 + 6H20
The raw materials for photosynthesis are Carbon dioxide and water and the end products are carbohydrate and oxygen and water.Oxygen is liberated into the atmosphere as a by-product during this process. This evolved oxygen comes from water and not from carbon dioxide.