Photosynthesis is the process in which light energy is converted into chemical energy. Using the energy of light, carbohydrates such as sugars are synthesized from carbon dioxide and water. The process of photosynthesis occurs when green plants use the energy of light to convert carbon dioxide (CO2) and water (H2O) into carbohydrates. Light energy is absorbed by chlorophyll, a photosynthetic pigment of the plant, while air containing carbon dioxide and oxygen enters the plant through the leaf stomata. An extremely important by-product of photosynthesis is oxygen, on which most organisms depend. Glucose, a carbohydrate processed during photosynthesis, is mostly used by plants as an energy source to build leaves, flowers, fruits, and seeds. Molecules of glucose later combine with each other to form more complex carbohydrates such as starch and cellulose. The cellulose is the structural material used in plant cell walls. Photosynthesis provides the basic energy source for virtually all organisms.
This video explains how to show experimentally that light is necessary for photosynthesis.