Photosynthesis is the process by which green plants, and some bacteria and protozoans, use sunlight to convert carbon dioxide and water into useful fuel, in the form of sugar, releasing oxygen as a by-product. The sugar is used for the growth of the plant or stored in a variety of forms within the plant for future use. Plants need energy from light, carbon dioxide and water for photosynthesis to occur.
In most plants, an underground root system draws up the water that is essential for photosynthesis. The water is transported into the leaves through vascular veins in the stems of the plant, known as xylem. Leaves also possess tiny ports, called stoma, that are used to draw carbon dioxide directly into the leaf from the atmosphere.
The conversion of water and carbon dioxide takes place in specialized cells within the leaf. These cells contain organs known as chloroplasts. Chloroplasts, in turn, possess structures known as thylakoid membranes. Inside these membranes is chlorophyll, the green pigment used in the photosynthesis process.
Chlorophyll is the pigment within the chloroplast that makes photosynthesis possible. Chlorophyll absorbs all wavelengths of light, except green, which is why leaves are usually green in color. The chlorophyll provides the light energy necessary for photosynthesis to occur.
The chemistry of photosynthesis is relatively simple. Six water molecules and six carbon dioxide molecules are converted to one sugar molecule with six oxygen molecules left over. The light energy derived from sunlight is first converted, in a light reaction, to chemical energy and is temporarily stored as chemicals within the cell. These chemicals then interact with carbon dioxide in a reaction that does not require light, known as the Calvin cycle, to form sugar. The oxygen leaves the plant and goes into the air through the stomas.
The sugar that is a result of photosynthesis is used for other chemical reactions that are necessary for the continued growth and health of the plant. Sugars also can be converted to more complex sugars and starches and stored in roots, fruit and stems within the plant.