Chlorophyll, the pigment found in plants, some algae, and some bacteria that gives them their green color and that absorbs the light necessary for photosynthesis. Chlorophyll absorbs mainly violet-blue and orange-red light. The great abundance of chlorophyll in leaves and its occasional presence in other plant tissues, such as stems, causes these plant parts to appear green. In some leaves, chlorophyll is masked by other pigments. In fall, chlorophyll wanes in the leaves of trees, and other pigments predominate.

Chlorophyll is a large molecule composed mostly of carbon and hydrogen. At the center of the molecule is a single atom of magnesium surrounded by a nitrogen-containing group of atoms called a porphyrin ring. The structure somewhat resembles that of the active constituent of hemoglobin in the blood. A long chain of carbon and hydrogen atoms proceeds from this central core and attaches the chlorophyll molecule to the inner membrane of the chloroplast, the cell organelle in which photosynthesis takes place. As a molecule of chlorophyll absorbs a photon of light, its electrons become excited and move to higher energy levels (see Photochemistry). This initiates a complex series of chemical reactions in the chloroplast that enables the energy to be stored in chemical bonds.

Several kinds of chlorophyll exist. They differ from each other in details of their molecular structure and absorb slightly different wavelengths of light. The most common type is chlorophyll a, making up about 75 percent of the chlorophyll in green plants. It is also found in cyanobacteria (formerly known as blue-green algae) and in more complex photosynthetic cells. Chlorophyll b is an accessory pigment present in plants and other complex photosynthetic cells; it absorbs light energy of a different wavelength and transfers it to chlorophyll a for ultimate conversion to chemical energy. Other chlorophylls, of minor importance, are found in some bacteria.