The generic name for the intensely colored green pigments which are the photoreceptors of light energy in photosynthesis. These pigments belong to the tetrapyrrole family of organic compounds.

Five closely related chlorophylls, designated a through e, occur in higher plants and algae. The principal chlorophyll (Chl) is Chl a, found in all oxygen-evolving organisms; photosynthetic bacteria, which do not evolve O₂, contain instead bacteriochlorophyll (Bchl). Higher plants and green algae contain Chl b, the ratio of Chl b to Chl a being 1:3. Chlorophyll c (of two or more types) is present in diatoms and brown algae. Chlorophyll d, isolated from marine red algae, has not been shown to be present in the living cell in large enough quantities to be observed in the absorption spectrum of these algae. Chlorophyll e has been isolated from cultures of two algae, Tribonema bombycinum and Vaucheria hamata. In higher plants the chlorophylls and the above-mentioned pigments are contained in lipoprotein bodies, the plastids. Cell plastids Photosynthesis

Chlorophyll molecules have three functions: They serve as antennae to absorb light quanta; they transmit this energy from one chlorophyll to another by a process of “resonance transfer;” and finally, this chlorophyll molecule, in close association with enzymes, undergoes a chemical oxidation (that is, an electron of high potential is ejected from the molecule and can then be used to reduce another compound). In this way the energy of light quanta is converted into chemical energy.

The chlorophylls are cyclic tetrapyrroles in which four 5-membered pyrrole rings join to form a giant macrocycle. Chlorophylls are members of the porphyrin family, which plays important roles in respiratory pigments, electron transport carriers, and oxidative enzymes.

It now appears that the chlorophyll a group may be made up of several chemically distinct Chl a species.

The two major pigments of protoplasm, green chlorophyll and red heme, are synthesized from ALA (?-aminolevulinic acid) along the same biosynthetic pathway to protoporphyrin. ALA is converted in a series of enzymic steps, identical in plants and animals, to protoporphyrin. Here the pathway branches to form (1) a series of porphyrins chelated with iron, as heme and related cytochrome pigments; and (2) a series of porphyrins chelated with magnesium which are precursors of chlorophyll.

Chlorophylls reemit a fraction of the light energy they absorb as fluorescence. Irrespective of the wavelength of the absorbed light, the emitted fluorescence is always on the long-wavelength side of the lowest energy absorption band, in the red or infrared region of the spectrum.

The fluorescent properties of a particular chlorophyll are functions of the structure of the molecule and its immediate environment. Thus, the fluorescence spectrum of chlorophyll in the living plant is always shifted to longer wavelengths relative to the fluorescence spectrum of a solution of the same pigment. This red shift is characteristic of aggregated chlorophyll.