Chlorophyll is a family of green pigments involved in photosynthesis, the biological conversion of light into chemical energy. It occurs in plants, algae, and cyanobacteria, where it functions within organized pigment–protein complexes. Chlorophyll absorbs light and participates in the initial processes that power photosynthetic electron transport. Its molecular variants differ in their chemical structures, absorption properties, and distribution among organisms. Chlorophyll is therefore not a single substance, although the name is often used collectively for the pigments responsible for the green appearance of vegetation. (ncbi.nlm.nih.gov)
Molecular structure
Chlorophylls are magnesium-containing tetrapyrroles: their central ring system derives from four nitrogen-containing pyrrole units. In chlorophyll a, a magnesium ion is coordinated by four nitrogen atoms within a chlorin macrocycle, a partially reduced relative of a porphyrin. An extensive conjugated electronic system allows the molecule to absorb visible light. Chlorophyll a has the molecular formula C₅₅H₇₂MgN₄O₅. (pmc.ncbi.nlm.nih.gov)
Chlorophylls a and b also possess a long, hydrophobic phytol-containing side chain. This contributes to their association with membrane-bound proteins. The light-absorbing ring and the side chain have distinct functions: the ring supplies the principal photochemical properties, while the side chain helps organize the pigment within its biological environment. Chlorophyll c differs structurally, including generally lacking this long phytol chain. (ncbi.nlm.nih.gov)
Major forms and distribution
Chlorophyll a is the principal pigment of most oxygen-producing photosynthetic organisms. It participates in both light harvesting and reaction-center photochemistry. Chlorophyll b, found in land plants and several algal groups, serves mainly as an accessory light-harvesting pigment. It differs from chlorophyll a by having a formyl group in place of a methyl group at one position on the ring. This small structural change alters its absorption spectrum. (ncbi.nlm.nih.gov)
Chlorophyll c occurs in several aquatic photosynthetic groups and comprises multiple related forms. Chlorophylls d and f, associated with particular cyanobacteria, absorb farther into the red or far-red region than chlorophyll a. These pigments extend the wavelengths available for photosynthesis in environments with different light conditions. Related bacteriochlorophylls occur in various photosynthetic bacteria and are distinguished from the chlorophylls typical of oxygenic photosynthesis. (pmc.ncbi.nlm.nih.gov)
Light absorption and green coloration
Chlorophylls a and b absorb strongly in the blue and red regions of the visible spectrum and less strongly in much of the green region. Chlorophyll-containing tissues consequently appear green because a greater proportion of green light escapes absorption and is reflected or transmitted. This does not mean that green light is entirely unavailable for photosynthesis: some is absorbed, and its deeper penetration into leaves influences where excitation occurs. (pubchem.ncbi.nlm.nih.gov)
Absorption spectra depend on the pigment’s molecular structure and surroundings, including the solvent used for an extracted sample and its interactions with proteins in living tissue. Measurements of isolated chlorophyll therefore do not exactly reproduce the optical properties of an intact leaf. Accessory pigments, including carotenoids, absorb additional wavelengths and can transfer excitation energy to chlorophyll. (pmc.ncbi.nlm.nih.gov)
Function in photosynthesis
In plants and algae, chlorophyll is concentrated in the thylakoid membranes of chloroplasts. Pigments are organized into photosystems, each containing light-harvesting antenna complexes and a reaction center. Antenna chlorophylls absorb photons and pass excitation energy through neighboring pigments toward the reaction center. This transfer of excitation is distinct from the transfer of an electron between molecules. (ncbi.nlm.nih.gov)
At a reaction center, excitation enables charge separation: an electron is transferred to an acceptor, initiating an electron transport chain. In oxygenic photosynthesis, photosystem II obtains replacement electrons through the oxidation of water, releasing oxygen. Photosystem I supplies another light-driven excitation step. Together, these processes support the formation of ATP and NADPH, which provide energy and reducing power for carbon fixation, including the Calvin cycle. Chlorophyll thus powers the light reactions rather than directly converting carbon dioxide into sugar. (ncbi.nlm.nih.gov)
Biosynthesis and breakdown
Plants synthesize chlorophyll through a regulated series of enzyme-catalyzed reactions. Its pathway shares early intermediates with heme synthesis. A key branching step inserts magnesium into protoporphyrin IX rather than the iron used in heme. Subsequent reactions modify the ring and attach the hydrophobic side chain. Regulation coordinates pigment production with chloroplast development and the assembly of chlorophyll-binding proteins. (pmc.ncbi.nlm.nih.gov)
During leaf senescence and the ripening of some fruits, chlorophyll undergoes controlled degradation. Removal of magnesium and the phytol chain, followed by cleavage of the macrocycle and further modifications, produces catabolites that often accumulate in vacuoles. This dismantling removes the green pigment and contributes to the visible color changes of aging leaves and ripening fruit. (pmc.ncbi.nlm.nih.gov)
Measurement and ecological applications
Chlorophyll fluorescence is red and far-red light emitted following excitation. Absorbed energy can enter photochemistry, be dissipated as heat, or be re-emitted as fluorescence. Because these pathways compete, fluorescence measurements provide information about photosynthetic function. Laboratory and field instruments examine fluorescence intensity and its changes over time, often using controlled illumination; interpretation depends on measurement conditions and the physiological state of the sample. (pmc.ncbi.nlm.nih.gov)
In aquatic research, chlorophyll a concentration is widely used to estimate phytoplankton abundance. Satellite instruments infer near-surface concentrations from ocean-color measurements, including relationships between blue and green reflectance. These observations reveal spatial patterns and seasonal changes across the ocean, supporting studies of marine primary production. Satellite-derived chlorophyll is an indirect optical estimate, not a direct count of organisms or a direct measurement of their carbon-fixation rate. (science.nasa.gov)