What Is Chlorophyll?

Chlorophyll is a family of green photosynthetic pigments found in plants, algae, and cyanobacteria that absorb light energy and transfer it into the electron transport chain that drives photosynthesis. Each molecule is built around a chlorin ring, a cyclic tetrapyrrole holding a magnesium ion at its center, with a long hydrophobic phytol tail that anchors the pigment in the thylakoid membrane. The pigment was isolated and named in 1817 by Pierre Joseph Pelletier and Joseph Bienaimé Caventou, its structure was established by Hans Fischer in the 1930s, and Robert Burns Woodward completed a total synthesis in 1960.

Several variants exist and differ in the substituents on the ring. Chlorophyll a is present in every oxygenic photosynthetic organism and is the pigment that performs the primary charge separation. Chlorophyll b broadens the absorbed spectrum in plants and green algae, chlorophyll c appears in diatoms and dinoflagellates, and chlorophylls d and f extend absorption into the far red in certain cyanobacteria. For engineering purposes chlorophyll matters less as a chemical and more as an optical signal, since its absorption and fluorescence bands make plant and algal biomass measurable at a distance.

Optical Properties

Chlorophyll a in solvent absorbs strongly near 430 nanometers in the blue and near 662 nanometers in the red, with a pronounced minimum across the green between roughly 500 and 600 nanometers. That minimum is why vegetation looks green: the green photons are scattered back rather than absorbed. Chlorophyll b shifts its peaks to about 453 and 642 nanometers, so a mixture covers more of the visible band than either pigment alone. In the intact leaf, pigment-protein binding and self-shading broaden and flatten these features relative to the extracted spectrum, which is why laboratory extinction coefficients cannot be applied directly to canopy reflectance.

A small fraction of absorbed energy is re-emitted as fluorescence, with peaks near 685 and 740 nanometers. Because that emission competes with photochemistry and heat dissipation, its intensity reports on photosynthetic state rather than on pigment content alone. Pulse-amplitude-modulated fluorometers exploit this in the laboratory and the field, and satellite instruments retrieve solar-induced fluorescence by measuring the filling-in of solar Fraunhofer lines at those wavelengths.

Role in Photosynthesis

Most chlorophyll molecules in a cell do no chemistry at all. They act as antenna pigments in light-harvesting complexes, absorbing photons and passing the excitation by resonance energy transfer toward a reaction center within picoseconds. At the reaction centers of photosystem II and photosystem I, specialized chlorophyll pairs designated P680 and P700 perform charge separation, injecting an electron into an acceptor chain. Photosystem II replaces its lost electron by oxidizing water, releasing molecular oxygen, while the electron flow across the thylakoid membrane builds the proton gradient that powers ATP synthesis. Antenna size, pigment composition, and the ratio of the two photosystems all adjust with light conditions, which makes chlorophyll content a useful proxy for a plant's nitrogen status and stress level.

Measurement and Remote Sensing

Chlorophyll concentration is one of the most widely retrieved geophysical variables in Earth observation. Ocean color instruments estimate near-surface chlorophyll a from the ratio of water-leaving radiance in blue and green bands, using empirical relationships calibrated against shipboard measurements. The MODIS chlorophyll a data product applies such a band-ratio algorithm to produce concentrations in milligrams per cubic meter, and NASA has maintained a continuous satellite chlorophyll record since the launch of SeaWiFS in 1997, extended by the hyperspectral Ocean Color Instrument aboard the PACE mission in 2024. Operational monitoring programs such as NOAA's chlorophyll a ecosystem indicator use these retrievals to track primary productivity, seasonal bloom timing, and harmful algal events. On land, red-edge and near-infrared reflectance indices serve a parallel role, estimating leaf chlorophyll from airborne, satellite, and low-cost handheld sensors.

Applications

Chlorophyll measurement and chlorophyll-derived design have applications in a range of fields, including:

• Satellite ocean color monitoring of phytoplankton biomass and carbon uptake
• Precision agriculture, where canopy indices guide nitrogen application
• Water quality management and harmful algal bloom early warning
• Plant phenotyping and controlled-environment horticulture lighting design
• Biosensors and dye-sensitized photovoltaic devices using porphyrin-type pigments