Chlorophyll

Reader's Guide

Chlorophyll is central to photosynthesis, allowing plants to absorb energy from light. It is arranged in photosystems embedded in thylakoid membranes, where it serves three functions: absorbing light, transferring energy via resonance to a reaction center pair, and performing charge separation to produce protons and electrons. These electrons drive the electron transport chain, ultimately oxidizing water to produce O2 gas—the source of nearly all atmospheric oxygen. Chlorophyll absorbs light most strongly in blue and red portions of the spectrum, reflecting green light, which gives plants their color. Two types exist in green plants: chlorophyll a and b. Chlorophyll content can be measured via solvent extraction, fluorescence ratios, or optical sensors like Dualex and SPAD, enabling non-destructive assessment of plant health.

Did You Know?

• Chlorophyll a has a methyl group where chlorophyll b has a formyl group, affecting their absorption spectra.

A Century of Unraveling

The story of chlorophyll's identity spans nearly two centuries of painstaking chemistry.

The Three-Act Engine of Light

Within the thylakoid membranes of chloroplasts, chlorophyll molecules are organized into and around photosystems, where they perform a three-part choreography. The overwhelming majority — sometimes several hundred per photosystem — act as light-harvesting antennas, soaking up photons. That captured energy is then relayed by resonance energy transfer to a dedicated chlorophyll pair at the reaction center. There, the final act unfolds: charge separation, in which an electron is ripped from the chlorophyll in an oxidation event and handed off into an electron transport chain. Two photosystem units, P700 and P680 (named for their red-peak absorption wavelengths in nanometers), operate in concert. The electron that ultimately reduces the oxidized P680+ is stripped from water, releasing O2 and H+ — the very process responsible for nearly all oxygen in Earth's atmosphere. The resulting proton gradient across the thylakoid membrane drives ATP synthesis and the reduction of NADP+ to NADPH, a universal reagent that powers the conversion of CO2 into sugars and other biosynthetic reactions.

A Magnesium Heart in a Chlorin Frame

All known chlorophylls share a defining architectural feature: a fifth, ketone-bearing ring appended to the four pyrrole-like rings of the parent chlorin. This places them in the chlorin family — reduced cousins of the porphyrins that carry iron in hemoglobin — and they even share a common biosynthetic pathway with porphyrins, including the precursor uroporphyrinogen III. Where hemes bind iron at their N4 center, most chlorophylls instead coordinate a magnesium ion, with axial ligands often omitted in structural diagrams for clarity. A long phytyl side chain (C20H39O) is typically attached to the ring system. Chlorophyll a, the most widely distributed form in terrestrial plants, differs from chlorophyll b by bearing a methyl group where chlorophyll b carries a formyl group — a subtle substitution that shifts the absorption spectrum and lets plants capture a broader slice of visible light. Beyond these two primary types, accessory pigments within antenna complexes absorb photons at wavelengths outside chlorophyll's narrower range, funneling additional electrons into the photosystem.

Why the World Is Green

The name chlorophyll itself is a small etymological tribute to its most visible trait: the Greek χλωρός (khloros) means "pale green," and φύλλον (phyllon) means "leaf." That greenness is, in a sense, a byproduct of the molecule's optical selectivity. Chlorophylls absorb light most strongly in the blue and red portions of the electromagnetic spectrum but are poor absorbers of green and near-green wavelengths. The unabsorbed green light is diffusively reflected by structures such as cell walls, giving chlorophyll-containing tissues their characteristic hue. This pigment family is found in cyanobacteria and in the chloroplasts of algae and plants, where it drives oxygenic photosynthesis — a process distinct from the anoxygenic photosynthesis carried out by bacteriochlorophylls in certain bacteria. The enzymes that build chlorophyll are thought to have evolved roughly 2.3 billion years ago, making this molecule one of the oldest and most consequential biochemical innovations in Earth's history. Because a single chlorophyll molecule has a small probability of triggering charge separation under any given light intensity, antenna pigment proteins work cooperatively to funnel energy to the reaction center.