Chlorophyll

Chlorophyll is a group of green pigments and photochemical catalysts present in cyanobacteria and within the chloroplasts of algae and plants. The name comes from the Greek words *khloros* ("pale green") and *phyllon* ("leaf"). These pigments enable plants to capture energy from light and are essential for oxygenic photosynthesis, a process that differs from the anoxygenic photosynthesis carried out by bacteria using related molecules called bacteriochlorophylls.

Chlorophylls absorb light most strongly in the blue and red parts of the spectrum, but they are poor absorbers of green and near-green light. This is why chlorophyll-containing tissues appear green: green light is reflected or scattered by structures like cell walls rather than being absorbed. In green plants, two types of chlorophyll—a and b—are found in the photosystems.

**History** Chlorophyll was first isolated and named in 1817 by Joseph Bienaimé Caventou and Pierre Joseph Pelletier. In 1906, magnesium was discovered within chlorophyll, marking the first detection of that element in living tissue. German chemist Richard Willstätter conducted foundational work from 1905 to 1915, and Hans Fischer determined the general structure of chlorophyll a in 1940. By 1960, most of its stereochemistry was known, and Robert Burns Woodward published a total synthesis of the molecule. The final stereochemical details were completed by Ian Fleming in 1967, and Woodward and co-authors released an updated synthesis in 1990. In 2010, chlorophyll f was reported in cyanobacteria and other oxygenic microorganisms that form stromatolites; its molecular formula is C₅₅H₇₀O₆N₄Mg, and its structure was deduced as (2-formyl)-chlorophyll a using NMR, optical, and mass spectra.

**Photosynthesis** Chlorophyll is critical for photosynthesis, allowing plants to absorb energy from light. Chlorophyll molecules are arranged in and around photosystems embedded in the thylakoid membranes of chloroplasts. Within these complexes, chlorophyll performs three functions. First, the vast majority of chlorophyll molecules (up to several hundred per photosystem) absorb light. Second, they transfer that energy via resonance energy transfer to a specific chlorophyll pair in the reaction center. Third, that pair carries out charge separation, producing unbound protons (H⁺) and electrons (e⁻) that drive biosynthesis.

The two accepted photosystem units are photosystem I and photosystem II, with reaction centers named P700 and P680, respectively, after the wavelength (in nanometers) of their red-peak absorption maximum. The identity, function, and spectral properties of chlorophyll types in each photosystem are distinct and shaped by the surrounding protein structure. The reaction center absorbs light energy and transfers it, causing charge separation: the photon’s energy moves an electron, oxidizing the chlorophyll. The high-energy electron is donated to an electron transport chain. The charged reaction center (P680⁺) is then reduced by accepting an electron stripped from water, a process that ultimately produces O₂ and H⁺ through several intermediates. This reaction is the source of nearly all oxygen in Earth’s atmosphere. Photosystem I typically works in series with photosystem II; the P700⁺ of photosystem I is usually reduced by electrons from photosystem II via many intermediates in the thylakoid membrane, though the electron source can vary.

The electron flow from reaction center chlorophylls pumps H⁺ ions across the thylakoid membrane, creating a proton-motive force used mainly to produce ATP or to reduce NADP⁺ to NADPH. NADPH is a universal agent for reducing CO₂ into sugars and other biosynthetic reactions. Reaction center chlorophyll–protein complexes can absorb light and perform charge separation without other pigments, but the chance of a single chlorophyll doing so under typical light is small. Therefore, other chlorophylls and antenna pigment proteins cooperatively absorb and funnel light energy to the reaction center. Besides chlorophyll a, accessory pigments in these antenna complexes absorb photons at wavelengths outside chlorophyll’s narrow absorption spectrum, delivering additional electrons to the photosystem. The evolution of the enzymes that produce chlorophyll is thought to have occurred 2.3 billion years ago.

**Chemical structure** Several chlorophylls are known. All are defined as derivatives of the parent chlorin by having a fifth, ketone-containing ring beyond the four pyrrole-like rings. Most chlorophylls are classified as chlorins, which are reduced relatives of porphyrins (found in hemoglobin). They share a common biosynthetic pathway with porphyrins, including the precursor uroporphyrinogen III. Unlike hemes, which contain iron bound to the N₄ center, most chlorophylls contain magnesium.