Chlorophyll

Chlorophyll is the family of green pigments that capture light energy for photosynthesis in plants, algae and cyanobacteria. Each molecule has a ring of carbon and nitrogen atoms holding a central magnesium ion, with a long hydrocarbon tail that anchors it in membranes.

Chlorophyll's ring is a chlorin, a relative of the porphyrin ring of heme in which magnesium replaces iron. Chlorophyll a, the main form, has a methyl group where chlorophyll b has an aldehyde. The tail makes the molecule fat-soluble, so it sits in the thylakoid membranes of the chloroplast, bound to proteins in light-harvesting complexes.

Chlorophylls absorb blue light near 430 to 450 nanometers and red light near 660 to 680 nanometers, and they absorb green light poorly, so reflected green light gives leaves their color. Accessory pigments, the carotenoids and chlorophyll b, extend the range of light used and pass the energy on. In autumn, chlorophyll is broken down faster than it is replaced, and the yellow and orange carotenoids show through.

Most chlorophyll collects light and funnels it to special chlorophyll a pairs in the reaction centers of photosystem II and photosystem I. There an excited electron is passed down the electron transport chain, which makes the ATP and NADPH used in the Calvin cycle. Satellites measure the green-red contrast of vegetation and the color of the sea to estimate the activity of phytoplankton.

• Photosynthesis — the process chlorophyll drives
• Chloroplast — the organelle that holds it
• Heme — the iron analog of the ring