Leaves look green because chlorophyll rejects the green light
Chlorophyll soaks up blue and red light and handles green poorly, so green is what bounces back to our eyes. Its real job is splitting water with captured sunlight, a reaction that releases the oxygen behind practically all the oxygen in Earth's air.
The name joins Greek words for pale green and leaf. These green, light-powered catalysts occur in cyanobacteria and inside the chloroplasts of plants and algae, and they drive the oxygen-producing kind of photosynthesis; related bacteriochlorophylls in other bacteria do not release oxygen. Green plants use two main forms, chlorophyll a and b, which differ by a single side group yet absorb slightly different colours, widening the range of light a leaf can use.
Inside the chloroplast, chlorophylls sit in photosystems embedded in membranes called thylakoids. Most molecules, up to several hundred per photosystem, act as antennae, catching photons and passing the energy by resonance toward a special pair at the reaction centre. There the energy knocks loose an electron. In photosystem II, whose centre is called P680 after its red absorption peak in nanometres, the electron-hungry chlorophyll refills itself by stripping electrons from water, splitting it into oxygen and protons. The electron flow pumps protons across the membrane to make ATP and NADPH, which the plant then uses to turn carbon dioxide into sugar. A second unit, photosystem I with its P700 centre, usually works in series with the first.
Structurally, chlorophyll is a ring system closely related to the heme in our blood, made through a shared biochemical pathway. The key difference is at the centre: heme holds iron, while most chlorophylls hold magnesium. That magnesium, found in 1906, was the first time the element had been detected in living tissue. The enzymes that build chlorophyll are thought to have evolved about 2.3 billion years ago.
Joseph Bienaimé Caventou and Pierre Joseph Pelletier isolated and named the pigment in 1817. Hans Fischer worked out chlorophyll a's general structure in 1940, Robert Burns Woodward published a total synthesis in 1960, and in 2010 a new variant, chlorophyll f, was reported in microbes that build stromatolites.
Source: Chlorophyll
