Every leaf runs on a bacterium swallowed two billion years ago
Roughly two billion years ago, an early complex cell engulfed a photosynthetic cyanobacterium, perhaps as a meal, and failed to digest it. The captive survived, kept making sugar for its host and became the chloroplast. That single ancient merger lies behind nearly every green plant and alga alive today.
Chloroplasts are organelles packed with chlorophyll that capture sunlight, split water to release oxygen and store the energy chemically, which the Calvin cycle then uses to build sugars from carbon dioxide. They also make fatty acids and amino acids and play a part in plant immunity. A cell may hold just one, as in some single-celled algae, or up to 100 in plants such as wheat. They are surprisingly mobile, shifting around the cell in response to the colour and strength of light.
Their bacterial past shows. Like mitochondria, chloroplasts carry their own DNA, separate from the nucleus, and they multiply only by dividing; a cell cannot build one from scratch, so each daughter cell must inherit some. Over time many of the captured bacterium's genes moved into the host's nucleus, letting the host control its tenant. Andreas Schimper noticed in 1883 how closely chloroplasts resemble cyanobacteria, and the Russian biologist Konstantin Mereschkowski proposed the engulfment theory in 1905. Mitochondria are thought to have arrived earlier by a similar route, which is why every eukaryote has mitochondria but only some have chloroplasts.
Nearly all chloroplasts descend from that one event, whose closest living relative may be Gloeomargarita lithophora. The exception is the amoeba Paulinella, which captured a different cyanobacterium between about 90 and 140 million years ago. Many other organisms got chloroplasts secondhand by swallowing red or green algae, leaving plastids wrapped in three or four membranes; some even retain a shrunken remnant of the eaten alga's nucleus, called a nucleomorph.
Hugo von Mohl gave the first clear description in 1837, calling them grains of chlorophyll. Schimper named them chloroplastids, and Eduard Strasburger adopted chloroplast in 1884, from Greek words meaning green and the one who forms.
Source: Chloroplast
