Chloroplast

Chloroplasts are a type of plastid—a specialized organelle—found mainly in plant and algal cells, where they perform photosynthesis. They are packed with chlorophyll pigments that capture sunlight, convert that energy into chemical energy, and split water molecules to release oxygen. This chemical energy then powers the Calvin cycle, which builds sugars and other organic compounds from carbon dioxide. Beyond photosynthesis, chloroplasts handle tasks like making fatty acids, synthesizing amino acids, and contributing to plant immune responses. The number of chloroplasts in a cell varies widely: some single-celled algae have just one, while plants like Arabidopsis and wheat can have up to a hundred.

These organelles are highly dynamic, constantly moving and circulating inside cells, and their behavior shifts with environmental cues like light color and intensity. Plant cells cannot create new chloroplasts from scratch; instead, they must inherit them during cell division. This inheritance traces back to an ancient event where a photosynthetic cyanobacterium was engulfed by an early eukaryotic cell. Because of this endosymbiotic origin, chloroplasts, like mitochondria, have their own DNA separate from the cell’s nucleus. With the sole exception of the amoeboid *Paulinella chromatophora*, all chloroplasts stem from a single such endosymbiotic event. Yet they appear in a vast range of distantly related organisms, thanks to many subsequent secondary and even tertiary endosymbioses.

The first clear description of a chloroplast came from Hugo von Mohl in 1837, who called them “grains of chlorophyll.” In 1883, Andreas Franz Wilhelm Schimper named them “chloroplastids,” and Eduard Strasburger shortened it to “chloroplasts” in 1884. The word itself comes from Greek *chloros* (green) and *plastes* (one who forms).

Chloroplasts evolved from cyanobacteria through a process called organellogenesis. Cyanobacteria are a diverse group of gram-negative bacteria that perform oxygenic photosynthesis and, like chloroplasts, have thylakoid membranes packed with photosynthetic pigments, including chlorophyll a. This origin was first proposed by Russian biologist Konstantin Mereschkowski in 1905, building on Schimper’s 1883 observation that chloroplasts closely resemble cyanobacteria. Chloroplasts are found only in plants, algae, and a few *Paulinella* species. Mitochondria are thought to have arisen from a similar endosymbiosis, when an aerobic prokaryote was engulfed.

About two billion years ago, a free-living cyanobacterium entered an early eukaryotic cell—either as food or a parasite—but escaped the phagocytic vacuole and stayed inside. This mutualistic arrangement, called endosymbiosis, gave the host sugar from photosynthesis. Over time, the cyanobacterium was assimilated: many of its genes were lost or transferred to the host’s nucleus, and the host began making some cyanobacterial proteins and shipping them back to the chloroplast, giving the host control. Chloroplasts that trace directly to a cyanobacterial ancestor (without a later endosymbiosis) are called primary plastids. It’s now widely accepted that, except for *Paulinella chromatophora*, all primary chloroplasts come from a single endosymbiotic event, with the closest living relative of that ancestral cyanobacterium likely being *Gloeomargarita lithophora*. Separately, about 90–140 million years ago, a similar event happened in *Paulinella* with a cyanobacterium from the genus *Prochlorococcus*; this independently evolved organelle is often called a chromatophore. Chloroplasts are thought to have appeared after mitochondria, since all eukaryotes have mitochondria but not all have chloroplasts—a sequence known as serial endosymbiosis.

Many other organisms acquired chloroplasts through secondary endosymbiosis, by engulfing a red or green alga that already had a primary chloroplast. These are called secondary plastids. Because of this second engulfment, secondary chloroplasts have extra membranes beyond the original two: typically three or four, including the two cyanobacterial membranes and sometimes the alga’s own cell membrane and even its nucleus.