Chloroplast
A chloroplast (/ˈklɔːrəˌplæst, -plɑːst/)[1][2] is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy-storage molecules ATP and NADPH while freeing oxygen from water in the cells. The ATP and NADPH is then used to make organic molecules from carbon dioxide in a process known as the Calvin cycle. Chloroplasts carry out a number of other functions, including fatty acid synthesis, amino acid synthesis, and the immune response in plants. The number of chloroplasts per cell varies from one, in unicellular algae, up to 100 in plants like Arabidopsis and wheat.
A chloroplast is characterized by its two membranes and a high concentration of chlorophyll. Other plastid types, such as the leucoplast and the chromoplast, contain little chlorophyll and do not carry out photosynthesis.
Chloroplasts are highly dynamic—they circulate and are moved around within plant cells, and occasionally pinch in two to reproduce. Their behavior is strongly influenced by environmental factors like light color and intensity. Chloroplasts, like mitochondria, contain their own DNA, which is thought to be inherited from their ancestor—a photosynthetic cyanobacterium that was engulfed by an early eukaryotic cell.[3] Chloroplasts cannot be made by the plant cell and must be inherited by each daughter cell during cell division.
With one exception (the amoeboid Paulinella chromatophora), all chloroplasts can probably be traced back to a single endosymbiotic event, when a cyanobacterium was engulfed by the eukaryote. Despite this, chloroplasts can be found in an extremely wide set of organisms, some not even directly related to each other—a consequence of many secondary and even tertiary endosymbiotic events.
The word chloroplast is derived from the Greek words chloros (χλωρός), which means green, and plastes (πλάστης), which means "the one who forms".[4]
The first definitive description of a chloroplast (Chlorophyllkörnen, "grain of chlorophyll") was given by Hugo von Mohl in 1837 as discrete bodies within the green plant cell.[5] In 1883, Andreas Franz Wilhelm Schimper would name these bodies as "chloroplastids" (Chloroplastiden).[6] In 1884, Eduard Strasburger adopted the term "chloroplasts" (Chloroplasten).[7][8][9]
(Top) 10-nm-thick STEM tomographic slice of a lettuce chloroplast. Grana stacks are interconnected by unstacked stromal thylakoids, called "stroma lamellae". Round inclusions associated with the thylakoids are plastoglobules. Scalebar = 200 nm. See.[127]
(Bottom) Large-scale 3D model generated from segmentation of tomographic reconstructions by STEM. grana = yellow; stroma lamellae = green; plastoglobules = purple; chloroplast envelope = blue. See.[127]