Chloroplasts are small compartments found in the cells of algae and plants that contain the molecular machinery for harvesting sunlight and converting its energy into food. This process is called photosynthesis, and the food it produces enables plants and algae to grow and ultimately feed the rest of us.
Chloroplasts are derived from cyanobacteria that were engulfed by primitive cells many hundreds of millions of years ago. Instead of being digested and used as food, the cyanobacteria were kept alive to produce food for their host cells by photosynthesis. The bacteria in effect became solar-powered food factories for their host cells. Over time the bacteria became so highly specialised for photosynthesis and integrated into their host cells, that they could no longer survive on their own - they had become chloroplasts.
Plant and algal cells cannot make new chloroplasts from scratch, they can only arise from the division of pre-existing chloroplasts. Therefore it is very important that before a cell divides its chloroplasts also divide so they can be distributed to the daughter cells. Chloroplasts can pinch themselves into two equal parts, just like bacteria (binary fission) which is not surprising given they evolved from bacteria. One of the most important bacterial cell division proteins is FtsZ. FtsZ can polymerize and forms rings on the inside of the chloroplast. These rings somehow constrict and with the help of other proteins, divide the organelle. Chloroplasts appear to contain more than one type of FtsZ protein, and they both form a ring on the inside of the chloroplast at the dividing region. There is also a ring on the outside of the chloroplast (called the outer plastid dividing ring) that helps squeeze the organelle from the oustide during division. The outer ring proteins may be derived from the host and, though we know they have a filamentous appearance under the electron microscope their protein genes are not yet known.
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