Chloroplast

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Chloroplast, structure in the cells of plants and algae where photosynthesis takes place. Chloroplasts are mostly disk-shaped organelles, 4 to 6 micrometers in diameter. They occur most abundantly in leaf cells, where they can apparently orient themselves to light. Perhaps 40 to 50 chloroplasts exist in one cell and 500,000 in each sq mm (0.06 sq in) of leaf surface. Each chloroplast is enclosed in a double membrane. Internally, it consists of a ground substance called the stroma, which is traversed by a complex network of interconnected disks called thylakoids. Many of the thylakoids are stacked like saucers; the stacks are called grana. Molecules of chlorophyll, which absorb light for photosynthesis, are attached to the thylakoids. The light energy captured by the chlorophyll is converted to adenosine triphosphate, or ATP, in a series of chemical reactions that take place in the grana. Chloroplasts also contain small starch grains that temporarily store the products of photosynthesis.

In plants, chloroplasts develop in the presence of light from small, colorless organelles called proplastids. As cells divide in the growing parts of a plant, the proplastids inside them divide by fission. Thus, the daughter cells have the ability to produce chloroplasts. In algae, chloroplasts divide directly, without developing from proplastids. The self-reproducing ability of chloroplasts, their bacteria-like DNA and ribosomes, and their close similarity regardless of the type of cell they inhabit, suggest that they were once independent organisms that come to exist in symbiosis with the plant cell as host.

See also Cell.


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