Chloroplasts Contain Internal Compartments in Which Photosynthesis Takes Place

Except for vacuoles, chloroplasts are the largest and the most characteristic organelles in the cells of plants and green algae. They can be as long as 10 ^m and are typically 0.5-2 ^m thick, but they vary in size and shape in different cells, especially among the algae. In addition to the double membrane that bounds a chloroplast, this organelle also contains an extensive internal system of interconnected membrane-limited sacs called thylakoids, which are flattened to form disks (Figure 5-27). Thylakoids often form stacks called grana and are embedded in a matrix, the stroma. The thylakoid membranes contain green pigments

(chlorophylls) and other pigments that absorb light, as well as enzymes that generate ATP during photosynthesis. Some of the ATP is used to convert CO2 into three-carbon intermediates by enzymes located in the stroma; the intermediates are then exported to the cytosol and converted into sugars. I

The molecular mechanisms by which ATP is formed in mitochondria and chloroplasts are very similar, as explained in Chapter 8. Chloroplasts and mitochondria have other features in common: both often migrate from place to place within cells, and they contain their own DNA, which encodes some of the key organellar proteins (Chapter 10). The proteins encoded by mitochondrial or chloroplast DNA are synthesized on ribosomes within the organelles. However, most of the proteins in each organelle are encoded in nuclear DNA and are synthesized in the cytosol; these proteins are then incorporated into the organelles by processes described in Chapter 16.

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