Except for vacuoles, chloroplasts are the largest and the most characteristic organelles in the cells of plants and green algae (see Figure 1-19). The endosymbiont hypothesis (see Chapter 12) posits that these organelles originated by endocytosis of a primitive photosynthetic bacterium. Chloroplasts can be as long as 10 μm and are typically 0.5–2 μm thick, but they vary in size and shape in different cells, especially among the algae. In addition to the inner and outer membranes that bound a chloroplast, this organelle also contains an extensive internal system of interconnected membrane-limited vesicles called thylakoids, which are flattened to form disks. Thylakoids often form stacks called grana and are embedded in an aqueous matrix termed 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 carbon dioxide into three-carbon intermediates by enzymes located in the stroma; the intermediates are then exported to the cytosol and converted into sugars.

The molecular mechanisms by which ATP is formed in mitochondria and chloroplasts are very similar, as explained in Chapter 12. Besides being surrounded by two membranes, chloroplasts and mitochondria have other features in common: both often migrate from place to place within cells, and both contain their own DNA, which encodes some of the key organelle proteins (see Chapter 12). 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 13.