|Chloroplasts Resemble Mitochondria But Have an
Chloroplasts carry out their energy interconversions by chemiosmotic mechanisms in much the same way that mitochondria do, and they are organized on the same principles (Figures 14-39 and 14-40). They have a highly permeable outer membrane, a much less permeable inner membrane, in which special carrier proteins are embedded, and a narrow intermembrane space. The inner membrane surrounds a large space called the stroma, which is analogous to the mitochondrial matrix and contains various enzymes, ribosomes, RNA, and DNA.
There is, however, an important difference between the organization of mitochondria and that of chloroplasts. The inner membrane of the chloroplast is not folded into cristae and does not contain an electron-transport chain. Instead, the electron-transport chain as well as the photosynthetic light-absorbing system and an ATP synthase are all contained in a third distinct membrane that forms a set of flattened disclike sacs, the thylakoids (see Figure 14-39). The lumen of each thylakoid is thought to be connected with the lumen ofother thylakoids, thereby defining a third internal compartment called the thylakoid space, which is separated from the stroma by the thylakoid membrane.
The structural similarities and differences between mitochondria and chloroplasts are illustrated in Figure 14-41. Superficially, the chloroplast resembles a greatly enlarged mitochondrion in which the cristae have been converted into a series of interconnected submitochondrial particles in the matrix space. The knobbed end of the chloroplast ATP synthase, where ATP is made, protrudes from the thylakoid membrane into the stroma, just as it protrudes into the matrix from the membrane of each mitochondrial crista.