Читать книгу Cell Biology - Stephen R. Bolsover - Страница 41

Like nuclei, mitochondria are encapsulated by an outer and inner membrane (Figure 2.4). Perhaps the most distinctive feature of mitochondria is that the inner membrane is markedly elaborated and folded to increase its surface area. These shelf‐like projections, named cristae, make mitochondria among the most easily recognizable organelles (e.g. Figure 1.4 on page 8). The number of cristae, like the number of mitochondria themselves, depends upon the energy budget of the cell in which they are found. In muscle cells, which must contract and relax repeatedly over long periods of time, there are many mitochondria that contain numerous cristae; in fat cells, which generate little energy, there are few mitochondria and their cristae are less well developed. This gives a clue as to the function of mitochondria: they are the cell's power stations. Mitochondria produce the molecule adenosine triphosphate ( ATP ) (page 35), the cell's main energy currency that provides the energy to drive a host of cellular reactions and mechanisms. Mitochondria make ATP through the process of oxidative phosphorylation whereby oxygen is used to pass electrons from energy intermediates to a series of protein complexes on the inner mitochondrial membrane known as the electron transport chain. This results in the transfer of H⁺ out of the mitochondria and the generation of a concentration and voltage gradient. This gradient is subsequently tapped into by a protein known as ATP synthase which, as its name suggests, produces ATP. This process is essential for aerobic life and is the reason we breathe. We will return to ion gradients and the uses the cell puts them to in Chapter 9.

Figure 2.4. The mitochondrion.

The great majority of proteins of the mitochondrion are encoded by nuclear genes and synthesized in the cytoplasm. But some of the information necessary for the function of mitochondria is stored within the organelle itself. Mitochondria contain many small circular DNA molecules (Table 1.1 on page 7) that are very different from the long, linear DNA molecules in the nucleus. This is strong evidence for the endosymbiotic theory of the origin of mitochondria (page 7), which proposes that the small circular DNA molecules found in mitochondria are all that is left of the chromosomes of the original symbiotic bacteria. Mitochondria also contain ribosomes (again, more like those of bacteria than the ribosomes in the cytoplasm of their own cell) which allows synthesis of a small subset of mitochondrial proteins.