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Energy Conversion: Mitochondria and Chloroplasts 14
To maintain their high degree of organization in a universe that is constantly drifting toward chaos, cells have a constant need for a plentiful supply of ATP, as we have explained in Chapter 2. In eukaryotic cells, most of the ATP that powers life processes is produced by specialized, membrane-enclosed, energy-converting organelles. These are of two types. Mitochondria, which occur in virtually all cells of animals, plants, and fungi, burn food molecules to produce ATP by oxidative phosphorylation. Chloroplasts, which occur only in plants and green algae, harness solar energy to produce ATP by photosynthesis. In electron micrographs, the most striking features of both mitochondria and chloroplasts are their extensive internal membrane systems. These internal membranes contain sets of membrane protein complexes that work together to produce most of the cell’s ATP. In bacteria, simpler versions of essentially the same protein complexes produce ATP, but they are located in the cell’s plasma membrane (Figure 14–1).
Comparisons of DNA sequences indicate that the energy-converting organelles in present-day eukaryotes originated from prokaryotic cells that were endocytosed during the evolution of eukaryotes (discussed in Chapter 1). This explains why mitochondria and chloroplasts contain their own DNA, which still encodes a subset of their proteins. Over time, these organelles have lost most of their own genomes and become heavily dependent on proteins that are encoded by genes in the nucleus, synthesized in the cytosol, and then imported into the organelle. And the eukaryotic cells now rely on these organelles not only for the ATP they need for biosynthesis, solute transport, and movement, but also for many important biosynthetic reactions that occur inside each organelle.
The common evolutionary origin of the energy-converting machinery in mitochondria, chloroplasts, and prokaryotes (archaea and bacteria) is reflected in the fundamental mechanism that they share for harnessing energy. This is known as chemiosmotic coupling, signifying a link between the chemical bond-forming reactions that generate ATP (“chemi”) and membrane transport processes