Molecular Biology of the Cell - Volume 1 - Alberts B., Bray D., Lewis J., Raff M., Roberts K., Watson J. 1994

Molecular Organization of Cells
Energy Conversion: Mitochondria and Chloroplasts

Cell/35.html">Mitochondria, present in all Eukaryotic Cells, and Plastids, unique to plants (of which METABOLISM/14.html">Chloroplasts are of greatest interest), convert energy into forms that can be used to drive intracellular reactions. The specific function of these Organelles is reflected in the most striking feature of their Morphology—an Abundance of internal membranes. Membranes perform two key Functions in these energy-converting organelles. First, they carry out Electron transport processes, through which the energy of oxidation reactions (see Section 2.2.4) is converted into more useful forms, primarily ATP. Second, the membranes form large internal compartments within the organelle, housing Enzymes that catalyze other intracellular reactions.

Without mitochondria, an animal cell could only obtain ATP through anaerobic Glycolysis. However, The conversion of glucose to Pyruvate during glycolysis (Section 2.3.2) releases only a small fraction of the total Free energy that can be obtained from The oxidation of sugars. In mitochondria, the metabolism of sugars (and Fatty acids) is completed: pyruvate (like fatty acids) is oxidized by molecular oxygen (O₂) to CO₂ and H₂O. The energy released by this oxidation is used so efficiently that about 36 molecules of ATP are produced for every molecule of glucose oxidized, whereas glycolysis yields only 2 ATP molecules per glucose molecule. Chloroplasts are also highly efficient "machines" for ATP production, but their energy source is sunlight rather than sugars and fatty acids. Despite this fundamental difference, Mitochondria and chloroplasts are similarly organized and synthesize ATP in the same way.

The common pathway by which mitochondria, chloroplasts, and even Bacteria convert energy for biological purposes is based on a process known as Chemiosmotic Coupling. This process begins when "energy-rich" electrons are transferred from strong electron Donors along a chain of electron carriers embedded in an ion-impermeable membrane. During this transport along the electron-transport chain, electrons that were either excited by sunlight or extracted during the oxidation of nutrients successively drop to lower energy levels. Part of the released energy is used to pump protons from one side of the membrane to the other, thereby generating an electrochemical proton gradient across the membrane. The energy of this gradient drives Reactions Catalyzed by enzymes embedded in the same membrane (Fig. 7-1). In mitochondria and chloroplasts, most of the energy is used to convert ADP and P_i into ATP, although some of it is spent on transporting specific metabolites into and out of the organelle. In contrast, in bacteria, the electrochemical gradient serves as an equally important direct energy source as the ATP synthesized with its help: the gradient's energy drives not only many transport processes but also the rapid rotation of bacterial flagella that propel The Cell (Section 12.5.4).

Energy-converting eukaryotic organelles are believed to have evolved from Prokaryotic Cells that were engulfed by primitive eukaryotes and entered into Symbiosis with them 1.5 billion years ago. This explains why mitochondria and chloroplasts have their own DNA, which encodes some of these organelles' Proteins. Over time, however, mitochondria and chloroplasts have lost most of their own genomes and have become entirely dependent on proteins encoded by nuclear genes, which are synthesized in the Cytosol and only then imported into the organelle. Conversely, host cells have become dependent on these organelles, which provide a significant portion of the ATP required for Biosynthesis, Active Transport of ions and solutes, and motility, as well as containing A number of enzymes that catalyze various biosynthetic reactions.

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Fig. 7-1. Chemiosmotic coupling is used by all cells to convert energy. The energy of sunlight or nutrient oxidation is first used to generate a transmembrane electrochemical proton gradient. This gradient then serves as the energy source for various processes occurring in mitochondria, chloroplasts, and bacterial cells.