Cellular Energetics

The most important molecule for capturing and transferring free energy in biological systems is adenosine triphosphate, or ATP (see Figure 2-24). Cells use the energy released during hydrolysis of the terminal "high-energy" phosphoanhydride bond in ATP to power many energetically unfavorable processes. Examples include the synthesis of proteins from amino acids and of nucleic acids from nucleotides (Chapter 4), transport of molecules against a concentration gradient by ATP-powered pumps (Chapter 7), contraction of muscle (Chapter 19), and movement (beating) of cilia (Chapter 20). Although other high-energy molecules occur in cells, ATP is the universal "currency" of chemical energy; it is found in all types of organisms and must have occurred in the earliest life-forms.

This chapter focuses on how cells generate the high-energy phosphoanhydride bond of ATP from ADP and inorganic phosphate (HPO42"). This endergonic reaction, which is the reverse of ATP hydrolysis and requires an input of 7.3 kcal/mol to proceed, can be written as

where P(2" represents inorganic phosphate (HPO42"). The energy to drive this reaction is produced primarily by two main processes: aerobic oxidation, which occurs in nearly all cells, and photosynthesis, which occurs only in leaf cells of plants and certain single-celled organisms.

In aerobic oxidation, fatty acids and sugars, principally glucose, are metabolized to carbon dioxide (CO2) and water (H2O), and the released energy is converted to the chemical energy of phosphoanhydride bonds in ATP. In animal cells and most other nonphotosynthetic cells, ATP is generated mainly by this process. The initial steps in the oxidation of

Computer-generated model of a section of a mitochondrion from chicken brain, based on a three-dimensional electron tomogram. [T Frey and C. Mannella, 2000, Trends Biochem. Sci. 25:319.]

glucose, called glycolysis, occur in the cytosol in both eu-karyotes and prokaryotes and do not require oxygen (O2). The final steps, which require oxygen, generate most of the ATP. In eukaryotes, these later stages of aerobic oxidation occur in mitochondria; in prokaryotes, which contain only a plasma membrane and lack internal organelles, many of the final steps occur on the plasma membrane. The final stages of fatty acid metabolism sometimes occur in mitochondria and generate ATP; in most eukaryotic cells, however, fatty acids are metabolized to CO2 and H2O in peroxisomes without production of ATP.

In photosynthesis, light energy is converted to the chemical energy of phosphoanhydride bonds in ATP and stored in the chemical bonds of carbohydrates (primarily sucrose and starch). Oxygen also is formed during photosynthesis. In

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