Chapter 3: Cell Metabolism
Chapter Summary
THE CENTRAL ROLE OF ENZYMES AS BIOLOGICAL CATALYSTS
The Catalytic Activity of Enzymes: Virtually all chemical reactions within cells are catalyzed by enzymes.
Mechanisms of Enzymatic Catalysis: Enzymes increase reaction rates by binding substrates in the proper position, by altering the conformation of substrates to approach the transition state, and by participating directly in chemical reactions.
Coenzymes: Coenzymes function in conjunction with enzymes to carry chemical groups between substrates.
Regulation of Enzyme Activity: The activities of enzymes are regulated to meet the physiological needs of the cell. Enzyme activity can be controlled by the binding of small molecules, by interactions with other proteins, and by covalent modifications.
METABOLIC ENERGY
Free Energy and ATP: ATP serves as a store of free energy, which is used to drive energy-requiring reactions within cells.
The Generation of ATP from Glucose: The breakdown of glucose provides a major source of cellular energy. In aerobic cells, the complete oxidation of glucose yields 36 to 38 molecules of ATP. Most of this ATP is derived from electron transport reactions in which O2 is reduced to H2O.
The Derivation of Energy from Other Organic Molecules: ATP can also be derived from the breakdown of organic molecules other than glucose. Because fats are more reduced than carbohydrates, they provide a more efficient form of energy storage.
Photosynthesis: The energy required for the synthesis of organic molecules is ultimately derived from sunlight, which is harvested by plants and photosynthetic bacteria. In the first stage of photosynthesis, energy from sunlight is used to drive the synthesis of ATP and NADPH, coupled to the oxidation of H2O to O2. The ATP and NADPH produced by these reactions are then used to synthesize glucose from CO2 and H2O.
THE BIOSYNTHESIS OF CELL CONSTITUENTS
Carbohydrates: Glucose can be synthesized from other organic molecules, using energy and reducing power in the forms of ATP and NADH, respectively. Additional ATP is then needed to drive the synthesis of polysaccharides from simple sugars.
Lipids Lipids are synthesized from acetyl CoA, which is formed from the breakdown of carbohydrates.
Proteins: The amino acids are synthesized from intermediates in glycolysis and the citric acid cycle. Their polymerization to form proteins requires additional energy in the form of ATP and GTP.
Nucleic Acids: Purine and pyrimidine nucleotides are synthesized from carbohydrates and amino acids. Their polymerization to DNA and RNA is driven by the use of nucleoside triphosphates as activated precursors.
