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| 8.1 The laws of thermodynamics describe how energy changes. The Flow of Energy in Living Things • Energy is the capacity to do work. Kinetic energy is energy of motion, while potential energy is stored energy. (p. 144) • Oxidation is the process whereby an atom or molecule loses an electron, and reduction is the process whereby an atom or molecule gains an electron. Oxidation-reduction reactions always take place together. (p. 145) The Laws of Thermodynamics • The First Law of Thermodynamics states that energy cannot be created or destroyed; it can only change from one form to another. (p. 146) • During each energy conversion, some energy dissipates into the environment as heat. (p. 146) • The Second Law of Thermodynamics states that disorder (entropy) in the universe is continuously increasing. (p. 146) • Entropy is always increasing because, as more energy is used, more energy is converted to heat. (p. 146) Free Energy • The amount of energy available to break and form chemical bonds is referred to as a molecule's free energy. (p. 147) • Reactions in which the products contain less free energy than the reactants tend to proceed spontaneously. (p. 147) Activation Energy • Activation energy is the extra energy required to destabilize existing chemical bonds. (p. 148) • Catalysts increase the rate of reaction by lowering the activation energy, but do not alter the proportions of the reactants or products. (p. 148) 8.2 Enzymes are biological catalysts. Enzymes • Cells use proteins (enzymes) to lower activation energies. Thousands of different enzymes are known, each catalyzing one or a few specific chemical reactions. (p. 149) How Enzymes Work • Most enzymes are globular proteins with one or more active sites where specific substrates can bind, forming an enzyme-substrate complex. (p. 150) Enzymes Take Many Forms • Multienzyme complexes are noncovalently bonded assemblies of enzymes that catalyze different steps of a reaction sequence, and can offer significant advantages in catalytic efficiency. (p. 151) • Not all biological catalysts are proteins; for example, certain reactions can be catalyzed by ribosomes. (p. 151) Factors Affecting Enzyme Activity • Several factors can affect the activity of an enzyme, including temperature, pH, inhibitors and activators, and enzyme cofactors. (pp. 152—153) 8.3 ATP is the energy currency of life. ATP • The chief energy currency all cells use is a molecule called adenosine triphosphate (ATP). (p. 154) • Because of electrostatic repulsion, the covalent bonds between phosphates are unstable; thus, ATP is an excellent energy donor. (p. 154) 8.4 Metabolism is the chemical life of a cell. Biochemical Pathways: The Organizational Units of Metabolism • Metabolism refers to the total of all chemical reactions carried out by an organism. (p. 155) • Biochemical pathways occur when the product of one reaction becomes the substrate for the next reaction. (p. 155) • To operate efficiently, a biochemical pathway must be coordinated and regulated by the cell, and regulation often takes place as feedback inhibition. (p. 156) |