Concepts of Human Anatomy & Physiology   5/e   Van De Graaff/Fox
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I. Enzymes as Catalysts

Concept: Enzymes are biological catalysts that increase the rate of chemical reactions. Most enzymes are proteins, and their catalytic action results from their complex structure. The great diversity of protein structure allows different enzymes to be specialized in their action.

II. Control of Enzyme Activity

Concept: The rate of an enzyme-catalyzed reaction depends on numerous factors, including the concentration of the enzyme and the pH and temperature of the solution. Genetic control of enzyme concentration, for example, affects the rate of progress along particular metabolic pathways and thus regulates cellular metabolism.

III. Bioenergetics

Concept: Living organisms require the constant expenditure of energy to maintain their complex structures and processes. Central to life processes are chemical reactions that are coupled, so that the energy released by one reaction is incorporated into the products of another reaction. The transformation of energy in living systems is largely based on reactions that produce and destroy molecules of ATP and on oxidation-reduction reactions.

IV. Glycolysis and the Lactic Acid Pathway

Concept: In cellular respiration, energy is released by the stepwise breakdown of glucose and other molecules, and some of this energy is used to produce ATP. The complete combustion of glucose requires the presence of oxygen and yields 30 ATP per glucose. However, some energy can be obtained in the absence of oxygen by the pathway that leads to the production of lactic acid. This process results in a net gain of 2 ATP per glucose.

V. Aerobic Respiration

Concept: In the aerobic respiration of glucose, pyruvic acid is formed by glycolysis and then converted into acetyl coenzyme A. This begins the cyclic metabolic pathway of the Krebs cycle. As a result of these pathways, a large number of reduced NAD and FAD (NADH and FADH2) is generated. These reduced coenzymes provide electrons for an energy–generating process that drives the formation of ATP.

VI. Metabolism of Lipids and Proteins

Concept: Triglycerides can be hydrolyzed into glycerol and fatty acids. The latter are of particular importance because they can be converted into numerous molecules of acetyl CoA that can enter Krebs cycles and generate a large amount of ATP. Amino acids derived from proteins also may be used for energy. This involves deamination (removal of the amine group) and the conversion of the remaining molecule into either pyruvic acid or one of the Krebs cycle molecules

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