Energy and Energy Conversions
► Energy is the capacity to do work. Potential energy is the energy of state or position; it includes the energy stored in chemical bonds. Kinetic energy is the energy of motion (and related forms such as electric energy, light, and heat).
► Potential energy can be converted to kinetic energy, which can do work. Review Figure 6.1
► Living things, like everything else, obey the laws of thermodynamics. The first law of thermodynamics tells us that energy cannot be created or destroyed. The second law of thermodynamics tells us that the quantity of energy available to do work
(free energy) decreases and unusable energy (associated with entropy) increases. Review Figure 6.2
► Changes in free energy, total energy, temperature, and entropy are related by the equation AG = AH - TAS.
► Exergonic reactions release free energy and have a negative AG. Endergonic reactions take up free energy and have a positive AG. Endergonic reactions proceed only if free energy is provided. Review Figure 6.3
► The change in free energy (AG) of a reaction determines its point of chemical equilibrium, at which the forward and reverse reactions proceed at the same rate. For exergonic reactions, the equilibrium point lies toward completion (the conversion of all reactants into products). Review Figure 6.4
ATP: Transferring Energy in Cells
► ATP (adenosine triphosphate) serves as an energy currency in cells. Hydrolysis of ATP releases a relatively large amount of free energy. Review Figure 6.5
► The ATP cycle couples exergonic and endergonic reactions, transferring free energy from the exergonic to the endergonic reaction. Review Figures 6.6, 6.7. See web/CD Activity 6.1
Enzymes: Biological Catalysts
► The rate of a chemical reaction is independent of AG, but is determined by the size of the energy barrier. Catalysts speed reactions by lowering the energy barrier. Review Figures 6.8, 6.9
► Enzymes are biological catalysts, proteins that are highly specific for their substrates. Substrates bind to the active site, where catalysis takes place, forming an enzyme-substrate complex. Review Figure 6.10
► At the active site, a substrate can be oriented correctly, chemically modified, or strained. As a result, the substrate readily forms its transition state, and the reaction proceeds. Review Figures 6.11, 6.12. See web/CD Activity 6.2
► The active site where substrate binds determines the specificity of an enzyme. Upon binding to substrate, some enzymes change shape, facilitating catalysis. Review Figures 6.13, 6.14
► Some enzymes require cofactors to carry out catalysis. Prosthetic groups are permanently bound to the enzyme. Coenzymes are not usually bound to the enzyme. They can be considered substrates, as they are changed by the reaction and then released from the enzyme. Review Table 6.1 and Figure 6.15
► Substrate concentration affects the rate of an enzyme-catalyzed reaction. Review Figure 6.16
► Metabolism is organized into pathways in which the product of one reaction is a reactant for the next reaction. Each reaction in the pathway is catalyzed by an enzyme.
► Enzyme activity is subject to regulation. Some inhibitors react irreversibly with enzymes and block their catalytic activity. Others react reversibly with enzymes, inhibiting their action only temporarily. A compound closely similar in structure to an enzyme's normal substrate may competitively inhibit the action of the enzyme. Review Figures 6.17, 6.18. See web/CD Tutorial 6.1
► Allosteric regulators bind to a site different from the active site and stabilize the active or inactive form of an enzyme. Many such enzymes have multiple subunits. Review Figure 6.19. See web/CD Tutorial 6.2
► For allosteric enzymes, plots of reaction rate versus substrate concentration are sigmoid, in contrast to plots of the same variables for nonallosteric enzymes. Review Figure 6.20
► The end product of a metabolic pathway may inhibit the allosteric enzyme that catalyzes the commitment step of that pathway. Review Figure 6.21
► Enzymes are sensitive to their environment. Both pH and temperature affect enzyme activity. Review Figures 6.22, 6.23
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.