Metabolism the Totality of an Organism’s

metabolic pathway begins with a speciûc molecule, which is then altered in a series of deûned steps, resulting • Metabolism: the totality of an organism’s chemical in a certain product. Each step of the pathway is catalyzed reactions. by a speciûc enzyme. • Bioenergetics: The study of how energy üows through 1. Catabolic pathways release energy by breaking down complex molecules into simpler compounds. 2. Anabolic living pathways consume energy to build complicated organisms. molecules from simpler ones. • Energy: the capacity to cause change. 2. Energy exists in various forms, and the work of life • Thermodynamics: The study of the energy depends on the ability of cells to transform energy from transformations that occur in a collection of matter. one type into another. • Entropy: a measure of disorder or randomness. 1. Kinetic energy: Energy associated with the relative • Spontaneous process: a process that can occur without motion of objects. Heat, or thermal energy, is kinetic an input of energy. Does not imply that such a process energy associated with the random movement of atoms or would occur quickly. molecules. • Free energy is the portion of a system’s energy that can 2. Potential energy: The energy that matter possesses perform because of its location or structure. Chemical energy is a work when temperature and pressure are uniform term used by biologists to refer to the potential energy throughout the system. available for release in a chemical reaction. • Energy coupling: the use of an exergonic press to drive 3. An isolated system, such as that approximated by liquid an in a thermos bottle, is unable to exchange either energy or endergonic one. matter with its surroundings. In an open system, energy • Enzyme: a macromolecule that acts as a catalyst. and matter can be transferred between the system and its • Substrate: the reactant an enzyme acts on. The enzyme surroundings. binds to its 4. First law of thermodynamics: Energy can be substrate, forming an enzyme transferred and transformed, but it cannot be created or substrate complex. destroyed. (Principle of conservation of energy). 5. Second law of thermodynamics: Every energy transfer Chapter 8: An Introduction to Metabolism or transformation increases the entropy of the universe. For a process to occur spontaneously, it must increase the entropy of the universe. 1. An organism’s metabolism transforms matter and 1. It is true that cells create ordered structures from less energy, subject to the laws of thermodynamics. 1. A organized starting materials. However, an organism also takes in organized forms of energy from the surroundings 3. ATP powers cellular work by coupling exergonic and replaces them with less ordered forms. 2. On a larger reactions to endergonic reactions. scale, energy üows into an ecosystem in the form of light 1. A cell does three main kinds of work: and exists in the form of heat. 3. The entropy of a 1. Chemical work: the pushing of endergonic reactions, particular system, such as an organism, may actually which would not occur spontaneously. decrease as long as the total entropy of the universe—the 2. Transport work: the pumping of substances across system plus its surroundings4increases. membranes against the direction of spontaneous 2. The free-energy change of a reaction tells us whether movement. 3. Mechanical work: such as the beating of or not the reaction occurs spontaneously. 1. The change cilia, the contraction of muscle cells, and the movement of chromosomes during cellular reproduction. in free energy, ∆G, can be calculated for a chemical 2.Adenosine triphosphate contains the sugar ribose, reaction with the following formula: ∆G = ∆H 3 T ∆S when ∆H is the change in the system’s enthalpy, ∆S is the with the nitrogenous base adenine and a chain of three phosphate groups bounded to it. The bonds between the change in the system’s entropy, and T is the absolute phosphate groups of ATP can be broken by hydrolysis. temperature in Kelvin units. Only experiments with a When the terminal phosphate bond is broken, 7.3 kcal of negative ∆G are spontaneous. 2. We can think of free energy as a measure of a system’s energy per mole is released. All three phosphate groups instability4its tendency to change to a more stable state. are negatively charged. These like charges are crowded Unless something prevents it, systems will move towards together, and their mutual repulsion contributes to the instability of this region of the ATP molecule. The greater stability. triphosphate tail of ATP is the chemical equivalent of a 3. As a reaction proceeds towards equilibrium, the free energy of the mixture of reactants and products decreases. compressed spring. Free energy increases when a reaction is somehow pushed 3.The cell’s proteins harness the energy released during away from equilibrium. Because a system at equilibrium ATP hydrolysis in several ways to perform the three types of cellular work4chemical, transport, and mechanical. cannot spontaneously change, it can do no work. A process is spontaneous and can perform work only when it Thus, the cell is able to use the energy released by ATP hydrolysis directly to drive chemical reactions that, by is moving towards equilibrium. themselves, are endergonic. This usually involves the 4. An exergonic reaction proceeds with a net release of transfer of a phosphate group from ATP to some other free energy. Because the chemical mixture loses free molecule, such as the reactant. The recipient of the energy, ∆G is negative for an exergonic reaction. 5. An endergonic reaction is one that absorbs free energy phosphate group is then said to be phosphorylated. 4. ATP is a renewable resource that can be regenerated by from its surroundings. Because this kind of reaction essentially stores free energy in molecules, ∆G is positive. the addition of phosphate to ADP. The free energy required to phosphorylate ADP comes from exergonic Such reactions are nonspontaneous. breakdown reactions in the cell. The chemical potential 6. Reactions in an isolated system eventually reach equilibrium and can then do no work. Because systems at energy temporarily stored in ATP drives most cellular equilibrium are at a minimum of G and can do no work, a work. cell that has reached metabolic equilibrium is dead. The 4. Enzymes speed up metabolic reactions by lowering fact that metabolism as a whole is never at equilibrium is energy barriers 1. The laws of thermodynamics tell us what will and will one of the deûning features of life. 7. A living cell is not in equilibrium. The constant üow of not happen under given conditions but say nothing about the rate of these processes. materials in and out of the cell keeps the metabolic 2. Every chemical reaction between molecules involves pathways from ever reaching equilibrium. both bond breaking and bond forming. 3.The initial investment of energy for starting a reaction is known as the free energy of activation, or activation energy, abbreviated EA. We can think of activation energy as the amount of energy needed to push the reactants over an energy barrier, or hill, so that the