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UNIT 1 Chapter 5 Metabolism, Energy, and Life I. Metabolism: a) Definition: the sum of an organism’s chemical reactions (life functions) which help in maintaining homeostasis. b) Life Functions 1. Nutrition: the process in which organic molecules (food) are obtained (ingested) and used (digested). 2. Transport: the movement or distribution of material from one location to another. Examples include active and passive transport. 3. Respiration: the process in which energy (ATP) is produced by the catabolism of organic compounds through a series of interconnected chemical reactions. Examples include anaerobic and aerobic respiration. 4. Excretion: the removal of potentially toxic, metabolic wastes. Examples of metabolic wastes include: CO2, H2O, and nitrogenous wastes. 5. Growth: the increase In size of a given organism. This occurs by the increase in cell size and the increase in cell number. 6. Synthesis: the process in which small simple molecules are combined to make larger, more complex molecules. 7. Regulation: the coordination and control of the chemical reactions needed to maintain homeostasis. 8. Reproduction: the process in which the number of organisms within a population are increased. Reproduction is only necessary for the survival of the species, not the individual. II. Life Functions and Metabolic Pathways • The metabolism of a cell involves an intricate network of chemical reactions. • The metabolic functions are arranged into a series of interconnected pathways. • The rate of these pathways is governed by the presence of biological catalysts called enzymes. a) Types of Metabolic Pathways 1. Catabolic Pathways: • release energy by the lysis of complex molecules into simpler compounds. Ex: Cellular Respiration 2. Anabolic Pathways: • consume energy to build complicated molecules from simpler ones. Ex: Protein Synthesis from the dehydration synthesis of amino acids. FIGURE 1: Anabolism (Photosynthesis) and Catabolism (Cellular Respiration) III. Thermodynamics and Life Key Concept: An adequate discussion of metabolism is not possible without an understanding energy, its uses, its transformations, and its laws. a) Thermodynamics • Thermodynamics is the study of energy. • Energy is the ability to bring about change or to do work. It exists in many forms, such as heat, light, chemical energy, and electrical energy. b. Potential vs. Kinetic energy 1. Potential energy in biochemical systems represents stored chemical bond energy. 2. Kinetic energy represents the energy of motion of the chemicals in living systems usually generated by the breaking of chemical bonds in biochemical systems. c) Laws of Thermodynamics 1. First Law of Thermodynamics: •Energy cannot be created or destroyed but it is can be changed from one form to another. It is always conserved. •The total amount of energy and matter in the Universe remains constant, merely changing from one form to another. 2. Second Law of Thermodynamics states that "in all energy exchanges, if no energy enters or leaves the system, the potential energy of the state will always be less than that of the initial state." • Closed systems will have a tendency to go from a level low disorder to high disorder. Entropy is a level of disorder within a given system. • Energy is naturally converted from a more to a less usable form. d) Free Energy and Spontaneity 1. Spontaneous Reactions: chemical reactions that occur without the addition of an outside source of energy. 2. Non-spontaneous Reactions : chemical reactions that require an external energy source to occur. e) Gibbs Free Energy • Free energy is the portion of a system’s energy that can perform work when temperature is uniform throughout the system. • Free energy is the form of usuable energy needed to perform work. • By evaluating the entropy (S) , enthalpy (H) , and temperature (T) of a system, one has the ability to determine the amount of free energy (G) available to perform work. NOTE: •Enthalpy (H) indicates whether a reaction is exothermic or endothermic. •If DH = (+), the reaction is endothermic and energy is absorbed. •If DH =(-), the reaction is exothermic and energy is released. 1) Gibbs Free Energy Equation DG = DH - TD S If DG = (-), the reaction is spontaneous. If DG = (+), the reaction is non-spontaneous. 2) Conditions Favoring Spontaneous Reactions DH = (-) and DS = (+) F) Exergonic Reactions Energy releasing processes, ones that "generate" energy, are termed exergonic reactions. Figure 1 Time-energy graphs of an exergonic reaction (top) and endergonic Figure 2: Exergonic Reaction G)Endergonic Reactions: • Reactions that require energy to initiate the reaction are known as endergonic reactions. Figure 3: Endergonic Reaction Note: •All natural processes tend to proceed in such a direction that the disorder or randomness of the universe increases (the second law of thermodynamics). • Endergonic = Endothermic • Exergonic = Exothermic H) Coupled Reactions • For non-spontaneous reactions to occur, energy is needed. • Non-spontaneous reactions must be coupled with a spontaneous (exothermic or exergonic) reaction to occur. Ex: Photosynthesis: The spontaneous nuclear fission of hydrogen on the sun releases solar energy (exergonic reaction). This energy (light) is absorbed by photosynthetic organisms to synthesized glucose from carbon dioxide and water (endergonic reaction). IV. Energy in Living Systems a) Adenosine Triphosphate (ATP) 1. Characteristics • Energy-carrying molecule in living systems. • Composed of nitrogen-containing base (adenine), a fivecarbon sugar (ribose), and three phosphate groups. • The energy in ATP is stored in the high-energy bonds between terminal and second phosphate group. Adenine Phosphate Groups Ribose • ATP is generated by the process of cellular respiration. V. ENZYMES Substrate Active Site Enzyme a) Characteristics: 1. Enzymes are biochemical catalysts. They control the rate of chemical reactions by lowering the amount of energy needed to initiate the chemical reaction. 2. An important characteristic of catalysts is that they are NOT USED UP or changed when catalyzing a chemical reaction, thus they are available to repeat the catalysis repeatedly ACTIVATION ENERGY Figure 1. Activation energy. The substrate contains a certain amount of trapped, or potential, energy. To release that energy, activation energy has to be added. The amount of energy needed to initiate the reaction can be lessened if an enzyme is present. 3. Enzymes are made of protein. Due to this fact, they display quite a complex, three-dimensional shape that is specific for particular substrate molecules. 4. The area of an enzyme that is responsible for interacting with a substrate is the active site. 5. The active site of an enzyme is complementary to the substrate molecule. 6. The substrate will temporarily bind to the enzyme at the active site. 7. The enzyme will cause steric strain on the substrate causing it to become chemically unstable. 8. The instability of the substrate molecule will cause it to react with less energy than previously needed. 9. Catalysts cannot cause energetically unfavorable reactions to occur. They are only effective in catalyzing reactions that are already spontaneous. 10. The model that describes the interaction between enzyme and substrate is called the Lock and Key Model. 11. The active site of enzymes can be changed slightly to allow substrate molecules to bind. • This change in the enzyme’s active site is caused by the presence of the substrate. • This model of enzyme-substrate action is called the induced fit hypothesis. D:\ICIB.exe 12. The active site dictates whether the enzyme has the ability to catalyze a chemical reaction. • There are many factors that can affect the active site. • This is the key to understanding the regulation of enzyme activity. b) Factors Affecting Enzyme Activity KEY IDEA: Enzymes have evolved to function optimally at a particular pH, temperature, and salt concentration. Changes in any/all of these variables will result in enzyme denaturation. This effectively changes the enzyme’s ability to catalyze reactions. 1. pH vs. Enzyme Activity • Indicates the acidity/alkalinity of a solution by measuring the concentration of hydrogen ions. • Most enzymes function best at a pH range between 6-8. • Exceptions include the enzyme pepsin which functions to hydrolyze protein in the acidic fluids of the stomach. The higher levels of acidity in the stomach causes a conformational change in the enzyme’s active site. • This results in the change of the inactive pepsin to the active pepsinogen. Table II pH for Optimum Activity Enzyme pH Optimum Lipase (pancreas) 8.0 Lipase (stomach) 4.0 - 5.0 Lipase (castor oil) 4.7 Pepsin 1.5 - 1.6 Trypsin 7.8 - 8.7 Urease 7.0 Invertase 4.5 Maltase 6.1 - 6.8 Amylase (pancreas) 6.7 - 7.0 Amylase (malt) 4.6 - 5.2 Catalase 7.0 2. Temperature vs. Enzyme Activity • Temperature is a measure of the average kinetic energy of matter. • Increases in temperature increases the KE and thus increases the velocity of the particles of matter. • By increasing temperature, the probability of effective collisions between reactants and between enzyme and reactants increases, thus increasing the reaction rate. Temperature vs. Enzyme Activity Denaturation Enzyme Activity Temperature • However, when the temperature of a system exceeds the level to which the enzyme is adapted to functioning, the enzyme denatures and the reaction rate decreases. OVERVIEW: By changing the pH, temperature, and salinity of a system in which enzymes are functioning, a destabilization of the tertiary structure of the enzyme ensues. This causes a change in the enzyme’s active site resulting in denaturation. 3. Substrate Concentration and Enzyme Activity • An increase in substrate concentration will cause a greater probability of a reaction with enzymes. • This will cause an increase in enzyme activity until all of the enzymes are functioning at which point the activity remains constant. 4. Enzyme Concentration vs. Enzyme Activity • An increase in enzyme concentration with an abundance of substrate will cause the reaction rate to increase. Enzyme Concentration vs. Enzyme Activity c) Metabolism Regulation Via Controlling Enzyme Activity • Allosteric Regulation: Certain molecules bind to a binding site, distinct from the active site. These molecules can act as activators or inhibitors by changing the active site configuration. • Feedback Inhibition: a method of metabolic control in which the end-product of a metabolic pathway acts as an inhibitor of an enzyme within that pathway. D:\ICIB.exe 1. Types of Inhibition • Competitive Inhibition: the process in which a molecule similar in shape to the substrate binds to the active site, effectively blocking the substrate molecule. Ex: Alcohol metabolism • Noncompetitive Inhibition: the process in which a molecule binds to a site different from the active site on the enzyme. This interaction changes the shape of the active site blocking the substrate molecule.