PART II BIOENERGETICS AND METABOLISM Metabolism involves o Obtaining chemical energy by capturing solar energy or degrading energy rich nutrients from the environment o Converting nutrient molecules into the cell s own characteristic molecules o Polymerizing monomeric precursors into macromolecules proteins nucleic acids and polysaccharides o Synthesizing and degrading biomolecules required for specialized Living organisms can be divided into two large groups use carbon dioxide from the atmosphere as their sole cellular functions o Autotrophs source of carbon Photosynthetic bacteria Green algae Vascular plants o Heterotrophs form of relatively complex organic molecules must obtain carbon from their environment in the Multicellular animals Most microorganisms Many autotrophs are photosynthetic Heterotrophs obtain their energy from the degradation of organic nutrients produced by autotrophs All living organisms also require a source of nitrogen o Bacteria and plants can generally use ammonia or nitrate o Vertebrates must obtain nitrogen in the form of amino acids or other organic compounds o Only a few organisms cyanobacteria and many species of soil bacteria are capable of utilizing atmospheric nitrogen There are cycles of carbon oxygen and nitrogen in our atmosphere o These cycles are driven by a flow of energy through the biosphere beginning with the capture of solar energy by photosynthetic organisms and the use of this energy to generate energy rich carbohydrates and other organic nutrients o These nutrients are then used as energy sources by heterotrophs In all metabolic processes there is a loss of useful free energy and an increase in the amount of unusable energy heat and entropy cell or organism the sum of all the chemical transformations taking place in a Metabolism o Occurs through a series of enzyme catalyzed reactions metabolic pathways o The precursor is converted into a product through a series of metabolic intermediates called metabolites o Intermediary metabolism metabolic pathways that interconvert precursors metabolites and products of low molecular weight the combined activities of all the Catabolism the degradative phase of metabolism o Release energy some of which is conserved as ATP and reduced Anabolism complex molecules small simple precursors are built up into larger and more electron carriers o The rest of the energy is lost as heat o Convergent o Also called biosynthesis o Require an input of energy o Divergent Some pathways are linear some are branched others are cyclic Anabolic and catabolic pathways are suppressed when the other is active o They do not have the exact same enzymes o They must include one step that is thermodynamically favorable to make them irreversible Metabolism is regulated at several levels o The availability of substrate o Allosteric regulation o Growth factors and hormones in multicellular organisms Chapter 13 Bioenergetics and Biochemical Reaction Types 13 1 Bioenergetics and Thermodynamics Energy transductions changes of one form of energy into another o Bioenergetics is the quantitative study of energy transductions in living cells and of the nature and function of the chemical processes underlying these transductions Biological Energy Transformations Obey the Laws of Thermodynamics The First Law of Thermodynamics for any physical or chemical change the total amount of energy in the universe remains constant Energy may change form or it may be transported from one region to another but it cannot be created or destroyed The Second Law of Thermodynamics in all natural processes the entropy of Gibbs free energy G the universe increases during a reaction at constant temperature and pressure expresses the amount of energy capable of doing work o When the free energy change is negative the reaction is exergonic and proceeds spontaneously o The free energy change is positive in endergonic reactions o Units are J mol or cal mol Enthalpy H the heat content of the reacting system o Reflects the number and kinds of bonds in the reactants and products o If the change in enthalpy is negative the reaction is exothermic and vice versa o Units are J mol or cal mol Entropy S system o Units are J mol K a quantitative expression for the randomness or disorder in a Cells Require Sources of Free Energy Cells function at essentially constant temperature and pressure Cells must use free energy not heat o Heterotrophs acquire free energy from nutrient molecules o Photosynthetic cells acquire free energy from absorbed solar o Both transform this energy into ATP and other energy rich radiation compounds Standard Free Energy Change Is Directly Related to the Equilibrium Constant When a reacting system is not at equilibrium the tendency to move toward equilibrium represents a driving force the magnitude of which can be expressed as the free energy change for the reaction o The standard free energy change is the driving force when the reaction is occurring at standard conditions Standard transformed constants biochemical standard state o Written with a prime o Are also referred to as standard free energy changes physical constants based on the The standard free energy change is a constant o It is simply an alternative way of expressing a reaction s equilibrium constant eq 1 G o 0 eq 1 G o 0 eq 1 G o 0 If K If K If K Relatively small changes in G o correspond to large changes in K G o is the difference between the free energy content of the products and the eq free energy content of the reactants under standard conditions o If G o is negative the products contain less free energy than the reactants and the reaction will proceed spontaneously under standard conditions Actual Free Energy Changes Depend on Reactant and Product Concentrations The standard free energy change tells us in which direction and how far a given reaction must go to reach equilibrium when the initial concentration of each component is 1 0 M the pH is 7 0 the temperatures is 25 oC and the pressure is 101 3 kPa o Thus it is a constant The actual free energy change is a function of reactant and product concentrations and of the temperature The criterion for spontaneity of a reaction is the value of the actual free energy change not the standard free energy change Some thermodynamically favorable reactions do not occur at measurable rates The free energy change for a reaction is independent of the pathway by which the reactions occurs reaction proceeds o Enzymes cannot change