CHAPTER 8 NOTES Living cell generates thousands of different reactions Metabolism is the totality of an organism s chemical reactions required to maintain life Types of metabolic reactions Anabolic building molecules requires energy Ex bldg of protein from aa Catabolic breaking molecules releases energy Ex cellular respiration A sequence of chemical reactions where the product of one reaction serves as a substrate for the next is called a metabolic pathway or biochemical pathway Each step is catalyzed by a specific enzyme Metabolism transforms matter and energy follows laws of thermodynamics Energy is the capacity to cause change o Types of energy important in biological systems are potential energy chemical atomic and kinetic energy light heat and sound o Measuring energy calorie cal amt heat required to raise 1 g of water 1 C kcal 1000 cal Calorie in food o Energy to light Bioluminescence Thermodynamics is the science of energy transformations o 1st Law Energy can be transformed or transferred but is never created or destroyed conservation of energy o 2nd Law In every energy transfer some energy becomes unusable heat which increases entropy or disorder of the universe Energy transformations proceed spontaneously to convert matter from a more ordered less stable form to a less ordered more stable form Spontaneous reactions occur without energy input quickly or slowly o it increases the entropy of the universe Entropy may decrease in an organism but the universe s total entropy increases Cells create ordered struc from less ordered materials It takes a lot of energy to maintain this order thus organisms live under the expense of free energy Free energy is the portion of a system s energy that is able to perform work when temperature and pressure is uniform throughout the system as in a living cell o Referred to as Gibbs free energy G in a living cell refers to the amount of energy actually available to break and subsequently form other chemical bonds Biologists want to know which reactions occur spontaneously and which require input of energy What factors determine the spontaneity of a reaction o Entropy and Enthalpy What ties the two together is o Gibbs Free Energy The change in this free energy G predicts whether the reaction is spontaneous or requires energy o G H T S o G change in free energy o H change in enthalpy total energy o S change in entropy o T temperature in degrees Kelvin G spontaneous G NOT spontaneous If the products of a reaction have less potential energy in their chemical bonds than the reactants do then the reaction releases energy and is exergonic The G for the reaction is negative and the reaction will proceed spontaneously without any energy input If the products of a reaction have more potential energy in their chemical bonds than the reactants do then the reaction requires energy and is endergonic The G for the reaction is positive and the reaction will not proceed without an energy input Systems with high potential energy are not very stable and as they give up energy going to a lower energy more stable state some of the energy released in the process can be captured to do work This work may be in the form of motion or it may be in the formation of chemical bonds Reactions in a closed system eventually reach equilibrium and then do no work Cells are open systems experiencing a constant flow of materials A defining feature of life is that metabolism is never at equilibrium In cells processes with very large negative Gs are usually broken down in smaller steps where energy can be captured in manageable chunks Cells manage energy resources by energy coupling the use of an exergonic process to drive an endergonic one o Most energy coupling in cells is mediated by ATP o ATP is the cell s energy shuttle o Provides energy for cellular functions o Hydrolysis of last phosphate on ATP by addn of H2O o G 7 3 kcal mole o The chemical potential energy temporarily stored in ATP drives most cellular work o The energy to phosphorylate ADP comes from catabolic reactions in the cell How ATP performs work in a cell Coupled exergonic and endergonic reactions to create overall exergonic G Transfers one phosphate group from ATP to reactant phosphorylation When phosphate group is displaced work can occur Glucose phosphate glucose phosphate H2O G 3 3 Kcal mole Endergonic ATP H2O ADP Pi G 7 3 Kcal mole Exergonic Coupled reaction Glucose ATP glucose phosphate ADP G 4 0 Kcal mole Exergonic Phosphate directly transferred to Glucose is called Phosphorylation A spontaneous reaction is not necessarily a fast reaction Catalyst agent that speeds up the rate of a chemical reaction without being consumed during the reaction Enzymes most common protein catalysts in living cells o speed up reactions o channel the reaction in certain directions o allow the cell to control the reactions o Sucrase very specifically breaks sucrose down into glucose and fructose o Recall Chemical rxn btn molecules involves bond breaking forming o The initial energy needed to start a chemical rxn is called Activation energy EA o EA often supplied in the form of heat from the surroundings o How do enzymes speed up reactions By lowering the EA barrier They do not affect the rxn s G or equilibrium o Reactants that Enzymes act on is Substrate o Enzymes bind to Substrates Enzyme substrate complex o Substrate binds Enzymes at very specific sites where the 3D shape of the substrate fits the 3D shape of the enzyme This is where the reaction occurs and it is called the active site Active site can lower Ea and speed up a reaction by 1 Orienting substrates correctly 2 Bonding to the substrate directly 3 Providing a favorable microenvironment 4 Straining substrate bonds Induced fit of a substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction o An enzyme s activity o can be affected by General environmental factors such as Temperature and pH Chemicals that specifically influence the enzyme Other Enzyme helpers Cofactor non protein usually inorganic ion but could be organic temporarily binds to enzyme Coenzyme organic molecule that participates in reaction but left unchanged afterward Includes vitamins Prosthetic groups small molecules permanently attached to the enzyme and help catalyze o Molecules other than the substrate also can bind to enzymes and affect their activity Competitive inhibitors bind to the active site and block substrate access Non competitive
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