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Chapter 4

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Biology 1202B
Richard Gardiner

Chapter 4: Energy and Enzymes Energy and the laws of Thermodynamics - Energy driven processes - Energy: the capacity to do work Energy Exists in Different Forms and States - Energy can exist in many different forms - Can be converted readily from one form to another - Kinetic energy: energy possessed by an object because it is in motion - Potential energy: energy stored, the energy an object has because of its location of chemical structure The Law of thermodynamics describes the energy flow in natural systems - the study of energy and its transformations is called thermodynamics - surrounding: outside the system - three different type of systems o isolated: does not exchange energy or matter o closed system: exchange energy and not matter o open system: exchange energy and matter The first law of thermodynamics - Energy flow between systems and the surroundings led to the formulation of two fundamental laws of thermodynamics that apply equally to living cells and to stars - First law of thermodynamics o Energy can be transformed from one form into another or transferred from one place to another, but can not be created or destroyed o Also called the principle of the conservation of energy The Second Law of thermodynamics - Each time energy is transformed from one form into another some of the energy is lost and unavailable to do work - Lost to the surrounding as heat - Cellular respiration, cells are able to convert only about 40% of the potential energy in glucose; remained is lost as heat - The unusable energy that is produced during energy transformations results in an increase in the disorder or formation results in an increase in the disorder or randomness of the universe - Disorder is a quantity called entropy - Second law of thermodynamics, o The total disorder (entropy) of a system and its surrounding always increases - The physcal disintegration of an organized system is the second law in action o Systems will move spontaneously towards arrangement with greater entropy o It takes energy to maintain low entropy Life and the second law of thermodynamics - Life goes against the second law o Things don’t become more random in a living cell; they become more ordered - Living things bring energy and matter and use them to generate order out of disorder - To generate order, living things give off heat and by products of metabolism such as carbon dioxide that are much less ordered and increase the disorder, or entropy of the surroundings - Living organism can be thought of as islands of low entropy in a sea (the universe) that is constantly becoming more random and disordered Free energy and Spontaneous Reactions - Chemical or physical reaction will occur without an input of energy  spontaneous reactions Energy content and Entropy Contribute to making a Reaction Spontaneous - Reaction tend to be spontaneous if the products have less potential energy then the reactants - Reactions tend to be spontaneous when the products are less ordered than the reactants The change in Free energy indicates whether a reaction is spontaneous - the portion of a system energy that is available to do work is called free energy (G) o G=H-T(delta)S o H is the change in enthalpy o S is the change in entropy of the system o T is the absolute temperature o For a reaction to be spontaneous the G must be negative - G represents the difference between the free energy of the final state compared with the initial state and that a negative G indicates that the products have less free energy than the reactants - Systems that have high free energy are less stable than systems that have less free energy - Systems will spontaneously change into a more stable state but cannot spontaneously change into being less stable - Concentration gradient Life and Equilibrium - Another term for maximum stability is equilibrium - As a system moves towards equilibrium, the free energy of the system becomes progressively lower and reaches its lowest point and maximum stability when the system is at equilibrium (G is zero) - Equilibrium point being at the bottom - The more negative the G the further towards completion the reaction will move before equilibrium is established - Positive G, the reaction would run backwards Metabolic pathways consist of Exergonic and Endergonic Reactions - Exergonic reaction: releases free energy, G is negative because the product contain less free energy than the reactants - Endergonic reaction: the products contain more free energy than the reactants; G is positive - Reaction tend to be part of a metabolic pathway, which is series of sequential reaction in which the products of one reaction are used immediately as the reactants for the next reaction series - Catabolic pathway: energy is released by the break down The energy Currency of the Cell: ATP - Assembly of complex molecules - These reaction have a positive G and are called endergonic and they may be part of both catabolic and anabolic pathways ATP Hydrolysis Releases Free Energy - ATP: contain large amounts of free energy because they possess what are called high energy phosphate bonds o Five carbon sugar, ribose, linked to the nitrogenous base adenine and a chain of three phosphate groups - There negative charges strongly repel each other, making the bonding arrangements unstable - Spontaneous reaction that relieves the repulsion and releases large amounts of free energy - ATP is a hydrolysis reaction - Results in the formation of adenosine diphosphate ADP can be further hydrolyzed to adenosine mono phosphate AMP o However this releases less energy then the hydrolysis of ATP ATP and Energy coupling - Hydrolysis reactions releases free energy that warms up the water in the surrounding - During shivering in muscle tissues to maintain body heat - Energy coupling: ATP is brought close contact with other reactions involved in endergonic reactions - The terminal phosphate group is transferred to the reactant molecules - This transfer is a phosphorylated, makes the molecule less stable - Energy coupling requires the action of an enzyme to bring ATP and reactant molecules in close association - The enzymes has a specific site on it that binds both the ATP and the reactant molecules allowing for transfer of the phosphate group Regeneration of ATP - Breakdown of ATP into ADP and Pi is an exergonic reaction that can be coupled to make other endergonic reaction proceed spontaneously - ATP is an renewable resourse - Recombining ADP and Pi - ATP synthesis from ADP and Pi is an energy requiring endergonic process - ATP cycle - Atypical cell maintains an ATP concentration that is about 1000 times greater than ADP – vary far from equilibrium The role of Enzymes in Biological Reactions - that a reaction is spontaneous does not mean it proceeds rapidly The Activation Energy Represents a Kinetic Barrier - For bonds to be broken they must first be strained or otherwise made less stable so that bond breakage can actually occur - Thus even though a reaction is spontaneous (negative G) the reaction will not actually start unless a relatively small boost of energy is added - This initial energy is known as the activation energy - Transition state: where bonds are unstable and are ready to be broken Enzymes Accelerate Reaction by Reducing the Activation Energy - Catalyst: chemical agent that speeds up the rate of reaction with out itself taking part in the reaction o Enzymes - The greater the activation energy barrier, the slower the reaction will proceed - Since the rate of a reaction will proportional to the number of reactant molecules that can acquire the necessary energy to get to the transition state, enzymes make it possible for a greater proportion of reactant molecule to attain the activation energy - Enzymes lower the activation energy of a reaction, they do not alter the change in free energy (G) of the reaction - Enzymes do not speed up the rate of spontaneous (exergonic) reaction - Enzymes do not supply free energy to a reaction - Enzymes cannot make an endergonic reaction proceed spontaneously - ATP hydrolysis can be used to make an endergonic reaction proceed spontaneously, but alone, an enzyme cannot - Enzymes do not change
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