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Chapter 4

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Wilfrid Laurier University
Matthew Smith

Chapter 4 A Spontaneous Endothermic Reaction  How can melting of ice be a spontaneous reaction if it is endothermic?  Melting of ice increases entropy  Phase changes result in an increase in entropy o Solid  liquid  gas Free Energy (∆G)  Energy available to do work o ∆G = ∆H – T∆S  ∆ (delta) = change (final state – initial state)  ∆G = change in free energy (Gibbs Free Energy)  ∆H = change in enthalpy  T = absolute temperature (degree Kelvin)  ∆S = change in entropy  As reaction goes to completion, its influence by 2 factors: o Change in energy content: ∆H o Changes in entropy: ∆S  For spontaneous reaction, ∆G < 0  -∆G means that the products have less free energy then the reactions Chemical Reactions and Equilibrium  Equilibrium is maximum stability  Equilibrium point is reached when reactants are converted to products and products are converted back to reactants at equal rates  ∆G = 0 Equilibrium in Living systems  Living systems are open  ∆G of life always negative as organisms take in energy-rich molecules (or light, if photosynthetic) and use them to do work  Organisms reach equilibrium, ∆G = 0, Only when they die Metabolic Pathways and Reactions  Two groups of reactions: o Exergonic reaction where ∆G is negative because products contain led free energy than reactants o Endergonic reactions where ∆G is positive because products contain more free energy than reactants Free Energy Summary  Free energy changes when the potential and/or entropy of substances changes  Chemical reactions run in the direction that lowers the entropy in the system Metabolic Pathways and Reactions  Metabolic Pathway: series of sequential reactions in which products of one reaction are used immediately as reactants for the next reaction in the series  Catabolic pathway: Energy is release by breakdown of complex molecules to simpler compounds  Anabolic pathway: Consumes energy to build complicated molecules from simpler ones. (ex. Protein synthesis) 4.3 The energy currency of the cell: ATP  ATP (adenosine triphosphate) o ATP hydrolysis releases free energy that can be used as a source of energy for the cell  ATP and Energy Coupling  Hydrolysis of ATP is an exergonic reaction that can be coupled to make otherwise endergonic reactions process spontaneously  Coupling reactions require enzymes  ATP/ADP Cycle! Enzymes  Just because a reaction is spontaneous does not mean that it proceeds rapidly  Enzymes are a special group of proteins that can alter the speed of a reaction Activation Energy  Initial input of energy to start a reaction, even if it is spontaneous  Activation Energy, E :Ainitial energy investment required to start a reaction  Molecules that gain necessary activation energy occupy the transition state Biological Catalysts  Catalyst: Chemical agent that speeds up the rate of reaction without itself taking part in the reaction  Enzymes: Are biological catalysts o Increase the rate of a reaction by lowering activation energy of a reaction o Remains unchanged Enzyme Specificity  Active site of enzyme combines briefly with reactants (substrates)  only one specific site where the substrate will combine with the enzyme.  Enzyme is released unchanged  during the reaction, the enzyme will change shape, however, by the end, it will look the same as it did before the reaction.  The induced fit hypothesis is a modification of the “lock and key” hypothesis. This hypothesizes that the active sit if not a rigid lock, but that it changes its confirmation in order to bind its substrate most optimally when the substrate is present  Intrinsically disorder proteins  instead of having a defined shape, they are randomly existing amino acids. This randomness is important for their binding. When they bind to a substrate, they change from the amino acid chain to a structured, tight fit around the substrate Catalytic Cycle of Enzymes  Enzyme Cofactors  Enzyme cofactors o Inorganic ions or organic non-protein groups necessary for catalysis to occur  Cofactors: Metallic ions (magnesium, iron, copper, zinc)  Coenzymes: organic cofactors such as vitamins (ex. Vitamin C). o Only when bound to cofactor, structure of enzyme is altered so it cannot bind to the substrate Transition State  During catalysis, the substrate and active site from an intermediate transition state  Enzymes achieve this transition state via 3 major mechanisms i. Bringing the reacting molecules into close proximity ii. Exposing the reactant molecules to altered environments that promote their interactions iii. Changing the shape of a substrate molecule iv. Formation of the transition state Enzyme and Substrate Concentrations  In presence of excess substrate, rate
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