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Biology - Chapter 8 - Metabolism
Introduction
• The cell extracts energy and applies energy to perform work
• Some organisms even convert energy to light, as in bioluminescence
• Energy is the capacity to cause specific physical change and chemical change
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Thermodynamic spontaneity can be measure by changes in either entropy or free energy

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Entropy
o A change in entropy , ΔS, represents a change in the degree of randomness or disorder of
the components of the system
o The value of ΔSuniverse is positive for every spontaneous process or reaction
o While the change in entropy of the universe is a valid measure of the spontaneity of a process,
the change in entropy of the system is not
Free-Energy Change , ΔG
o A living system's free energy is energy that can do work when the temperature and pressure are
uniform
ΔG = ΔH - TΔS
o Enthalpy is the amount of heat content used or released in a system at constant pressure
▪ ΔH = Hproducts - Hreactants
• Negative ΔH (reaction is exothermic)
• Positive ΔH (reaction is endothermic)
o Only processes with a negative ΔG are spontaneous
o Spontaneous processes can be harassed to perform work

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Free Energy, Stability, and Equilibrium
o Free Energy is a measure of a system's instability, its tendency to change to a more stable state
o During a spontaneous change, free energy decreases and the stability of a system increases
o Equilibrium is a state of maximum

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Exergonic and Endergonic Reactions in Metabolism
o An exergonic reaction proceeds with a net release of free energy and its spontaneous (energyyielding)
o An endergonic reaction absorbs free energy from its surroundings and its nonspontaneous
(energy-requiring)

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Equilibrium and Metabolism
o Reactions in a closed system eventually reach equilibrium and then do not work
o Cells are not in equilibrium; they are open systems experience a constant flow of materials
o A defining feature of life is that metabolism is never at equilibrium

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A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction
An enzyme is a catalytic protein
Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reaction

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The Activation Energy Barrier
o Every chemical reaction between molecules involves bond breaking and bond forming
o The initial energy needed to start a chemical reaction is called the free energy of activation,
or activation energy
o Activation energy is often supplied in the form of thermal energy that the reactant
molecules absorb from their surroundings
How Enzymes Lower the Activation Energy Barrier
o Enzymes catalyze reactions by lowering the activation energy barrier
o Enzymes do not affect the change in free energy (ΔG); instead they hasten reaction would
eventually occur

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Substrate Specificity of Enzymes
o The reactant that an enzyme acts on is called the enzyme's substrate
o The enzyme binds to its substrate, forming an enzyme-substrate complex
o The active site is the region on the enzyme where the substrate binds
o Induced fit of a substrate brings chemical groups of the active site into positions that
enhance their ability to catalyze the reaction

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Catalysis in the Enzyme's Active Site
o In an enzymatic reaction, the substrate binds to the active site of the enzyme
o The active site can lower an activation energy barrier by:

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Orienting substrates correctly
Straining substrate bonds
Providing a favorable microenvironment
Covalently bonding to the substrate

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Effects of Local Conditions on Enzyme Activity
o An enzyme's activity can be affected by:
▪ General environmental factors, such as temperature and pH
▪ Chemicals that specifically influence the enzyme

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Effects of Temperature and pH
o Each enzyme has an optimal temperature in which it can function
o Each enzyme has an optimal pH in with it can function
o Optimal conditions favor the most active shape for the enzyme molecule

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Cofactors
o Cofactors or prosthetic groups are protein enzyme helpers
o Cofactors may be inorganic (such as metals) or organic
o An organic cofactor is a coenzyme
▪ Coenzymes include vitamins

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Enzyme Inhibitors
o Competitive inhibitors bind to the active site of an enzyme, competing with the substrate
o Noncompetitive inhibitors bind to another part of an enzyme, causing the enzyme to
change shape and making the active site less effective
o Inhibitors: toxins, poisons, pesticides, and antibiotics

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The Evolution of Enzymes
o Enzymes are proteins encoded by genes
o Changes (mutations) in genes lead to changes in amino acid composition of an enzyme
o Altered amino acids in enzymes may alter their substrate specificity
o Under new environment conditions a novel form of an enzyme might be favored



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