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Energy and Metabolism
Chapter 8
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Outline
• Flow of Energy in Living Things• Laws of Thermodynamics• Free Energy• Activation Energy• Enzymes
– Forms– Activity
• ATP• Biochemical Pathways
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Flow of Energy in Living Things
• Energy - the capacity to do work– kinetic - energy of motion– potential - stored energy
• Thermodynamics - changes in heat– calorie - heat required to raise the
temperature of one gram of water one degree Celsius
kilocalorie = 1000 calories
Potential energy Kinetic energy
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Flow of Energy in Living Things
• Oxidation - Reduction– Oxidation occurs when an atom or molecule loses an
electron.– Reduction occurs when an atom or molecule gains an
electron. Redox reactions occur because every electron that is
lost by an atom through oxidation is gained by some other atom through reduction.
Gain of electron (reduction)
Low energy
e–
A B
High energy
Loss of electron (oxidation)
A
o o
B
+ –
A* B*
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Laws of Thermodynamics
• First Law of Thermodynamics– Energy cannot be created or destroyed,
but only change form. During each conversion, some of the
energy dissipates into the environment as heat.
Heat is defined as the measure of the random motion of molecules.
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Laws of Thermodynamics
• Second Law of Thermodynamics
– The disorder (entropy) in the universe is continuously increasing.
Energy transformations proceed spontaneously to convert matter from a more ordered, less stable form, to a less ordered, more stable form.
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Free Energy
• Free energy refers to the amount of energy actually available to break and subsequently form other chemical bonds.
– Gibbs’ free energy (G) change in free energy endergonic - any reaction that
requires an input of energy exergonic - any reaction that
releases free energy
Reactant
Product
Energymust besupplied. E
ner
gy
sup
plie
dE
ner
gy
rele
ased
Reactant
Product
Energy isreleased.
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Activation Energy
• Activation energy refers to the extra energy required to destabilize existing chemical bonds and initiate a chemical reaction.
– catalyst - substance that lowers the activation energy
cannot violate laws of thermodynamics.direction of a chemical reaction is
determined solely by the difference in free energy between the reactants and the products
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Activation Energy and Catalysis
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Enzymes Are Biological Catalysts
• Enzymes are proteins that carry out most catalysis in living organisms.
– Unique three-dimensional shape enables an enzyme to stabilize a temporary association between substrates.
Because the enzyme itself is not changed or consumed in the reaction, only a small amount is needed, and can then be reused.
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Enzymes
• Most enzymes are globular proteins with one or more active sites.
– Substrates bind to the enzyme at these active sites, forming an enzyme-substrate complex.
The substrate,sucrose, consistsof glucose andfructose bondedtogether.
1The substratebinds to theenzyme, formingan enzyme-substratecomplex.
2
The binding ofthe substrateand enzymeplaces stress onthe glucose-fructose bond,and the bondbreaks.
3
Products arereleased, andthe enzyme isfree to bindothersubstrates.
4Bond
Enzyme
Active site
H2O
Glucose Fructose
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Enzyme Forms
• A multienzyme complex is composed of several enzymes, catalyzing the different steps of a sequence of reactions, that are associated with one another.
– subunits work in concert, providing significant advantages in catalytic efficiency
• RNA catalysts “ribozymes”
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Factors Affecting Enzyme Activity
• Temperature– Rate of an enzyme-
catalyzed reaction increases with temperature, but only up to an optimum temperature.
• pH– Ionic interactions also
hold enzymes together.
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Optimumtemperature forhuman enzyme
Optimum temperature for enzyme fromhotsprings prokaryote
Optimum pHfor pepsin
Temperature of reaction (°C)
pH of reaction
Optimum pHfor trypsin
40 50 60 70 80
1 2 3 4 5 6 7 8 9Ra
te o
f re
ac
tio
nR
ate
of
rea
cti
on
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Factors Affecting Enzyme Activity
• Inhibitors and activators– inhibitor - substance that binds to an enzyme and
decreases its activity competitive inhibitors - compete with the substrate for
the same active site noncompetitive inhibitors - bind to the enzyme in a
location other than the active site allosteric sites - specific binding sites acting as
on/off switches
Competitive inhibition Noncompetitive inhibition
Competitiveinhibitorinterfereswith activesite ofenzyme sosubstratecannot bind
Allosteric inhibitorchanges shape ofenzyme so it cannotbind to substrate
Enzyme
Substrate
Enzyme
Substrate
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Factors Affecting Enzyme Activity
– activator - substances that bind to allosteric sites and keep the enzymes in their active configurations
increase enzyme activitycofactors - chemical components that
facilitate enzyme activity coenzyme
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ATP
• Adenosine triphosphate (ATP) is the chief energy currency of the cell.
– Each molecule is a nucleotide composed of ribose, adenine, and a triphosphate group.
energy stored in the triphosphate group
cell uses ATP to drive endergonic reactions
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Biochemical Pathways
• Biochemical pathways are the organizational units of metabolism.
– Metabolism is the total of all chemical reactions carried out by an organism.
anabolism - reactions that expend energy
catabolism - reactions that harvest energy
Product
Enzyme 1
Enzyme 3
Enzyme 4
Substrate
Enzyme 2
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Biochemical Pathway
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Regulation of Biochemical Pathways
• Biochemical pathways must be coordinated and regulated to operate efficiently.
– advantageous for cell to temporarily shut down biochemical pathways when their products are not needed
feedback inhibition - When the cell produces increasing quantities of a particular product, it automatically inhibits its ability to produce more.
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Feedback Inhibition
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Feedback Inhibition
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Summary
• Flow of Energy in Living Things• Laws of Thermodynamics• Free Energy• Activation Energy• Enzymes
– Forms– Activity
• ATP• Biochemical Pathways
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