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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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