1 Energy and Metabolism Chapter 6. 2 Flow of Energy Thermodynamics – Branch of chemistry concerned...

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Energy and Metabolism

Chapter 6

2

Flow of Energy

• Thermodynamics– Branch of chemistry concerned with energy

changes• Cells are governed by the laws of physics and

chemistry

• Energy – capacity to do work– 2 states

1. Kinetic – energy of motion2. Potential – stored energy

– Many forms – mechanical, heat, sound, electric current, light, or radioactivity

– Heat the most convenient way of measuring energy• 1 calorie = heat required to raise 1 gram of water 1ºC

• calorie or Calorie?

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a. Potential energy b. Kinetic energy

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

• Energy flows into the biological world from the sun

• Photosynthetic organisms capture this energy• Stored as potential energy in chemical bonds

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Redox reactions

• Oxidation– Atom or molecule loses an electron

• Reduction– Atom or molecule gains an electron– Higher level of energy than oxidized form

• Oxidation-reduction reactions (redox)– Reactions always paired

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e–

A BA + B +A+ B–

Loss of electron (oxidation)

Gain of electron (reduction)

Lower energy Higher energy

Laws of thermodynamics

• First law of thermodynamics– Energy cannot be created or destroyed– Energy can only change from one form to another– Total amount of energy in the universe remains

constant– During each conversion, some energy is lost as

heat

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• Second law of thermodynamics– Entropy (disorder) is continuously increasing– Energy transformations proceed spontaneously to

convert matter from a more ordered/less stable form to a less ordered/ more stable form


Disorder happensspontaneously

Organizationrequires energy

© Jill Braaten

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Free energy

• G = Energy available to do work• G = H – TS

H = enthalpy, energy in a molecule’s chemical bonds

T = absolute temperature S = entropy, unavailable energy

ΔG = ΔH – TS• ΔG = change in free energy• Positive ΔG

– Products have more free energy than reactants– H is higher or S is lower– Not spontaneous, requires input of energy– Endergonic

• Negative ΔG – Products have less free energy than reactants– H is lower or S is higher or both– Spontaneous (may not be instantaneous)– Exergonic

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a.

b.

0

0

Course of Reaction

Products

G > 0

G < 0

Reactants

Reactants

Course of Reaction

Products

Free

Ene

rgy

(G)

Ener

gy R

elea

sed

Ener

gy S

uppl

ied

Free

Ene

rgy

(G)

Ener

gy R

elea

sed

Ener

gy S

uppl

ied

Energy isreleased

Energy mustbe supplied

Exergonic

Endergonic

Activation energy

• Extra energy required to destabilize existing bonds and initiate a chemical reaction

• Exergonic reaction’s rate depends on the activation energy required– Larger activation energy proceeds more slowly

• Rate can be increased 2 ways1. Increasing energy of reacting molecules (heating)2. Lowering activation energy

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ΔG

Ener

gy R

elea

sed

Ener

gy S

uppl

ied

Free

Ene

rgy

(G) Activation

energy

Activationenergy0

uncatalyzedcatalyzed

Course of Reaction

Product

Reactant

Catalysts

• Substances that influence chemical bonds in a way that lowers activation energy

• Cannot violate laws of thermodynamics– Cannot make an endergonic reaction spontaneous

• Do not alter the proportion of reactant turned into product

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ΔG

Ener

gy R

elea

sed

Ener

gy S

uppl

ied

Free

Ene

rgy

(G) Activation

energy

Activationenergy0

uncatalyzedcatalyzed

Course of Reaction

Product

Reactant

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ATP

• Adenosine triphosphate• Chief “currency” all cells use• Composed of– Ribose – 5 carbon sugar– Adenine– Chain of 3 phosphates• Key to energy storage• Bonds are unstable• ADP – 2 phosphates• AMP – 1 phosphate – lowest energy form

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AMP

CORE

O

O–

O

O

O

HH H

H

O

CC

NN

N

C

N

C

CHH

P O–

O P

O P O

ADP

ATP

Triphosphategroup

O–

CH2

High-energybonds

a.

Adenine NH2

Ribose

OHOH

b.

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ATP cycle

• ATP hydrolysis drives endergonic reactions– Coupled reaction results in net –ΔG (exergonic and

spontaneous)• ATP not suitable for long-term energy storage– Fats and carbohydrates better– Cells store only a few seconds worth of ATP

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+

+

Pi

Energy fromexergoniccellularreactions

ATP H2O

ADP

Energy forendergoniccellularprocesses

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Enzymes: Biological Catalysts• Most enzymes are protein– Some are RNA

• Shape of enzyme stabilizes a temporary association between substrates

• Enzyme not changed or consumed in reaction• Carbonic anhydrase– 200 molecules of carbonic acid per hour made without

enzyme– 600,000 molecules formed per second with enzyme

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Active site

a. b.Enzyme Enzyme–substrate complex

Substrate

Active site

• Pockets or clefts for substrate binding• Forms enzyme–substrate complex• Precise fit of substrate into active site• Applies stress to distort particular bond to

lower activation energy– Induced fit

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1. The substrate, sucrose, consists of glucose and fructose bonded together.

2. The substrate binds to the active site of the enzyme, forming an enzyme– substrate complex.

3. The binding of the substrate and enzyme places stress on the glucose– fructose bond, and the bond breaks.

4. Products arereleased, andthe enzyme isfree to bind othersubstrates.

BondGlucose

Fructose

Active site

Enzymesucrase

H2O

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• Enzymes may be suspended in the cytoplasm or attached to cell membranes and organelles

• Multienzyme complexes – subunits work together to form molecular machine– Product can be delivered easily to next enzyme– Unwanted side reactions prevented– All reactions can be controlled as a unit

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Nonprotein enzymes

• Ribozymes• 1981 discovery that certain reactions

catalyzed in cells by RNA molecule itself1. 2 kinds

1. Intramolecular catalysis – catalyze reaction on RNA molecule itself

2. Intermolecular catalysis – RNA acts on another molecule

Enzyme function

• Rate of enzyme-catalyzed reaction depends on concentrations of substrate and enzyme

• Any chemical or physical condition that affects the enzyme’s three-dimensional shape can change rate– Optimum temperature– Optimum pH

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a.

b.

Rate

of R

eacti

on

Optimum pH for pepsin

Rate

of R

eacti

on

pH of Reaction

Optimum pH for trypsin

1 2 3 4 5 6 7 8 9

30 40 50 60 70 80

Temperature of Reaction (˚C)

Optimum temperaturefor human enzyme

Optimum temperature for enzymefrom hotsprings prokaryote

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Inhibitors

• Inhibitor – substance that binds to enzyme and decreases its activity

• Competitive inhibitor– Competes with substrate for active site

• Noncompetitive inhibitor– Binds to enzyme at a site other than active site– Causes shape change that makes enzyme unable

to bind substrate

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a. Competitive inhibition b. Noncompetitive inhibition

Enzyme Enzyme

Allosteric site

Activesite

Competitive inhibitor interfereswith active site of enzyme sosubstrate cannot bind

Allosteric inhibitor changesshape of enzyme so it cannotbind to substrate

Activesite


Substrate Substrate

Inhibitor Inhibitor

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Allosteric Enzymes

• Allosteric enzymes – enzymes exist in active and inactive forms

• Most noncompetitive inhibitors bind to allosteric site – chemical on/off switch

• Allosteric inhibitor – binds to allosteric site and reduces enzyme activity

• Allosteric activator – binds to allosteric site and increases enzyme activity

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Metabolism

• Total of all chemical reactions carried out by an organism

• Anabolic reactions/anabolism– Expend energy to build up molecules

• Catabolic reactions/catabolism– Harvest energy by breaking down molecules

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Biochemical pathways

• Reactions occur in a sequence• Product of one reaction is the substrate for

the next• Many steps take place in organelles

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Initial substrate

Intermediatesubstrate A

Intermediatesubstrate B

Intermediatesubstrate C

End product

Enzyme1

Enzyme2

Enzyme3

Enzyme4

Feedback inhibition

• End-product of pathway binds to an allosteric site on enzyme that catalyzes first reaction in pathway

• Shuts down pathway so raw materials and energy are not wasted

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a. b.

Enzyme 1

Enzyme 2

Enzyme 3

Enzyme 1

Enzyme 2

Enzyme 3End product End product

Initialsubstrate

Intermediatesubstrate A

Intermediatesubstrate B

Initialsubstrate

1 Energy and Metabolism Chapter 6. 2 Flow of Energy Thermodynamics – Branch of chemistry concerned...

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Transcript of 1 Energy and Metabolism Chapter 6. 2 Flow of Energy Thermodynamics – Branch of chemistry concerned...