Ch06 lecture

Copyright © 2009 Pearson Education, Inc..

Lectures by

Gregory AhearnUniversity of North Florida

Chapter 6

Energy Flow in the Life of a Cell

Copyright © 2009 Pearson Education Inc.

5.1 What Is Energy?

Energy is the capacity to do work.• Synthesizing molecules• Moving objects• Generating heat and light


5.1 What Is Energy?

Types of energy• Kinetic: energy of movement• Potential: stored energy

Fig. 5-1


5.1 What Is Energy?

First Law of Thermodynamics• “Energy cannot be created nor destroyed, but

it can change its form.”• Example: potential energy in gasoline can be

converted to kinetic energy in a car, but the energy is not lost


5.1 What Is Energy?

Second Law of Thermodynamics• “When energy is converted from one form to

another, the amount of useful energy decreases.”

• No process is 100% efficient.• Example: more potential energy is in the

gasoline than is transferred to the kinetic energy of the car moving

• Where is the rest of the energy? It is released in a less useful form as heat—the total energy is maintained.


5.1 What Is Energy?

Matter tends to become less organized.• There is a continual decrease in useful

energy, and a build up of heat and other non-useful forms of energy.

• Entropy: the spontaneous reduction in ordered forms of energy, and an increase in randomness and disorder as reactions proceed

• Example: gasoline is made up of an eight-carbon molecule that is highly ordered

• When broken down to single carbons in CO2, it is less ordered and more random.


5.1 What Is Energy?

In order to keep useful energy flowing in ecosystems where the plants and animals produce more random forms of energy, new energy must be brought in.


5.1 What Is Energy?

Sunlight provides an unending supply of new energy to power all plant and animal reactions, leading to increased entropy.

Fig. 5-2


5.2 How Does Energy Flow In Chemical Reactions? Chemical reaction: the conversion of one

set of chemical substances (reactants) into another (products)• Exergonic reaction: a reaction that releases

energy; the products contain less energy than the reactants


energyreleased

reactants

products

Exergonic reaction

+

+

(a)

5.2 How Does Energy Flow In Chemical Reactions? Exergonic reaction

Fig. 5-3a


5.2 How Does Energy Flow In Chemical Reactions? Endergonic reaction: a reaction that requires

energy input from an outside source; the products contain more energy than the reactants


energyused

products

reactants

Endergonic reaction

+

+

(b)

5.2 How Does Energy Flow In Chemical Reactions? Endergonic reaction

Fig. 5-3b


5.2 How Does Energy Flow In Chemical Reactions? Exergonic reactions release energy.

• Example: sugar burned by a flame in the presence of oxygen produces carbon dioxide (CO2) and water

• Sugar and oxygen contain more energy than the molecules of CO2 and water.

• The extra energy is released as heat.


5.2 How Does Energy Flow In Chemical Reactions? Burning glucose releases energy.

Fig. 5-4

energyreleased

C6H12O6 6 O2

(glucose) (oxygen)

+

6 CO2

(carbondioxide)

6 H2O

(water)

+


5.2 How Does Energy Flow In Chemical Reactions? Endergonic reactions require an input of

energy.• Example: sunlight energy + CO2 + water in

photosynthesis produces sugar and oxygen• The sugar contains far more energy than the

CO2 and water used to form it.


5.2 How Does Energy Flow In Chemical Reactions? Photosynthesis requires energy.

Fig. 5-5

C6H12O6 6 O2

(glucose) (oxygen)

+

6 CO2

(carbondioxide)

6 H2O

(water)

+

energy


high

low

progress of reaction progress of reaction

energycontent

ofmolecules

Activation energy neededto ignite glucose

Energy level of reactants

glucose + O2

CO2 + H2O CO2 + H2O

glucose

Activationenergycapturedfromsunlight

Energy level of reactants

Burning glucose (sugar): an exergonic reaction Photosynthesis: an endergonic reaction(a) (b)

5.2 How Does Energy Flow In Chemical Reactions? All reactions require an initial input of energy.

• The initial energy input to a chemical reaction is called the activation energy.

Fig. 5-6


5.2 How Does Energy Flow In Chemical Reactions? The source of activation energy is the

kinetic energy of movement when molecules collide.

Molecular collisions force electron shells of atoms to mingle and interact, resulting in chemical reactions.


5.2 How Does Energy Flow in Chemical Reactions? Exergonic reactions may be linked with

endergonic reactions.• Endergonic reactions obtain energy from

energy-releasing exergonic reactions in coupled reactions.

• Example: the exergonic reaction of burning gasoline in a car provides the endergonic reaction of moving the car

• Example: exergonic reactions in the sun release light energy used to drive endergonic sugar-making reactions in plants


5.3 How Is Energy Carried Between Coupled Reactions? The job of transferring energy from one

place in a cell to another is done by energy-carrier molecules.• ATP (adenosine triphosphate) is the main

energy carrier molecule in cells, and provides energy for many endergonic reactions.


ADP

ATP

phosphate

energy

+

A P P P

A P P P

5.3 How Is Energy Carried Between Coupled Reactions? ATP is made from ADP (adenosine

diphosphate) and phosphate plus energy released from an exergonic reaction (e.g., glucose breakdown) in a cell.

Fig. 5-7


5.3 How Is Energy Carried Between Coupled Reactions? ATP is the principal energy carrier in cells.

• ATP stores energy in its phosphate bonds and carries the energy to various sites in the cell where energy-requiring reactions occur.

• ATP’s phosphate bonds then break yielding ADP, phosphate, and energy.

• This energy is then transferred to the energy-requiring reaction.


phosphateADP

energy

ATP+

A

A P P P

P P P

5.3 How Is Energy Carried Between Coupled Reactions? Breakdown of ATP releases energy.

Fig. 5-8


5.3 How Is Energy Carried Between Coupled Reactions? To summarize:

• Exergonic reactions (e.g., glucose breakdown) drive endergonic reactions (e.g., the conversion of ADP to ATP).

• ATP moves to different parts of the cell and is broken down exergonically to liberate its energy to drive endergonic reactions.


5.3 How Is Energy Carried Between Coupled Reactions? Coupled reactions

Fig. 5-9

endergonic(ATP synthesis)

exergonic(ATP breakdown)

exergonic(glucose breakdown)

endergonic(protein synthesis)CO2 + H2O + heat

glucose

aminoacids

protein

+ PP P

P PPA

A


5.3 How Is Energy Carried Between Coupled Reactions? A biological example of coupled reactions

• Muscle contraction (an endergonic reaction) is powered by the exergonic breakdown of ATP.

• During energy transfer in this coupled reaction, heat is given off, with overall loss of usable energy.


5.3 How Is Energy Carried Between Coupled Reactions? ATP breakdown is coupled with muscle

contraction.

Fig. 5-10

Energy released from ATPbreakdown exceeds theenergy used for musclecontraction, so the overallcoupled reaction is exergonic

++100 unitsenergyreleased

+

+

+ +80 unitsenergy releasedas heat

20 unitsenergy

+

contractedmuscle

contractedmuscle

relaxedmuscle

relaxedmuscle

Exergonic reaction:

Endergonic reaction:

Coupled reaction:

PADPATP

ADP PATP


5.3 How Is Energy Carried Between Coupled Reactions?

Animation—Energy and Chemical ReactionsPLAYPLAY


5.3 How Is Energy Carried Between Coupled Reactions? Electron carriers also transport energy

within cells.• Besides ATP, other carrier molecules

transport energy within a cell.• Electron carriers capture energetic electrons

transferred by some exergonic reaction.• Energized electron carriers then donate these

energy-containing electrons to endergonic reactions.


low-energyproducts

energized

depleted

low-energyreactants

high-energyreactants

NADH

high-energyproducts

e–

e–

NAD+ + H+

5.3 How Is Energy Carried Between Coupled Reactions? Common electron carriers are NAD+ and

FAD.

Fig. 5-11


5.3 How Is Energy Carried Between Coupled Reactions?

Animation—Energy and LifePLAYPLAY


PATHWAY 1

Initial reactant Intermediates Final products

enzyme 1 enzyme 2 enzyme 3 enzyme 4

PATHWAY 2

enzyme 5 enzyme 6

A B D E

F

C

G

5.4 How Do Cells Control Their Metabolic Reactions? Cell metabolism: the multitude of chemical

reactions going on at any specific time in a cell

Metabolic pathways: the sequence of cellular reactions (e.g., photosynthesis and glycolysis)

Fig. 5-12


5.4 How Do Cells Control Their Metabolic Reactions? At body temperature, many spontaneous

reactions proceed too slowly to sustain life.• A reaction can be controlled by controlling its

activation energy (the energy needed to start the reaction).

• At body temperature, reactions occur too slowly because their activation energies are too high.

• Molecules called catalysts are able to gain access to energy that is not produced spontaneously.


progress of reaction

low

high

energycontent

ofmolecules

Activation energywith catalyst

Activation energywithout catalyst

reactants

products

5.4 How Do Cells Control Their Metabolic Reactions? Catalysts reduce activation energy.

• Catalysts are molecules that speed up a reaction without being used up or permanently altered.

• They speed up the reaction by reducing the activation energy.

Fig. 5-13


5.4 How Do Cells Control Their Metabolic Reactions? Three important principles about all

catalysts• Catalysts speed up a reaction.• They speed up reactions that would occur

anyway, if their activation energy could be surmounted.

• Catalysts are not altered by the reaction.


5.4 How Do Cells Control Their Metabolic Reactions? Enzymes are biological catalysts.

• Almost all enzymes are proteins.• Enzymes are highly specialized, generally

catalyzing only a single reaction.• In metabolic pathways involving multiple

reactions, each reaction is catalyzed by a different enzyme.


5.4 How Do Cells Control Their Metabolic Reactions? The structure of enzymes allows them to

catalyze specific reactions.• Enzymes have an active site where the

reactant molecules, called substrates, enter and undergo a chemical change as a result.

• The specificity of an enzyme reaction is due to the distinctive shape of the active site, which only allows proper substrate molecules to enter.


5.4 How Do Cells Control Their Metabolic Reactions? How does an enzyme catalyze a reaction?

• Both substrates enter the enzyme’s active site.

• Substrates enter an enzyme’s active site, changing both of their shapes.

• The chemical bonds are altered in the substrates, promoting the reaction.

• The substrates change into a new form that will not fit the active site, and so are released.


5.4 How Do Cells Control Their Metabolic Reactions? The cycle of enzyme–substrate interactions

Fig. 5-14

substrates

active siteof enzyme

enzyme

Substrates enterthe active site in aspecific orientation

1

The substrates, bondedtogether, leave the enzyme;the enzyme is ready for anew set of substrates

3 The substrates andactive site change shape,promoting a reactionbetween the substrates

2


5.4 How Do Cells Control Their Metabolic Reactions?

Animation—EnzymesPLAYPLAY


5.4 How Do Cells Control Their Metabolic Reactions? Cells regulate metabolism by controlling

enzymes.• Allosteric regulation can increase or decrease

enzyme activity.• In allosteric regulation, an enzyme’s activity

is modified by a regulator molecule.• The regulator molecule binds to a special

regulatory site on the enzyme separate from the enzyme’s active site.


5.4 How Do Cells Control Their Metabolic Reactions? Binding of the regulator molecule modifies

the active site on the enzyme, causing the enzyme to become more or less able to bind substrate.

Thus, allosteric regulation can either promote or inhibit enzyme activity.


active site

substrate

enzyme

Enzyme structure

Many enzymes haveboth active sites andallosteric regulatorysites

allostericregulatory site

(a)

5.4 How Do Cells Control Their Metabolic Reactions? Enzyme structure

Fig. 5-15a


allostericregulatormolecule

Allosteric inhibition

An allosteric regulatormolecule causes theactive site to changeshape, so the substrateno longer fits

(b)

5.4 How Do Cells Control Their Metabolic Reactions? Allosteric inhibition

Fig. 5-15b


5.4 How Do Cells Control Their Metabolic Reactions? Competitive inhibition can be temporary or

permanent. Some regulatory molecules temporarily bind

directly to an enzyme’s active site, preventing the substrate molecules from binding.

These molecules compete with the substrate for access to the active site, and control the enzyme by competitive inhibition.


A competitive inhibitor moleculeoccupies the active site andblocks entry of the substrate

5.4 How Do Cells Control Their Metabolic Reactions? Competitive inhibition

Fig. 5-16

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