Microfilaments differ from microtubules in that microfilaments

A) are larger than microtubules. B) are found only in plants whereas microtubules are

found in plants and animal cells. C) are mainly composed of actin whereas

microtubules are composed of tubulin. D) anchor organelles, whereas microtubules primarily

function to help cells change shape and move. E) form the inner core of cilia and flagella whereas

microtubules regulate metabolism.

Cellular Energetics: Thermodynamics, ATP, and Enzyme

catalysisCampbell Biology

Chapter 5

Energy and Thermodynamics

Energy is the capacity to do work

• There are many forms of energy:– Kinetic energy– Potential energy– Chemical energy– Electrical energy

All Living Things Require and Consume Energy

• We get our energy from food

• Ultimate source of energy for all life on earth is the sun

The First Law of Thermodynamics

• Energy cannot be created or destroyed

• The amount of energy in the universe is constant

• Energy can be interconverted from one form to another:– Potential energy– Kinetic energy– Radiant energy

Potential energy

• Energy is the ability to do work

• Potential Energy of position

• Gravitational potential energy

• Chemical potential energy

Kinetic energy

• Energy of motion• KE= 1/2mv2

• Temperature is a measure of molecular kinetic energy

The 1st Law of Thermodynamics: Energy can be interconverted from one

form to another

More energy interconversions

The 2nd Law of Thermodynamics : The Law of Entropy

• Interconversions of energy are never 100% efficient

• Entropy!• Entropy is a measure of

disorder (i.e. chaos, randomness)

• Each interconversion of energy involves loss of usable energy

Entropy in Action

Biochemical reactions are inefficient

The price of minimizing entropy is the constant expenditure of free energy

• Given a finite amount of energy, each energy interconversion will result in an ever-increasing amount of unusable energy (entropy)

Closed systems will deplete themselves of usable (free) energy

Recognizing Entropy in the world

Which system has more entropy?

A B

Can living systems reduce entropy?

Recognizing Enthalpy

Enthalpy = Energy in chemical bondsB

Which systems have more Enthalpy?

These?

Or these?

Biochemical reactions are spontaneous only if ∆G is negative

• Reactions which release energy are exergonic

• Reactions which require energy are endergonic

∆ G = ∆H - T∆S• Only exergonic processes

with a negative ∆G are spontaneous

• Spontaneous processes can be harnessed to perform work

C6H12O6(s) + 6O2(g) 6CO2(g)+ 6H2O(l)

Will the Reaction happen?Well, is heat given off?Does entropy increase?

G = ∆H - T∆S∆H = enthalpy (heat in chemical bonds)

∆S= Degree of entropy (chaos) created by RxnT= Temperature at which Rxn occurs

If ΔG < 0, the reaction is spontaneous (it will happen)

Important: Spontanous ≠ fast

LE 5-2b

Heat

Glucose

Oxygen

Chemical reactions

ATP ATP

Energy for cellular work

Carbon dioxide

Water

Which of these diagrams depicts an endergonic reaction?

Energy released

Reactants

Products

Amount ofenergy

required

Po

ten

tial

en

erg

y o

f m

ole

cu

les

Po

ten

tial

en

erg

y o

f m

ole

cu

les

Reactants

Products

Amount ofenergy

releasedEnergy required

A B

LE 8-7a

G = 0

A closed hydroelectric system

G < 0

LE 8-7c

A multistep open hydroelectric system

G < 0G < 0

G < 0

In living things, a state of equilibrium most often means ___________.

A) Efficiency is optimizedB) The reaction is EndothermicC) Enthalpy is increasedD) Entropy is minimizedE) You are dead

ATP

A steer must eat over 100 pounds of grain to gain less than 10 pounds of

muscle tissue. This illustrates • A) the first law of thermodynamics. B) the second law of thermodynamics. C) that some energy is destroyed in every

energy conversion. D) that energy transformations are typically

100% efficient. E) None of the choices are correct.

Living cells manage to perform endergonic activities

• How is this possible?

ATP hydrolysis can be coupled to endergonic reactions to power cellular work

• A cell does three main kinds of work:– Mechanical– Transport– Chemical

• To do work, cells manage energy resources by energy coupling, the use of an exergonic process to drive an endergonic one

The Structure and Hydrolysis of ATP• ATP (adenosine triphosphate) is the cell’s energy shuttle• ATP provides energy for cellular functions• ATP is a nucleic acid monomer

ATP is the energy currency of all living things

Phosphate groups

Ribose

Adenine

ATP: Adenosine Triphosphate

LE 8-9

Adenosine triphosphate (ATP)

Energy

P P P

PPP i

Adenosine diphosphate (ADP)Inorganic phosphate

H2O

+ +

Phosphorylation can change the conformation of proteins

LE 8-10

Anabolic (building up) reactions are usually endergonic

Breakdown of ATP is exergonic

G = +3.4 kcal/mol

G = –7.3 kcal/mol

G = –3.9 kcal/mol

NH2

NH3Glu Glu

Glutamicacid

Coupled reactions: Overall G is negative;together, reactions are spontaneous

Ammonia Glutamine

ATP H2O ADP P i

+

+ +

How ATP Performs Work• ATP drives endergonic reactions by phosphorylation,

transferring a phosphate group to some other molecule, such as a reactant

• The recipient molecule is now phosphorylated

•Mechanical

•Transport

•Chemical

Three types of cellular work are powered by ATP

hydrolysis

The Regeneration of ATP• ATP is regenerated by addition of a phosphate group

to ADP• The energy to phosphorylate ADP comes from food• The chemical potential energy temporarily stored in

ATP drives most cellular work

LE 8-12

Pi

ADP

Energy for cellular work(endergonic, energy-consuming processes)

Energy from catabolism(exergonic, energy-yielding processes)

ATP

+

Enzymes

At which level of protein structure are interactions between R groups

most important?A) primaryB) secondaryC) tertiaryD) quaternaryE) the R groups are not related to the overall

structure of a protein

Sugar is an energy-rich molecule

• Breakdown of sugar is spontaneous

• C6H12O6(s) + 6O2(g) 6CO2(g)+ 6H2O(l)

Wood and paper are made of cellulose

• Cellulose is a polymer of glucose

• Why doesn’t our jar of sugar burst into flame?

Exergonic reactions still require activation energy

• Spontaneous ≠ fast• Ea is dependent on

temperature• At high temperatures,

reactions happen faster

Jumping bean analogy• Molecules are like

jumping beans• Temperature ≈ height of

jump• Living things cannot wait

for a good jump• After a long time, where

will the beans be?• All of them?• Will they ever stop

jumping?

Living things can use enzymes to speed up reactions

• Enzymes speed up reactions by lowering energy of activation

• They are catalysts

Catalysts speed up reactions

• Platinum is used in catalytic converters

• 2CO + 02 2CO2

• Catalysts are not consumed in a reaction

• They cannot add energy to a reaction

Enzymes are protein catalysts

• Catalysts- things added to chemical reactions which speed up those reactions

• Catalysts are not consumed in a reaction

• Catalysts cannot add energy to a reaction

• -ase: The enzyme suffix

Catalase

Enzymes can dramatically lower the energy of activation for a reaction

Reaction Course

Energyreactants

products

E a

E a

no enzyme

with enzyme

12Note that the equilibrium of the reaction is unaffected

How enzymes work Structure aids

function An active site

naturally fits substrate

Enzyme specificity depends on shape

Substrate Binding and Reaction

Some important Enzymes

Cellulase

NitrogenaseATP synthase

Catalysis in the Enzyme’s Active Site• In an enzymatic reaction, the substrate binds to the

active site• The active site can lower an EA barrier by

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

LE 5-6

Enzyme availablewith empty activesite

Active site

Glucose

Fructose

Products arereleased

Enzyme(sucrase)

Substrate(sucrose)

H2O

Substrate isconverted toproducts

Substrate bindsto enzyme withinduced fit

Factors Affecting Enzyme Activity

1. Salts2. Temperature3. pH4. Inhibitors and Activators

Effects of Temperature and pH

• Each enzyme has an optimal temperature in which it can function

• Each enzyme has an optimal pH in which it can function

• Tertiary structure can be radically altered by changes in pH

LE 8-18

Optimal temperature fortypical human enzyme

Optimal temperature forenzyme of thermophilic (heat-tolerant bacteria)

Temperature (°C)

Optimal temperature for two enzymes

0 20 40 60 80 100

Rate

of r

eacti

on

Optimal pH for pepsin(stomach enzyme)

Optimal pHfor trypsin(intestinalenzyme)

pH

Optimal pH for two enzymes

0

Rate

of r

eacti

on

1 2 3 4 5 6 7 8 9 10

Enzyme Inhibition

• Competitive inhibitors bind to the active site of an enzyme, competing with the substrate

• Noncompetitive (allosteric) inhibitors bind to another part of an enzyme, causing the enzyme to change shape (allostery) and making the active site less effective

Many drugs are enzyme inhibitors

• Protease inhibitors fight HIV

Can enzymes catalyze endothermic reactions?

• If so, how?• If not, why not?

Inhibition of an enzyme is irreversible when

A) a competitive inhibitor is involved. B) a noncompetitive inhibitor is involved. C) the shape of the enzyme is changed. D) bonds form between inhibitor and enzyme. E) None of the choices are correct.