Cellular Energetics – Respiration and Photosynthesis

Bioenergetics – our cells’ ability to release the energy in glucose, starch, and fat

We do this by chemical reactions catalyzed by enzymes

Exergonic reactions vs. endergonic reactions Exergonic – nutrients being oxidized in the

mitochondria Endergonic – plants using CO2 and water to

form sugars Activation energy – energy barrier that must

be broken for exergonic rxns to proceed.

Figure 8.6

(a) Exergonic reaction: energy released, spontaneous

(b) Endergonic reaction: energy required, nonspontaneous

Reactants

EnergyProducts

Progress of the reaction

Amount of energy

released(G 0)

ReactantsEnergy

Products

Amount of energy

required(G 0)

Progress of the reaction

Fre

e e

nerg

yFre

e e

nerg

y

Figure 8.8b

Adenosine triphosphate (ATP)

Energy

Inorganicphosphate

Adenosine diphosphate (ADP)

(b) The hydrolysis of ATP

How the Hydrolysis of ATP Performs Work

• The three types of cellular work (mechanical, transport, and chemical) are powered by the hydrolysis of ATP

• In the cell, the energy from the exergonic reaction of ATP hydrolysis can be used to drive an endergonic reaction

• Overall, the coupled reactions are exergonic • Obeys the laws of thermodynamics, but less

messy

© 2011 Pearson Education, Inc.

Figure 8.9

Glutamicacid

Ammonia Glutamine

(b)Conversionreactioncoupledwith ATPhydrolysis

Glutamic acidconversionto glutamine

(a)

(c)Free-energychange forcoupledreaction

Glutamicacid

GlutaminePhosphorylatedintermediate

GluNH3 NH2

Glu GGlu = +3.4 kcal/mol

ATP ADP ADP

NH3

Glu Glu

PP i

P iADP

GluNH2

GGlu = +3.4 kcal/mol

Glu GluNH3 NH2ATP

GATP = 7.3 kcal/molGGlu = +3.4 kcal/mol

+ GATP = 7.3 kcal/mol

Net G = 3.9 kcal/mol

1 2

Enzymes Lower activation energy Specificity Active site binds substrate in lock and

key fit – enzyme/substrate complex Induced fit – when enzyme changes its

shape to accommodate substrate Enzymes are not used up in the reaction Do not work alone – need co-enzymes

like vitamins, NAD, and NADP

Figure 8.13

Course ofreactionwithoutenzyme

EA

withoutenzyme EA with

enzymeis lower

Course ofreactionwith enzyme

Reactants

Products

G is unaffectedby enzyme

Progress of the reaction

Fre

e en

erg

y

Figure 8.14

Substrate

Active site

Enzyme Enzyme-substratecomplex

(a) (b)

Figure 8.15-3

Substrates

Substrates enter active site.

Enzyme-substratecomplex

Enzyme

Products

Substrates are heldin active site by weakinteractions.

Active site canlower EA and speedup a reaction.

Activesite is

availablefor two new

substratemolecules.

Products arereleased.

Substrates areconverted toproducts.

12

3

45

6

Factors affecting reaction rates

1. Temperature Increasing temp. increasing rxn rate Too much heat can damage the

enzyme – denature most human enzymes work at 37

degrees Celsius2. pH3. Enzyme concentration4. Substrate concentration

Figure 8.16

Optimal temperature fortypical human enzyme (37°C)

Optimal temperature forenzyme of thermophilic

(heat-tolerant)bacteria (77°C)

Temperature (°C)(a) Optimal temperature for two enzymes

Rate

of

reacti

on

Rate

of

reacti

on

120100806040200

0 1 2 3 4 5 6 7 8 9 10pH

(b) Optimal pH for two enzymes

Optimal pH for pepsin(stomachenzyme)

Optimal pH for trypsin(intestinal

enzyme)

Enzyme Regulation Allosteric regions on an enzyme can be

bound by inhibitors or activators Allosteric sites are subject to feedback

inhibition – where the product inhibits the rxn.

Competitive inhibition – when the allosteric inhibitor binds the active site of the enzyme

Non-competitive inhibition – when the inhibitor binds another site on the enzyme leading to a conformational change in the active site

Figure 8.17

(a) Normal binding (b) Competitive inhibition (c) Noncompetitive inhibition

Substrate

Activesite

Enzyme

Competitiveinhibitor

Noncompetitiveinhibitor

Figure 8.19

Regulatorysite (oneof four)

(a) Allosteric activators and inhibitors

Allosteric enzymewith four subunits

Active site(one of four)

Active form

Activator

Stabilized active form

Oscillation

Non-functionalactive site

Inactive formInhibitor

Stabilized inactiveform

Inactive form

Substrate

Stabilized activeform

(b) Cooperativity: another type of allosteric activation

Figure 8.21

Active siteavailable

Isoleucineused up bycell

Feedbackinhibition

Active site ofenzyme 1 isno longer ableto catalyze theconversionof threonine tointermediate A;pathway isswitched off. Isoleucine

binds toallostericsite.

Initial substrate(threonine)

Threoninein active site

Enzyme 1(threoninedeaminase)

Intermediate A

Intermediate B

Intermediate C

Intermediate D

Enzyme 2

Enzyme 3

Enzyme 4

Enzyme 5

End product(isoleucine)

Video on Enzymes

• Watch this video

Sources of ATP Photosynthesis Cellular respiration

Figure 9.6-3

Electronscarried

via NADH

Electrons carriedvia NADH and

FADH2

Citricacidcycle

Pyruvateoxidation

Acetyl CoA

Glycolysis

Glucose Pyruvate

Oxidativephosphorylation:electron transport

andchemiosmosis

CYTOSOL MITOCHONDRION

ATP ATP ATP

Substrate-levelphosphorylation

Substrate-levelphosphorylation

Oxidative phosphorylation

Figure 9.8

Energy Investment Phase

Glucose

2 ADP 2 P

4 ADP 4 P

Energy Payoff Phase

2 NAD+ 4 e 4 H+

2 Pyruvate 2 H2O

2 ATP used

4 ATP formed

2 NADH 2 H+

NetGlucose 2 Pyruvate 2 H2O

2 ATP

2 NADH 2 H+ 2 NAD+ 4 e 4 H+

4 ATP formed 2 ATP used

Figure 9.10

Pyruvate

Transport protein

CYTOSOL

MITOCHONDRION

CO2 Coenzyme A

NAD + HNADH Acetyl CoA

1

2

3

Figure 9.12-8

NADH

1

Acetyl CoA

CitrateIsocitrate

-Ketoglutarate

SuccinylCoA

Succinate

Fumarate

Malate

Citricacidcycle

NAD

NADH

NADH

FADH2

ATP

+ H

+ H

+ H

NAD

NAD

H2O

H2O

ADP

GTP GDP

P i

FAD

3

2

4

5

6

7

8

CoA-SH

CO2

CoA-SH

CoA-SH

CO2

Oxaloacetate

Figure 9.15

Proteincomplexof electroncarriers

(carrying electronsfrom food)

Electron transport chain

Oxidative phosphorylation

Chemiosmosis

ATPsynth-ase

I

II

III

IVQ

Cyt c

FADFADH2

NADH ADP P i

NAD

H

2 H + 1/2O2

H

HH

21

H

H2O

ATP

Figure 9.17

2 ADP 2 ATP

Glucose Glycolysis

2 Pyruvate

2 CO22

2 NADH

2 Ethanol 2 Acetaldehyde

(a) Alcohol fermentation (b) Lactic acid fermentation

2 Lactate

2 Pyruvate

2 NADH

Glucose Glycolysis

2 ATP2 ADP 2 Pi

NAD

2 H

2 Pi

2 NAD

2 H

Animations

• Glycolysis• Krebs cycle• Oxidative phosphorylation