1 Cellular Respiration: Harvesting Chemical Energy

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Cellular Respiration: Harvesting Chemical Energy

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Cellular Respiration

• Energy flows into an ecosystem as sunlight and leaves as heat

• Photosynthesis generates oxygen and organic molecules, which are used in cellular respiration

• All cells can harvest energy from organic molecules to power work

• To do this, they break down the organic molecules and use the energy that is released to make ATP from ADP and phosphate

ECOSYSTEM

Lightenergy

Photosynthesisin chloroplasts

Cellular respirationin mitochondria

Organicmolecules+ O2

CO2 + H2O

ATP

powers most cellular work

Heatenergy

3

Catabolic Pathways and Production of ATP• Heterotrophs live off the energy produced by

autotrophs - extracting energy from food via digestion and catabolism

• There are different catabolic pathways used in ATP production:• Fermentation - the partial degradation of sugars in

the absence of oxygen.• Cellular respiration - A more efficient and

widespread catabolic process that consumes oxygen as a reactant to complete the breakdown of a variety of organic molecules.

4

Catabolic Pathways and Production of ATP

• Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose:• The catabolism of glucose is exergonic with a

G of −686 kcal per mole of glucose.• Some of this energy is used to produce ATP,

which can perform cellular work

C6H12O6 + 6O2 6CO2 + 6H2O + Energy (ATP + heat)

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Redox Reactions• Catabolic pathways yield energy through the transfer

electrons from one reactant to another by oxidation and reduction

• Redox reactions• In oxidation - A substance loses electrons, or is oxidized• In reduction - A substance gains electrons, or is reduced

Na + Cl Na+ + Cl–

becomes oxidized(loses electron)

becomes reduced(gains electron)

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Oxidation of Organic Fuel Molecules During Cellular Respiration

• Cellular respiration provides the energy for the cell using the exergonic reaction:

• During cellular respiration glucose is oxidized and oxygen is reduced

• Glucose oxidation is accomplished in a series of steps

C6H12O6 + 6O2 6CO2 + 6H2O + Energy ~686kcal/mole

becomes oxidized

becomes reduced

7

Glucose Oxidation• If electron transfer is not stepwise

• A large release of energy occurs• As in the reaction of hydrogen and oxygen to form water

(a) Uncontrolled reaction

Fre

e en

ergy

, G

H2O

Explosiverelease of

heat and lightenergy

Figure 9.5 A

H2 + 1/2 O2

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• Glucose catabolism is a series of redox reactions that release energy by repositioning electrons closer to oxygen atoms.

• The high energy electrons are stripped from glucose and picked up by NAD+ and FAD.

Glucose Catabolism

NAD+H

O

O

O O–

O

O O–

O

O

O

P

P

CH2

CH2

HO OH

H

HHO OH

HO

H

H

N+

C NH2

HN

H

NH2

N

N

Nicotinamide(oxidized form)

NH2+ 2[H]

(from food)

Dehydrogenase

Reduction of NAD+

Oxidation of NADH

2 e– + 2 H+

2 e– + H+

NADH

OH H

N

C +

Nicotinamide(reduced form)

N

Figure 9.4

H

9

The Electron Transport Chain• Passes electrons in a series of steps instead of in one explosive reaction• Uses the energy from the electron transfer to form ATP• Eventually, the electrons, along with H+, are passed to a final acceptor.

2 H 1/2 O2

(from food via NADH)

2 H+ + 2 e–

2 H+

2 e–

H2O

1/2 O2

Controlled release of energy for synthesis of

ATPATP

ATP

ATP

Electro

n tran

spo

rt chain

Fre

e e

ne

rgy,

G

+

NADH

50

FADH2

40 FMNFe•S

I FADFe•S II

IIIQ

Fe•SCyt b

30

20

Cyt cCyt c1

Cyt aCyt a3

IV

10

0

Multiproteincomplexes

Fre

e en

erg

y (G

) re

lati

ve t

o O

2 (k

cal/m

ol)

H2O

O22 H+ + 1/2

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• If molecular oxygen (O2) is the final electron acceptor, the process is called aerobic respiration.

• If some other inorganic molecule is the final electron acceptor, the process is called anaerobic respiration.

• If an organic molecule is the final electron acceptor, the process is called fermentation.

Glucose Catabolism

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The Stages of Cellular Respiration

• Respiration is a cumulative function of three metabolic stages• Glycolysis - breaks down glucose into two

molecules of pyruvate• The Citric Acid Cycle (Kreb’s) - completes the

breakdown of glucose• Oxidative phosphorylation - driven by the

electron transport chain and Generates ATP

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Cellular Respiration

Electronscarried

via NADH

GlycolsisGlucos

ePyruvate

ATP

Substrate-levelphosphorylation

Electrons carried via NADH and

FADH2

Citric acid cycle

Oxidativephosphorylation:

electron transport and

chemiosmosis

ATPATP

Substrate-levelphosphorylation

Oxidativephosphorylation

MitochondrionCytosol

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Substrate Phosphorylation• Both glycolysis and the citric acid cycle can

generate ATP by substrate-level phosphorylation

Enzyme Enzyme

ATP

ADP

Product

Substrate

P

+

P

PEP

Enzyme ADP

Adenosine

P

P

P ATP

P

P

Adenosine

Pyruvate

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Glycolysis

• Glycolysis harvests energy by oxidizing glucose to pyruvate

• Glycolysis• Means “splitting of sugar”• Breaks down glucose into pyruvate• Occurs in the cytoplasm of the cell

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Glycolysis

• Occurs in the cytoplasm of the cell

• Results in the partial breakdown of glucose

• Anaerobic – no oxygen is used during glycolysis

• For each molecule of glucose that passes through glycolysis, the cell nets two ATP molecules.

Glycolysis Citricacidcycle

Oxidativephosphorylation

ATPATPATP

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Glycolysis

• Energy investment phase

Glucose

ATP

ADP

Hexokinase

ATP ATP ATP

Glycolysis Oxidationphosphorylation

Citricacidcycle

Glucose-6-phosphate

ATP/NADH Ledger

- 1 ATP

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Glycolysis

• Total of 2 ATP invested

Glucose

ATP

ADP

Hexokinase

ATP ATP ATP


Citricacidcycle

Glucose-6-phosphate

Phosphoglucoisomerase

Phosphofructokinase

Fructose-6-phosphate

ATP

ADP

Fructose-1, 6-bisphosphate

Aldolase

Isomerase

Dihydroxyacetonephosphate

Glyceraldehyde-3-phosphate

ATP/NADH Ledger

- 2 ATP

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Glycolysis

• Energy payoff phase

NAD+

Triose phosphatedehydrogenase

+ H+

NADH

1, 3-Bisphosphoglycerate

ADP

ATPPhosphoglycerokinase

Phosphoglyceromutase

2-Phosphoglycerate

3-Phosphoglycerate

NAD+


+ H+

NADH


ADP



2-Phosphoglycerate

3-Phosphoglycerate

ATP/NADH Ledger

- 2 ATP+ 2 ATP+ 2 NADH

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Glycolysis

• End-products of glycolysis are 2 pyruvate molecules

NAD+


+ H+

NADH


ADP



2-Phosphoglycerate

3-Phosphoglycerate

ADP

ATPPyruvate kinase

H2OEnolase

Phosphoenolpyruvate

Pyruvate

NAD+


+ H+

NADH


ADP



2-Phosphoglycerate

3-Phosphoglycerate

ADP

ATPPyruvate kinase

H2OEnolase

Phosphoenolpyruvate

Pyruvate

ATP/NADH Ledger

- 2 ATP+ 4 ATP+ 2 NADH

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• Occurs in the cytoplasm• Glucose converted to two 3-C

chains• Anaerobic - no oxygen• 2 ATP used, 4 ATP produced• Inefficient - net yield only 2

ATPs• Not discarded by evolution but

used as starting point for energy production

• If no O2 - Fermentation occurs

• End products: • 2 ATP• Pyruvate (3 C) • 2 x CO2• 2 x NADH

Glycolysis Summary

Energy investment phase

Glucose

2 ATP used2 ADP + 2 P

4 ADP + 4 P 4 ATP formed

2 NAD+ + 4 e– + 4 H+

Energy payoff phase

+ 2 H+2 NADH

2 Pyruvate + 2 H2O

2 Pyruvate + 2 H2O

2 ATP

2 NADH + 2 H+

Glucose

4 ATP formed – 2 ATP used

2 NAD+ + 4 e– + 4 H+

Net

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The Citric Acid (Krebs) CycleThe Krebs cycle is named after Hans Krebs and is a metabolic event that follows glycolysis. This process occurs in the fluid matrix of the mitochondrion, uses the pyruvic acid from glycolysis and is aerobic. To begin the Krebs cycle, pyruvic acid is converted to acetyl CoA.

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Oxidation of Pyruvate

• More energy can be extracted if oxygen is present• Within mitochondria, pyruvate is decarboxylated,

yielding acetyl-CoA, NADH, and CO2

CYTOSOL

Pyruvate

NAD+

MITOCHONDRION

Transport protein

NADH + H+

Coenzyme ACO2

Acetyl Co A

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The Citric Acid (Krebs) Cycle

• Occurs in the mitochondrial matrix

• Aerobic – although O2 is not used directly in this pathway, it will not occur unless enough is present in the cell.

• Main catabolic pathway• Acetyl-CoA is oxidized in

a series of nine reactions

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• AcetylCoA reacts with oxaloacetate using an enzyme called citrate synthase producing citric acid.

• Because of this, the Krebs cycle is sometimes called the citric acid cycle.

ATP ATP ATP


Citricacidcycle

Citricacidcycle

Citrate

Isocitrate

Oxaloacetate

Acetyl CoA

H2O

Krebs Cycle

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Krebs Cycle• The next 7 steps

decompose the citrate back to oxaloacetate,

• Citric acid is systematically decarboxylated and dehyrogenated in order to use up the acetyl groups that were attached to the oxaloacetate.

• This allows oxaloacetate and CoA to be used in the next cycle.

ATP ATP ATP


Citricacidcycle

Citricacidcycle

Citrate

Isocitrate

Oxaloacetate

Acetyl CoA

H2O

CO2

NAD+

NADH

+ H+

-Ketoglutarate

CO2NAD+

NADH

+ H+SuccinylCoA

Succinate

GTP GDP

ADP

ATP

FAD

FADH2

P i

Fumarate

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Krebs Cycle

• The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain

ATP ATP ATP


Citricacidcycle

Citricacidcycle

Citrate

Isocitrate

Oxaloacetate

Acetyl CoA

H2O

CO2

NAD+

NADH

+ H+

-Ketoglutarate

CO2NAD+

NADH

+ H+SuccinylCoA

Succinate

GTP GDP

ADP

ATP

FAD

FADH2

P i

Fumarate

H2O

Malate

NAD+

NADH

+ H+

ATP/NADH Ledger

+ 2 ATP+ 6 NADH+ 2 FADH2

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Krebs Cycle

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ETC and Oxidative Phosphorylation• Occurs along the inner mitochondrial membrane (IMM) in the

cristae of the mitochondrion• NADH/FADH2 molecules carry electrons from glycolysis and the

citric acid cycle to the inner mitochondrial membrane, where they transfer electrons to a series of membrane-associated proteins.

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The Pathway of Electron Transport

• Most of the chain’s components are proteins, which exist in multiprotein complexes

• The carriers alternate reduced and oxidized states as they accept and donate electrons

• Electrons drop in free energy as they go down the chain and are finally passed to O2, forming water

NADH

50

FADH2

40 FMN

Fe•S

I FAD

Fe•S II

IIIQ

Fe•S

Cyt b

30

20

Cyt c

Cyt c1

Cyt a

Cyt a3

IV

10

0


Fre

e e

ner

gy

(G

) re

lati

ve

to O

2 (

kca

l/m

ol)

H2O

O22 H+ + 1/2

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The Pathway of Electron Transport

• The electron transport chain generates no ATP

• The chain’s function is to break the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts

NADH

50

FADH2

40 FMN

Fe•S

I FAD

Fe•S II

IIIQ

Fe•S

Cyt b

30

20

Cyt c

Cyt c1

Cyt a

Cyt a3

IV

10

0


Fre

e e

ner

gy

(G

) re

lati

ve

to O

2 (

kca

l/m

ol)

H2O

O22 H+ + 1/2

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Electron Transport Phosphorylation • Electron transfer in the electron transport chain causes proteins to pump H+

from the mitochondrial matrix to the intermembrane space• The ETC uses energy from electrons to pump H+ across a membrane against

their concentration gradient - potential energy.• H+ then moves back across the membrane, passing through channels in ATP

synthase • ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP• This is an example of chemiosmosis, the use of energy in a H+ gradient to drive

cellular work

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LE 9-15

Protein complexof electroncarriers

H+

ATP ATP ATP

GlycolysisOxidative

phosphorylation:electron transportand chemiosmosis

Citricacidcycle

H+

Q

IIII

II

FADFADH2

+ H+NADH NAD+

(carrying electronsfrom food)

Innermitochondrialmembrane

Innermitochondrialmembrane

Mitochondrialmatrix

Intermembranespace

H+

H+

Cyt c

IV

2H+ + 1/2 O2 H2O

ADP +

H+

ATP

ATPsynthase

Electron transport chainElectron transport and pumping of protons (H+),

Which create an H+ gradient across the membrane

P i

ChemiosmosisATP synthesis powered by the flow

of H+ back across the membrane

Oxidative phosphorylation

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ATP • The energy stored in a H+ gradient

across a membrane couples the redox reactions of the electron transport chain to ATP synthesis

• The H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work

• Most of the ATP produced in cells is made by the enzyme ATP synthase

• The enzyme is embedded in the membrane and provides a channel through which protons can cross the membrane down their concentration gradient

• The energy released causes the rotor and the rod structures to rotate. This mechanical energy is converted to chemical energy with the formation of ATP

H+

ATP

ADP + Pi

Catalytichead

Intermembranespace

Mitochondrial matrix

Rod

Rotor

H+

H+

H+

H+ H+

H+H+

H+

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LE 9-14

INTERMEMBRANE SPACE

H+ H+

H+H+

H+

H+

H+

H+

ATP

MITOCHONDRAL MATRIX

ADP+

Pi

A rotor within the membrane spins as shown when H+ flows past it down the H+ gradient.

A stator anchored in the membrane holds the knob stationary.

A rod (or “stalk”) extending into the knob also spins, activating catalytic sites in the knob.

Three catalytic sites in the stationary knob join inorganic phosphate to ADP to make ATP.

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Summary of Glucose Catabolism

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Theoretical ATP Yield of Aerobic Respiration

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Catabolism of Proteins and Fats

• Proteins are utilized by deaminating their amino acids, and then metabolizing the product.

• Fats are utilized by beta-oxidation.

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Regulating Aerobic Respiration• Control of glucose catabolism occurs at two key points in the catabolic

pathway.• Glycolysis - phosphofructokinase• Pyruvate Oxidation – pyruvate decarboxylase

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Recycling NADH• As long as food molecules are available to

be converted into glucose, a cell can produce ATP.

• Continual production creates NADH accumulation and NAD+ depletion.

• NADH must be recycled into NAD+.• Aerobic respiration - oxygen as electron

acceptor• Fermentation - organic molecule

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Anaerobic Respiration

• Final electron acceptor is an inorganic molecule other than oxygen

• Some use NO3 -: E. coli

• Some use SO42-

• Important in nitrogen and sulfur cycles• ATP varies, less than 38 • Only part of Krebs cycle & ETC used

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Fermentation

• In some cases, the high energy electrons picked up by NAD+ during glycolysis are not donated to an ETC.

• Instead, NADH donates its extra electrons and H+ directly to an organic molecule, which serves as the final electron acceptor.

42

Fermentation

• Pyruvate converted to organic product

• NAD+ regenerated• Doesn’t require oxygen• Does not use Krebs cycle

or ETC• Organic molecule is final

electron acceptor• Produces 2 ATP max

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Alcohol Fermentation• Occurs in single-celled fungi called yeast• A terminal CO2 is removed from the pyruvic acid (3C)

produced during glycolysis, producing acetaldehyde (2C)• Acetaldehyde accepts 2 e- and a H+ from NADH, producing

ethanol and NAD+

Glucose

2 Pyruvic Acid

2 ATP

2 ADP C

2 Ethanol

2 Acetaldehyde

H OH

H

CH3

2 NAD+

OC

H

CH3

2 NADH

CO2O

H

O

CH3

GLYCOLYSIS

O

C

C

CO2+ 2 H+

2 NADH2 NAD+

2 Acetaldehyde

2 ATP2 ADP + 2 P i

2 Pyruvate

2

2 Ethanol

Alcohol fermentation

Glucose Glycolysis

44

Lactic Acid Fermentation• Used by most animal cells when O2 is not available• NADH donates 2 e- and a H+ directly to the pyruvate (3C)

produced during glycolysis, producing lactate (3C) and NAD+

C O

O–

2 Lactate

CH OH

Glucose

2 Pyruvate

GLYCOLYSIS

2 ATP

2 ADP

C OO -

C O

2 NAD+

2 NADH

CH3

CH3

CO2+ 2 H+

2 NADH2 NAD+

2 ATP2 ADP + 2 P i

2 Pyruvate

2

2 Lactate

Lactic acid fermentation

Glucose Glycolysis

45

Fermentation

Alcoholic fermentation and lactic acid fermentation each generate 2 ATP / glucose molecule compared to the theoretical maximum of 36 ATP per glucose during aerobic respiration.

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48

Extra Slides

49

Glycolysis

The energy investment phase

carbons

ATP/NADH Ledger

- 2 ATP

Energy coupling

ATP ADP + P: exergonic

Glu Glu-6-P :endergonic

50

Glycolysis

The energy payoff phase

ATP/NADH Ledger-2ATP

+2ATP

+2 NADH

Redox reactions

Energy coupling

51

Glycolysis

More energy coupling

ATP/NADH Ledger-2ATP+2ATP+2ATP

+2 NADH

End-products of glycolysis are 2 pyruvate molecules

52

53

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.

• During redox reactions, H+ are often transferred along with the electrons.

Gain of electron (reduction)Low energy

e–

A B

High energy

Loss of electron (oxidation)

Ao o

B+ –

A* B*

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Electron carriers• Molecules that pick up electrons from substances being oxidized and

donate them to substances being reduced.• For example, during the breakdown of glucose :

• Enzymes remove 2 H atoms (2p and 2e) from glucose • Both electrons and one proton are picked up by NAD+ to form

NADH • The other proton is released as a hydrogen ion (H+)

• Oxidized Form Reduced Form• NAD+ 2e- and 2H+ NADH + H+• FAD 2e- and 2H+ FADH2• NADP+ 2e- and 2H+ NADPH + H+

55

Product

H

H

H

H

NAD+

NAD

NAD

H

Energy-rich molecule

1. Enzymes that harvesthydrogen atoms have abinding site for NAD+

located near anotherbinding site. NAD+ andan energy-richmolecule bind tothe enzyme.

3. NADH thendiffuses away andis available toother molecules.

2. In an oxidation-reduction reaction,a hydrogen atomis transferred toNAD+, formingNADH.

Enzyme

NAD+NAD+

Use of chemical cofactor (NAD+)

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H+ H+ H+

H+

NAD+

FADH2

Q

C

e–e–

NADH

H2O+ 2H+ +

½O2

FAD

Electron Transport• An electron transport chain (ETC) is a series of electron

carriers that are embedded in a membrane and that pass electrons from one carrier to the next in a specific sequence:

57

Electron Flow

58

Electrontransport chain

Low energy

High energyEnergy forsynthesis of

Electrons from food

Formation of water

ATP

e–

e–

Electron Transport

• As electrons are passed from one carrier to the next in the ETC, some of their energy is released. This energy can be used to make ATP:

59

ATP

• Adenosine Triphosphate (ATP) is the energy currency of the cell.• Drives movement• Used in endergonic reactions

60

ATP - Energy Coupling

• ATP hydrolysis can be coupled to other reactions

Endergonic reaction: ∆G is positive, reaction is not spontaneous

∆G = +3.4 kcal/molGlu Glu

∆G = - 7.3 kcal/molATP H2O+

+ NH3

ADP +

NH2

Glutamicacid

Ammonia Glutamine

Exergonic reaction: ∆ G is negative, reaction is spontaneous

P

Coupled reactions: Overall ∆G is negative; together, reactions are spontaneous ∆G = –3.9 kcal/mol

Documents

1 Cellular Respiration: Harvesting Chemical Energy