09 cellrespiration text

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Cellular Respiration: Harvesting Chemical Energy

• Living cells

– Require transfusions of energy from outside sources to perform their many tasks

Figure 9.1


• Energy

– Flows into an ecosystem as sunlight and leaves as heat Light energy

ECOSYSTEM

CO2 + H2O

Photosynthesisin chloroplasts

Cellular respiration

in mitochondria

Organicmolecules

+ O2

ATP

powers most cellular work

Heatenergy

Figure 9.2


• Catabolic pathways yield energy by oxidizing organic fuels (exergonic)

One catabolic process, fermentationIs a partial degradation of sugars that occurs without oxygen


• Cellular respiration

– Is the most prevalent and efficient catabolic pathway

– Consumes oxygen and organic molecules such as glucose

– Yields ATP (Cells must regenerate ATP)


The Principle of Redox

• Redox reactions

– Transfer electrons from one reactant to another by oxidation and reduction

In oxidationA substance loses electrons, or is oxidized

In reductionA substance gains electrons, or is reduced

Na + Cl Na+ + Cl–

becomes oxidized(loses electron)

becomes reduced(gains electron)


Oxidation of Organic Fuel Molecules During Cellular Respiration

• During cellular respiration

– Glucose is oxidized and oxygen is reduced

C6H12O6 + 6O2 6CO2 + 6H2O + Energy

becomes oxidized

becomes reduced

Cellular respirationOxidizes glucose in a series of steps


• Electrons from organic compounds

– Are usually first transferred to NAD+, a coenzyme

NAD+

H

O

O

O O–

O

O O–

O

O

O

P

P

CH2

CH2

HO OHH

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

Enzyme


• 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

NADH, the reduced form of NAD+

Passes the electrons to the electron transport chain


• 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


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

ATP ATP

ATP

ATP

Electro

n tran

spo

rt chain

Fre

e en

ergy

, G

(b) Cellular respiration

+

Figure 9.5 B


The Stages of Cellular Respiration: A Preview

• Respiration is a cumulative function of three metabolic stages

– Glycolysis

– The citric acid cycle (kreb’s cycle)

– Oxidative phosphorylation (electron transport chain)


• Glycolysis

– Breaks down glucose into two molecules of pyruvate

• The citric acid cycle

– Completes the breakdown of glucose

Oxidative phosphorylationIs driven by the electron transport chainGenerates ATP


• An overview of cellular respiration

Figure 9.6

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


• Both glycolysis and the citric acid cycle

– Can generate ATP by substrate-level phosphorylation

Figure 9.7

Enzyme Enzyme

ATP

ADP

Product

SubstrateP

+


• 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


Glycolysis Citricacidcycle


ATP ATP ATP

2 ATP

4 ATP

used

formed

Glucose

2 ATP + 2 P

4 ADP + 4 P

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

2 Pyruvate + 2 H2O

Energy investment phase

Energy payoff phase

Glucose 2 Pyruvate + 2 H2O

4 ATP formed – 2 ATP used 2 ATP

2 NAD+ + 4 e– + 4 H + 2 NADH

+ 2 H+

Figure 9.8

• Glycolysis consists of two major phases

– Energy investment phase (preparatory phase) 2 ATP used

– Energy payoff phase – 4 ATP formed (2 ATP net)


A closer look at the energy investment phase

Dihydroxyacetonephosphate

Glyceraldehyde-3-phosphate

HH

H

HH

OHOH

HO HO

CH2OHH H

H

HO H

OHHO

OH

P

CH2O P

H

OH

HO

HO

HHO

CH2OH

P O CH2O CH2 O P

HOH HO

HOH

OP CH2

C OCH2OH

HCCHOHCH2

O

O P

ATP

ADPHexokinase

Glucose

Glucose-6-phosphate

Fructose-6-phosphate

ATP

ADP

Phosphoglucoisomerase

Phosphofructokinase

Fructose-1, 6-bisphosphate

Aldolase

Isomerase

Glycolysis

1

2

3

4

5

CH2OHOxidative

phosphorylation

Citricacidcycle

Figure 9.9 A

1. Glucose gains a phosphate from ATP and becomes glucose-6-phosphate

2. glucose-6-phosphate becomes fructose-6-phosphate

3. fructose-6-phosphate gains a P by phosphorylation of ATP and becomes fructose-1,6-diphosphate

4. and 5. Very unstable fructose-1,6-diphosphate breaks into two molecules of G3P (glyceraldehyde-3-phosphate)

1 ATP used – phosphorylation

1 ATP used – phosphorylation


A closer look at the energy payoff phase

6. G3P is oxidized, NAD+ is reduced to form NADH & the energy from this causes a P to bind to G3P changing it to 1,3-diphosphoglycerate

7. 1,3-diphosphoglycerate phosphorylates an ADP to form ATP and becomes 3-phosphoglycerate

8. 3-phosphoglycerate rearranges into 2-phosphoglycerate

2 NAD+

NADH2+ 2 H+

Triose phosphatedehydrogenase

2 P i

2P C

CHOH

O

P

O

CH2 O

2 O–

1, 3-Bisphosphoglycerate2 ADP

2 ATP

Phosphoglycerokinase

CH2 O P

2

C

CHOH

3-Phosphoglycerate

Phosphoglyceromutase

O–

C

C

CH2OH

H O P

2-Phosphoglycerate

2 H2O

2 O–

Enolase

C

C

O

PO

CH2

Phosphoenolpyruvate2 ADP

2 ATP

Pyruvate kinase

O–

C

C

O

O

CH3

2

6

8

7

9

10

Pyruvate

O

Figure 9.8 B

9. 2-phosphoglycerate goes through a dehydration reaction and becomes PEP (phosphoenolpyruvate)

10. another substrate level phosphorylation reaction producing ATP changes PEP into pyruvate

Redox reaction

2 ATP produced – phosphorylation

2 ATP produced – phosphorylation

Water lost


Glycolysis Output

• 1 glucose and 2 ATP go in

• 4 ATP (2 net ATP), 2 NADH (energy can only be used in presence of Oxygen), 2 H2O, 2H+ and 2 pyruvate come out



ATP ATP ATP

2 ATP

4 ATP

used

formed

Glucose

2 ATP + 2 P

4 ADP + 4 P

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

2 Pyruvate + 2 H2O

Energy investment phase

Energy payoff phase

Glucose 2 Pyruvate + 2 H2O

4 ATP formed – 2 ATP used 2 ATP

2 NAD+ + 4 e– + 4 H + 2 NADH

+ 2 H+

Figure 9.8


• The citric acid cycle completes the energy-yielding oxidation of organic molecules

• The citric acid cycle (or Kreb’s Cycle)

– Takes place in the matrix of the mitochondrion


• Before the citric acid cycle can begin

– Pyruvate must first be converted to acetyl CoA, which moves from the cytoplasm into the mitochondria

CYTOSOLMITOCHONDR

ION

NADH

+ H+NAD+

2

31

CO2Coenzym

e APyruvate

Acetyle CoA

SCoA

C

CH3

O

Transport protein

O–

O

O

C

C

CH3

Figure 9.10

1. pyruvate loses 1 carbon to form CO2

2. the 2C sugar is then oxidized and NAD+ is reduced to form an NADH

3. coenzyme A binds to the 2C sugar to form acetyl coenzyme A (acetyl CoA)


• An overview of the citric acid cycle

ATP

2 CO2

3 NAD+

3 NADH

+ 3 H+

ADP + P i

FAD

FADH2

Citricacidcycle

CoA

CoA

Acetyle CoA

NADH+ 3 H+

CoA

CO2

Pyruvate(from glycolysis,2 molecules per glucose)

ATP ATP ATP



Figure 9.11


Figure 9.12

Acetyl CoA

NADH

Oxaloacetate

CitrateMalate

Fumarate

SuccinateSuccinyl

CoA

-Ketoglutarate

Isocitrate

Citricacidcycle

S CoA

CoA SH

NADH

NADH

FADH2

FAD

GTP GDP

NAD+

ADP

P i

NAD+

CO2

CO2

CoA SH

CoA SH

CoAS

H2O

+ H+

+ H+ H2O

C

CH3

O

O C COO–

CH2

COO–

COO–

CH2

HO C COO–

CH2

COO–

COO–

COO–

CH2

HC COO–

HO CHCOO–

CH

CH2

COO–

HO

COO–

CH

HC

COO–

COO–

CH2

CH2

COO–

COO–

CH2

CH2

C O

COO–

CH2

CH2

C O

COO–

1

2

3

Glycolysis Oxidativephosphorylation

NAD+

+ H+

ATP

Citricacidcycle

Figure 9.12

1. acetyl CoA enters the cycle, CoA is removed and the remaining 2C acetyl group is bound to oxaloacetate (which is 4C), making citrate (a 6C molecule)

2. a redox reaction produces an NADH and releases one C as CO2 changing citrate into alpha-ketoglutarate (a 5C molecule)

3. another redox reaction and substrate level phosphorylation produces an ATP, another NADH and CO2, changing alpha-ketoglutarate into succinate (a 4C molecule)

A closer look at the citric acid cycle= intermediate molecule (short lived) = intermediate molecule (short lived)


A closer look at the citric acid cycle .. Cont.

• 4. succinate is oxidized and FAD is reduced to FADH2, forming malate

• 5. malate is oxidized, NAD+ is reduced to NADH, and oxaloacetate is produced (ready to start the cycle over again)

Acetyl CoA

NADH

Oxaloacetate

CitrateMalate

Fumarate

SuccinateSuccinyl

CoA

-Ketoglutarate

Isocitrate

Citricacidcycle

S CoA

CoA SH

NADH

NADH

FADH2

FAD

GTP GDP

NAD+

ADP

P i

NAD+

CO2

CO2

CoA SH

CoA SH

CoAS

H2O

+ H+

+ H+ H2O

C

CH3

O

O C COO–

CH2

COO–

COO–

CH2

HO C COO–

CH2

COO–

COO–

COO–

CH2

HC COO–

HO CHCOO–

CH

CH2

COO–

HO

COO–

CH

HC

COO–

COO–

CH2

CH2

COO–

COO–

CH2

CH2

C O

COO–

CH2

CH2

C O

COO–

4

5


NAD+

+ H+

ATP

Citricacidcycle

Figure 9.12


Total Output Citric Acid Cycle (total = 2 pyruvate)

• 4 CO2,

• 6 NADH,

• 2FADH2

• and 2 ATP

Acetyl CoA

NADH

Oxaloacetate

CitrateMalate

Fumarate

SuccinateSuccinyl

CoA

-Ketoglutarate

Isocitrate

Citricacidcycle

S CoA

CoA SH

NADH

NADH

FADH2

FAD

GTP GDP

NAD+

ADP

P i

NAD+

CO2

CO2

CoA SH

CoA SH

CoAS

H2O

+ H+

+ H+ H2O

C

CH3

O

O C COO–

CH2

COO–

COO–

CH2

HO C COO–

CH2

COO–

COO–

COO–

CH2

HC COO–

HO CHCOO–

CH

CH2

COO–

HO

COO–

CH

HC

COO–

COO–

CH2

CH2

COO–

COO–

CH2

CH2

C O

COO–

CH2

CH2

C O

COO–

4

5


NAD+

+ H+

ATP

Citricacidcycle

Figure 9.12


The Pathway of Electron Transport

• In the electron transport chain

• A series of oxidation-reduction reactions that result in phosphorylation

• Where: in the mitochondrial inner membrane (between the intermembrane space and mitochondrial matrix)

• Note: the cristae of the mitochondria membrane increase surface area and allow more space for this step & its mechanisms

• 2 parts: electron transport chain & ends with chemiosmosis

• Produces the most ATP, about 34

• At the end of the chain

– Electrons are passed to oxygen, forming water


Anatomy of a mitochondria


Overview

H2O

O2

NADH

FADH2

FMN

Fe•S Fe•S

Fe•S

O

FAD

Cyt b

Cyt c1

Cyt c

Cyt a

Cyt a3

2 H + + 12

I

II

III

IV

Multiproteincomplexes

0

10

20

30

40

50

Fre

e e

ner

gy

(G)

rela

tive

to O

2 (k

cl/m

ol)

Figure 9.13

1. NADH & FADH2 transfer electrons to the electron transport chain, a series of proteins that pass electrons along through redox reactions

2. energy from the electrons is used to pump H+ out of the matrix into the intermembrane space

3. oxygen is the final electron acceptor in the transport chain

4. H+ diffuse back into the matrix through an ATP synthase (this is chemiosmosis)

5. H+ binds to the O and forms water

Animation: http://vcell.ndsu.nodak.edu/animations/etc/movie.htm


Electron movement – Inner Membrane

NADH donates 2 electrons

Mitochondrial Matrix

Intermembrane

2 H+

Ubiquinone

Electrons passed

1 H+ at a time

cytochrome c

1 electron at a time

4 H+

Four electrons must be transferred to the oxidase complex in order for the next

major reaction to occur

2 H20

3 H+ to synthesize

one ATP from the substrates

ADP and Pi (inorganic

phosphate).

FADH2 donates another electron


Chemiosmosis: The Energy-Coupling Mechanism

• ATP synthase

– Is the enzyme that actually makes ATPINTERMEMBRANE SPACE

H+

H+

H+

H+

H+

H+ H+

H+

P i

+ADP

ATP

A rotor within the membrane spins clockwise whenH+ flows past it down the H+

gradient.

A stator anchoredin the membraneholds the knobstationary.

A rod (for “stalk”)extending into the knob alsospins, activatingcatalytic sites inthe knob.

Three catalytic sites in the stationary knobjoin inorganic Phosphate to ADPto make ATP. MITOCHONDRIAL MATRIXFigure 9.14

ATP synthase animation: http://vcell.ndsu.nodak.edu/animations/atpgradient/movie.htm


• The resulting H+ gradient

– Stores energy (potential)

– Drives chemiosmosis in ATP synthase

– Is referred to as a proton-motive force


• Chemiosmosis

– Is an energy-coupling mechanism that uses energy in the form of a H+ gradient across a membrane to drive cellular work


• Chemiosmosis and the electron transport chain

Oxidativephosphorylation.electron transportand chemiosmosis

Glycolysis

ATP ATP ATP

InnerMitochondrialmembrane

H+

H+H+

H+

H+

ATPP i

Protein complexof electron carners

Cyt c

I

II

III

IV

(Carrying electronsfrom, food)

NADH+

FADH2

NAD+

FAD+ 2 H+ + 1/2 O2

H2O

ADP +

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

which create an H+ gradient across the membrane

ChemiosmosisATP synthesis powered by the flowOf H+ back across the membrane

ATPsynthase

Q

Oxidative phosphorylation

Intermembranespace

Innermitochondrialmembrane

Mitochondrialmatrix

Figure 9.15


An Accounting of ATP Production by Cellular Respiration – A summary!

• There are three main processes in this metabolic enterprise

Electron shuttlesspan membrane

CYTOSOL 2 NADH

2 FADH2

2 NADH 6 NADH 2 FADH22 NADH

Glycolysis

Glucose2

Pyruvate

2AcetylCoA

Citricacidcycle

Oxidativephosphorylation:electron transport

andchemiosmosis

MITOCHONDRION

by substrate-levelphosphorylation

by substrate-levelphosphorylation

by oxidative phosphorylation, dependingon which shuttle transports electronsfrom NADH in cytosol

Maximum per glucose:About

36 or 38 ATP

+ 2 ATP + 2 ATP + about 32 or 34 ATP

or

Figure 9.16


• About 40% of the energy in a glucose molecule

– Is transferred to ATP during cellular respiration, making approximately 36 to 38 ATP

With the supply of NADH exhausted, the electron transport chain can no longer maintain the proton gradient that powers ATP synthase, and ATP synthesis comes to a stop.


• Fermentation enables some cells to produce ATP without the use of oxygen

• Glycolysis

– Can produce ATP with or without oxygen, in aerobic or anaerobic conditions

– Couples with fermentation to produce ATP

Both fermentation and cellular respirationUse glycolysis to oxidize glucose and other organic fuels to pyruvate


Types of Fermentation

• Fermentation consists of

– Glycolysis plus reactions that regenerate NAD+, which can be reused by glyocolysis

In alcohol fermentation

Pyruvate is converted to ethanol in two steps, one of which releases CO2

During lactic acid fermentation

Pyruvate is reduced

directly to NADH to form

lactate as a waste

product

2 ADP + 2 P1 2 ATP

GlycolysisGlucose

2 NAD+ 2 NADH

2 Pyruvate

2 Acetaldehyde 2 Ethanol

(a) Alcohol fermentation

2 ADP + 2 P1 2 ATP

GlycolysisGlucose

2 NAD+ 2 NADH

2 Lactate

(b) Lactic acid fermentation

H

H OH

CH3

C

O –

OC

C O

CH3

H

C O

CH3

O–

C O

C O

CH3O

C O

C OHH

CH3

CO22

Figure 9.17


Glucose

CYTOSOL

PyruvateNo O2 presentFermentation

O2 present Cellular respiration

Ethanolor

lactate

Acetyl CoA

MITOCHONDRION

Citricacidcycle

Figure 9.18

• Fermentation and cellular respiration

– Differ in their final electron acceptor


– Produces more ATP


• Glycolysis and the citric acid cycle connect to many other metabolic pathways

Amino acids

Sugars Glycerol Fattyacids

Glycolysis

Glucose

Glyceraldehyde-3- P

Pyruvate

Acetyl CoA

NH3

Citricacidcycle


FatsProteins Carbohydrates

Figure 9.19


Biosynthesis (Anabolic Pathways)

• The body

– Uses small molecules to build other substances (at the cost of ATP molecules)

• These small molecules

– May come directly from food or through glycolysis or the citric acid cycle


Regulation of Cellular Respiration via Feedback Mechanisms


– Is controlled by allosteric enzymes at key points in glycolysis and the citric acid cycle

Glucose

Glycolysis


Phosphofructokinase

Fructose-1,6-bisphosphateInhibits Inhibits

Pyruvate

ATPAcetyl CoA

Citricacidcycle

Citrate


Stimulates

AMP

+

– –

Figure 9.20

Example: excess ATP production serves to inhibit phosphofructokinase in glycolysis, AMP stimulates its production

AMP can be produced during ATP synthesis by the enzyme adenylate kinase by combining two ADP molecules:

2 ADP → ATP + AMP


Dihydroxyacetonephosphate

Glyceraldehyde-3-phosphate

HH

H

HH

OHOH

HO HO

CH2OHH H

H

HO H

OHHO

OH

P

CH2O P

H

OH

HO

HO

HHO

CH2OH

P O CH2O CH2 O P

HOH HO

HOH

OP CH2

C OCH2OH

HCCHOHCH2

O

O P

ATP

ADPHexokinase

Glucose

Glucose-6-phosphate


ATP

ADP

Phosphoglucoisomerase

Phosphofructokinase

Fructose-1, 6-bisphosphate

Aldolase

Isomerase

Glycolysis

1

2

3

4

5

CH2OHOxidative

phosphorylation

Citricacidcycle

Figure 9.9 A

2 NAD+

NADH2+ 2 H+

Triose phosphatedehydrogenase

2 P i

2P C

CHOH

O

P

O

CH2 O

2 O–

1, 3-Bisphosphoglycerate2 ADP

2 ATP

Phosphoglycerokinase

CH2 O P

2

C

CHOH

3-Phosphoglycerate

Phosphoglyceromutase

O–

C

C

CH2OH

H O P

2-Phosphoglycerate

2 H2O

2 O–

Enolase

C

C

O

PO

CH2

Phosphoenolpyruvate2 ADP

2 ATP

Pyruvate kinase

O–

C

C

O

O

CH3

2

6

8

7

9

10

Pyruvate

O

Figure 9.8 B


CYTOSOLMITOCHONDR

ION

NADH

+ H+NAD+

2

31

CO2Coenzym

e APyruvate

Acetyle CoA

SCoA

C

CH3

O

Transport protein

O–

O

O

C

C

CH3

Figure 9.10


Acetyl CoA

NADH

Oxaloacetate

CitrateMalate

Fumarate

SuccinateSuccinyl

CoA

-Ketoglutarate

Isocitrate

Citricacidcycle

S CoA

CoA SH

NADH

NADH

FADH2

FAD

GTP GDP

NAD+

ADP

P i

NAD+

CO2

CO2

CoA SH

CoA SH

CoAS

H2O

+ H+

+ H+ H2O

C

CH3

O

O C COO–

CH2

COO–

COO–

CH2

HO C COO–

CH2

COO–

COO–

COO–

CH2

HC COO–

HO CHCOO–

CHCH2

COO–

HO

COO–

CH

HC

COO–

COO–

CH2

CH2

COO–

COO–

CH2

CH2

C O

COO–

CH2

CH2

C O

COO–

4

5


NAD+

+ H+

ATP

Citricacidcycle

Figure 9.12


Oxidativephosphorylation.electron transportand chemiosmosis

Glycolysis

ATP ATP ATP

InnerMitochondrialmembrane

H+

H+H+

H+

H+

ATPP i

Protein complexof electron carners

Cyt c

I

II

III

IV

(Carrying electronsfrom, food)

NADH+

FADH2

NAD+

FAD+ 2 H+ + 1/2 O2

H2O

ADP +

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

which create an H+ gradient across the membrane

ChemiosmosisATP synthesis powered by the flowOf H+ back across the membrane

ATPsynthase

Q

Oxidative phosphorylation

Intermembranespace

Innermitochondrialmembrane

Mitochondrialmatrix

Figure 9.15