copyright © 2005 pearson education, inc. publishing as benjamin cummings biology, seventh edition...

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

Biology, Seventh EditionNeil Campbell and Jane Reece

Lectures by Chris Romero

Chapter 9Chapter 9

Cellular Respiration: Harvesting Chemical Energy


• Overview: Life Is Work

• Living cells

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


• The giant panda

– Obtains energy for its cells by eating plants

Figure 9.1


• Energy

– Flows into ecosystems as sunlight and leaves as heat

Light energy

ECOSYSTEM

CO2 + H2O

Photosynthesisin chloroplasts

Cellular respirationin mitochondria

Organicmolecules

+ O2

ATP

powers most cellular work

HeatenergyFigure 9.2


• Photosynthetic organisms trap a portion of the sunlight energy and transform it into chemical energy (organic molecules) with O2 is released.

• Cells use some of the chemical energy in organic molecules to make ATP; the energy source for cellular work.

• Energy leaves organisms as it dissipates as heat

• The products of respiration (CO2 and H2O) are the raw materials for photosynthesis.

• Photosynthesis produces glucose and oxygen, the raw materials for respiration


• Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels

Catabolic Pathways and Production of ATP

• Organic compounds store energy in their arrangement of atoms

• With the help of enzymes, a cell systematically degrades complex organic molecules that are rich in potential energy to simpler waste products that have less energy


• Fermentation a catabolic process

– Is a partial degradation of sugars that occurs without oxygen (anaerobic)

• Cellular respiration

– Is the most prevalent and efficient catabolic pathway

– Consumes oxygen and organic molecules such as glucose

– Yields ATP


Respiration can be summarized:

organic + oxygen → carbon + water + Energy compounds dioxide

cellular respiration is most often described as the oxidation of glucose:

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

The breakdown of glucose is exergonic (free energy change) (ΔG= – 686 kcal per mol)

– ΔG → the products of the chemical process store less energy than reactants and the reaction can happen spontaneously (without an input of energy)


• To keep working

– Cells must regenerate ATP from ADP + Pi

– To understand how cellular respiration accomplishes this, let’s examine the fundamental process known as oxidation and reduction


Redox Reactions: Oxidation and Reduction

• Why do the catabolic pathways that decompose glucose and other organic fuels yield energy?

– The answer is based on the transfer of electrons during the chemical reactions. The relocation of electrons releases energy stored in organic molecules, and this energy is used to synthesize ATP


The Principle of Redox

• Redox reactions

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

• In oxidation

– A substance loses electrons, or is oxidized

• In reduction

– A substance gains electrons, or is reduced


• Examples of redox reactions

Na + Cl Na+ + Cl–

becomes oxidized(loses electron)

becomes reduced(gains electron)

Xē + Y X + Yē

becomes oxidized

becomes reduced

We could generalize a redox reaction this way:

X is the electron donor, is called the reducing agent, it reduces Y which accepts the donated electronY is the electron acceptor, it oxidizes X by removing its electron


• Some redox reactions

– Do not completely exchange electrons

– Change the degree of electron sharing in covalent bonds

– An electron loses potential energy when it shifts from a less electronegative atom toward more electronegative one → this energy can be put to work

CH4

H

H

HH

C O O O O OC

H H

Methane(reducingagent)

Oxygen(oxidizingagent)

Carbon dioxide Water

+ 2O2 CO2 + Energy + 2 H2O

becomes oxidized

becomes reduced

Reactants Products

Figure 9.3


Oxidation of Organic Fuel Molecules During Cellular Respiration

• Oxidation of methane is the main combustion reaction that occurs at the burner of a gas stove

• During cellular respiration

– Glucose is oxidized and oxygen is reduced

C6H12O6 + 6O2 6CO2 + 6H2O + Energy

becomes oxidized

becomes reduced


• In general, organic molecules that have an abundance of hydrogen are excellent fuels because their bonds are a source of hilltop electrons whose energy may be released as these electrons fall down an energy gradient when they are transferred to oxygen

• By oxidizing glucose, respiration liberates stored energy from glucose and makes it available for ATP synthesis


Stepwise Energy Harvest via NAD+ and the Electron Transport Chain


– Oxidizes glucose in a series of steps each is catalyzed by an enzyme

– The hydrogen atoms are not transferred to oxygen, but instead are usually passed first to a coenzyme called NAD+ (nicotinamide adenine dinucleotide).

– NAD functions as coenzyme in the redox reactions thus is an oxidizing agent. It is found in all cells and helps in e transfer.


• Electrons from organic compounds

– Are usually first transferred to NAD+, a coenzyme

– NAD+ accept electrons and act as an oxidizing agent during respiration

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

How does NAD+ trap electrons from

glucose and other organic molecules? Enzymes called dehydrogenases removes a pair of hydrogen atoms (2 electrons and 2 protons) from the substrate (a sugar for example) thereby oxidizing it


• NADH, the reduced form of NAD+

– Passes the electrons to the electron transport chain

– Each NADH molecule formed during respiration represents stored energy that can be tapped to make ATP when the electrons complete their fall down an energy gradient from NADH to oxygen


How do electrons that are extracted from food and stored by NADH finally reach oxygen?

• 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


• The electron transport chain

– Passes electrons in a series of steps instead of one explosive reaction

– Uses the energy from the electron transfer to form ATP

– The transport chain consists of a number of molecules, mostly proteins, built into the inner membrane of a mitochondrion

– ET from NADH to O2 is an exergonic reaction with a free energy change of – 53 kcal/mol

– Food → NADH → ETC → Oxygen


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

– Oxidative phosphorylation

does not require O2

require O2


• Glycolysis

– Breaks down glucose into two molecules of pyruvate

• The citric acid cycle

– Completes the breakdown of glucose

• Oxidative phosphorylation

– Is driven by the electron transport chain

– Generates 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


An overview of cellular respiration

• During glycolysis each glucose molecule is broken down into 2 pyruvate molecules

• Pyruvate inters into mitochondria where it will be oxidized by the citric acid cycle to CO2.

• NADH and FADH2 transfer electrons from glucose to ETCs in the inner mitochondria membrane

• During oxidative phosphorylation, ETCs convert chemical energy to a form of energy used for ATP synthesis in a process called chemiosmosis.


• Both glycolysis and the citric acid cycle

– Can generate ATP by substrate-level phosphorylation

– Occurs when an enzyme transfers a phosphate group from a substrate molecule to ADP rather than adding an inorganic phosphate to ADP as in oxidative phosphorylation

Figure 9.7

Enzyme Enzyme

ATP

ADP

Product

SubstrateP

+


• Concept 9.2: Glycolysis harvests energy by oxidizing glucose to pyruvate

• Glycolysis (can occur in the presence or absence of O2)

– Means “splitting of sugar”

a 6-C sugar (glucose) to 3-C sugar (pyruvate)

– Breaks down glucose into pyruvate

– Occurs in the cytoplasm of the cell


• Glycolysis consists of two major phases

– Energy investment phase

– Energy payoff phaseGlycolysis Citric

acidcycle


ATP ATP ATP

2 ATP

4 ATP

used

formed

Glucose

2 ATP + 2 P

4 ADP + 4P

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


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

• A closer look at the energy investment phase

This is the reaction from which glycolysis Gets its name

This reaction never reaches equilibrium in the cell


2 NAD+

NADH2+ 2 H+

Triose phosphatedehydrogenase2 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

• A closer look at the energy payoff phase


• Concept 9.3: The citric acid cycle completes the energy-yielding oxidation of organic molecules

• The citric acid cycle

– Takes place in the matrix of the mitochondrion

– completes glucose oxidation by breaking down pyruvate derivitatives into carbon dioxide.


• Before the citric acid cycle can begin

– Pyruvate must first be converted to acetyl CoA, which links the cycle to glycolysis

CYTOSOL MITOCHONDRION

NADH + H+NAD+

2

31

CO2 Coenzyme APyruvate

Acetyle CoA

S CoA

C

CH3

O

Transport protein

O–

O

O

C

C

CH3

Figure 9.10


• 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

Glycolysis Citricacidcycle


Figure 9.11

For each turn of Krebs cycle, two carbons exit completely as CO2, three NADH and one FADH2 are formed.One ATP is made by substrate-level phosphorylation


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

4

5

6

7

8

Glycolysis Oxidativephosphorylation

NAD+

+ H+

ATP

Citricacidcycle

Figure 9.12

• A closer look at the citric acid cycle

NAD is reduced to NADH+

CoA is displaced by a phosphate groupWhich is transferred to GDP forming GTP

And then to ATP.


• During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis

• NADH and FADH2

– Donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation

Concept 9.4:


The Pathway of Electron Transport

– Electrons from NADH and FADH2

lose energy in several steps

– Couples this exergonic slide of electrons to ATP synthesis or oxidative phosphorylation

– The electron transport chain is made of electron carrier molecules embedded in the inner membrane of mitochondria.


– Each carrier in the chain has a higher electronegativity than the carrier before it, so electrons are pulled downhill towards oxygen

– Most carriers are protein molecules except for ubiquinone (Q)

– At the end of the chain, electrons are passed to oxygen, forming water


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

NADH is oxidized and flavoprotein is reduced as highenergy electrons from NADH are transferred to FMN

↓Flavoprotein is oxidized as it passes electrons to an iron sulfur protein, FeS.

↓FeS is oxidized as it pass electrons to ubiquinone Q

↓Q passes electrons on to a succession of electron carriers, most of which are cytochromes.

↓cyt a3 , the last cytochrome passes electrons to oxygen.


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


• At certain steps along the electron transport chain

– Electron transfer causes protein complexes to pump H+ from the mitochondrial matrix to the intermembrane space


• Chemiosmosis

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

• The resulting H+ gradient

– Stores energy

– Drives chemiosmosis in ATP synthase

– Is referred to as a proton-motive force


• 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 carriers

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

• During respiration, most energy flows in this sequence

– Glucose to NADH to electron transport chain to proton-motive force to ATP


• 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 38 ATP

1 ATP → - 7.3 kcal/mol

38 ATP X 7.3 / 686 = 40%


• Concept 9.5: Fermentation enables some cells to produce ATP without the use of oxygen


– Relies on oxygen to produce ATP

• In the absence of oxygen

– Cells can still produce ATP through fermentation


• Glycolysis

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

– Couples with fermentation to produce ATP


Types of Fermentation

• Fermentation consists of

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

• 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


Fermentation and Cellular Respiration Compared

• Both fermentation and cellular respiration

– Use glycolysis to oxidize glucose and other organic fuels to pyruvate

• Fermentation and cellular respiration

– Differ in their final electron acceptor


– Produces more ATP


• Pyruvate is a key juncture in catabolism

Glucose

CYTOSOL

Pyruvate

No O2 presentFermentation

O2 present Cellular respiration

Ethanolor

lactate

Acetyl CoA

MITOCHONDRION

Citricacidcycle

Figure 9.18


The Evolutionary Significance of Glycolysis

• Glycolysis

– Occurs in nearly all organisms

– Probably evolved in ancient prokaryotes before there was oxygen in the atmosphere


The Versatility of Catabolism

• Catabolic pathways

– Funnel electrons from many kinds of organic molecules into cellular respiration

• Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways


• The catabolism of various molecules from food

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

• 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


• The control of cellular respirationGlucose

Glycolysis

Fructose-6-phosphate

Phosphofructokinase

Fructose-1,6-bisphosphateInhibits Inhibits

Pyruvate

ATPAcetyl CoA

Citricacidcycle

Citrate


Stimulates

AMP

+

– –

Figure 9.20

copyright © 2005 pearson education, inc. publishing as benjamin cummings biology, seventh edition...

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