chap9 cellular respiration

8/8/2019 Chap9 Cellular Respiration

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CHAPTER 9:

CELLULAR RESPIRATION


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Living cells

Require transfusions of energy from outside sources

to perform their many tasks

Overview: Life Is Work

The giant panda Obtains energy for its cells by eating plants


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Energy

Flows into an ecosystem as sunlight and leaves as heat; thechemical elements essential to life are recycled

Light energy

ECOSYSTEM

CO2 + H2O

Photosynthesis

in chloroplasts

Cellular

respiration

in mitochondria

Organic

molecules+ O2

ATP

powers most cellular work

Heat

energ

y

How cells harvest the

chemical energy stored in

organic molecules & use it

to generate ATP, the

molecule that drives most

cellular work?


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Catabolic Pathways and Production of ATP

Metabolic pathways that release stored energy bybreaking down complex molecules catabolic pathways

The breakdown of organic molecules is exergonic

Catabolic pathways yield energy by oxidizing

organic fuels

Fermentation (catabolic process)

Is a partial degradation of sugars that occurs without

the use of oxygen


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Cellular respiration

Is the most prevalent and efficient catabolic pathway Consumes oxygen and organic molecules such as

glucose

Yields ATP

Catabolism is linked to work by a chemical drive shaft -

ATP

To keep working

Cells must regenerate its supply of ATP from ADPand Pi (Inorganic phosphate)


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Redox Reactions: Oxidation and Reduction

Catabolic pathways yield energy

Due to the transfer ofelectrons during the chemical

reactions

The relocation of electrons releases energy stored in

organic molecules

And this energy ultimately is used to synthesize ATP

by using enzyme.


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The Principle of Redox

Redox reactions

Transfer electrons from one reactant to another byoxidation 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)


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Some redox reactions

Do not completely exchange electrons

Change the degree of electron sharing in covalent bonds

CH4

H

H

HH

C O O O O OCH H

Methane

(reducing

agent)

Oxygen

(oxidizing

agent)

Carbon dioxide Water

+ 2O2CO

2 +Energ

y + 2 H2O

becomes oxidized

becomes

reduced

Reactants Products

Methane combustion as an energy-yielding

redox reaction

Energy must be added to pull an e-

away from an atom.

The more electronegative the atom(the stronger its pull on e-), the more

energy is required to take away an e-.

An e- loses potential energy when it

shifts from a less electronegative

atom to a more electronegative one.

A redox reaction that relocates e-

closer to O2 (eg. burning of methane),

releases chemical energy that can be

used for work.


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Stepwise Energy Harvest via NAD+ and the Electron

Transport Chain


Oxidizes glucose in a series of steps, each step

catalyzed by an enzyme.

At key steps, e- are stripped from the glucose, each

electron travels with a proton (H atom)

H atom from organic compounds are not transferred

directly to oxygen But are usually first transferred to NAD+, a coenzyme

As an electron acceptor, NAD+ functions as an oxidizing

agent during respiration


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NAD+H

O

O

O O

O

O O

O

O

O

P

P

CH2

CH2

HO OH

H

H

HO 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

H+

H+

NAD+ as an electron shuttle.

The enzymatic transfer of two electrons and

one proton (H+) from an organic molecule in

food to NAD+reduces the NAD+ to NADH;

the second proton (H+

) is released.


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Most of the electrons removed from food are transferred

initially to NAD+

NADH, the reduced form of NAD+

Passes the electrons to the electron transport chain


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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

Freeenergy,

G

H2O

Explosive

release of

heat and light

energy

H2 +1/2 O2


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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

Electrontr

ansp

ortc

hain

Freeenergy,G

(b) Cellular respiration

+

The electron transport chain

Passes electrons in a

series of steps instead ofin one explosive reaction

Uses the energy from the

electron transfer to form

ATP

e- removed from food are shuttled by

NADH to the top, higher-energy end

of the chain

At the bottom, lower-energy end,

oxygen captures these e- along with

H+, forming water.

In cellular respiration, e- travel this

downhill route

Food NADH Electron Transport

Chain oxygen


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The Stages of Cellular Respiration:A Preview

Respiration is a cumulative function of three metabolic

stages

Glycolysis: Breaks down glucose into two molecules of

pyruvate The citric acid cycle: Completes the breakdown of glucose

Oxidative phosphorylation (electron transport &

chemiosmosis): Is driven by the electron transport chain ,

Generates ATP


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An overview of cellular respiration

Electr

ons

carrie

d

via

NAD

H

GlycolysisGlucos

e

Pyruvate

ATP

Substrate-level

phosphorylation

Electrons carried

via NADH and

FADH2

Ci

tri

cac

id

cy

cl

e

Oxidative

phosphorylation:

electron

transport

and

chemios

mosis

ATPATP

Substrate-level

phosphorylationOxidative

phosphorylation

MitochondrionCytosol

Glycolysis occurs in

the cytosol.

Begins degradation

by breaking glucose

into 2 molecules of

pyruvate

The citric acid cycletakes place within

the mitochondrial

matrix.

Completes the

breakdown ofglucose by oxidizing

a derivative of

pyruvate to CO2

In the third stage of

respiration, the ETC

accepts e- from the 1st

two stages breakdown

products via NADH &

passes these e- from 1

molecule to another.

At the end of the chain,

e- are combined with

molecular O2 and H+ to

form water.

The energy released at

each step of the chain is

stored in a form the

mitochondrion can use

to make ATP.

This mode of ATP synthesis is called oxidative

phosphorylation because it is powered by the redox

reactions of the ETC.


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Glycolysis

Means splitting of sugar

Breaks down glucose into pyruvate Occurs in the cytoplasm of the cell

Glycolysis harvests energy by oxidizing

glucose to pyruvate


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Glycolysis consists oftwo major phases

Energy investment

phase

Energy payoff

phase

Glycolysis Citricacidcycle

Oxidativephosphorylation

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+

Net


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Dihydroxyacetonephosphate

Glyceraldehyde-3-phosphate

HH

H

HH

OH

OH

HOHO

CH2OH

HH

H

HO H

OHHO

OH

P

CH2O P

H

O

H

HO

HO

HHO

CH2OH

P O CH2 O CH2 O P

HO

H HO

HOH

OP CH2

C O

CH2OH

H

C

CHOH

CH2

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

CH2OH

Oxidative

phosphorylation

Citric

acid

cycle

A closer look at the energy investment phase

1 Glucose is phosphorylated by hexokinase.

Hexokinase transfers a P group from ATP to

the sugar.

2 Glucose-6-phosphate is rearranged to itsisomer, fructose-6-phosphate

3 Enzyme transfers a P group from ATP tothe sugar.

P groups on opposite sides, sugar is ready

to be split in half.

4 Enzyme cleaves the sugar molecule intotwo different 3-carbon sugars (isomers)

5 Net result of steps 4 and 5 is cleavage ofa 6-carbon sugar into two molecules of

glyceraldehyde-3-phosphate.

A closer look at the energy payoff phase


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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

CH2Phosphoenolpyruvate

2 ADP

2 ATP

Pyruvate kinase

O

C

C

O

O

CH3

2

6

8

7

9

10

Pyruvate

O

A closer look at the energy payoff phase

6 Sugar is oxidized by transfer of electronsand H+ to NAD+, forming NADH (exergonic).

Enzyme uses this energy to attach a P group to

the oxidized substrate.

7 Phosphate group is transferred to ADP toproduce ATP.

8 Enzyme relocates the P group.

9 Enolase causes a double bond to form in thesubstrate, extracting a water molecule, yielding

PEP.

10 Pyruvate kinase transfers P group from PEPto ADP.

Net gain of 2 ATP (2 ATP used, 4 ATP

produced)


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Less than a of chemical energy in glucose is released in

glycolysis; most energy is stockpiled in 2 molecules of

pyruvate

The citric acid cycle

Takes place in the matrix of the mitochondrion

The citric acid cycle completes the energy-yielding

oxidation of organic molecules

If molecular O2 is present, pyruvate enters mitochondrion;

enzymes of the citric acid cycle complete the oxidation ofthe organic fuel.


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Before the citric acid cycle can begin

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

to glycolysis

CYTOSOL MITOCHONDRION

NAD

H

+

H+NAD+

2

31

CO2Coenzyme

APyruvate

Acetyle

CoA

SCo

A

C

CH3

O

Transport protein

O

O

O

C

C

CH3

1 Pyruvatescarboxyl group

(-COO-) which is fully

oxidized and has little

chemical energy, is

released as CO2.

2 The remaining 2C fragment isoxidized to acetate.

An enzymetransfers the extracted

electrons to NAD+, storing energy in

the form of NADH.

3 Coenzyme A (sulfur-containing compound derived

from a B vitamin) is attached to

the acetate by an unstable bond.

This acetyl

group, very

reactive.

Acetyl CoA is

now ready to

feed its acetyl

group into thecitric acid

cycle for

further

oxidation.


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An overview of the citric acid cycle

ATP

2 CO2

3 NAD+

3 NADH

+ 3 H+

ADP + Pi

FAD

FADH2

Citric

acid

cycle

CoA

CoA

Acetyle CoA

NADH

+ 3 H+

CoA

CO2

Pyruvate

(from glycolysis,

2 molecules per glucose)ATP ATP ATP

Glycolysis Citricacidcycle

Oxidative

phosphorylation

The cycle generates 1 ATP per turn by

substrate-level phosphorylation, but

most of the chemical energy is

transferred to NAD+ and the related

coenzyme FAD during redox

reactions.

The reduced coenzymes, NADH and

FADH2, shuttle their high-energy

electrons to the ETC.

Summary of the inputs and outputs as pyruvate is broken

down to 3 CO2 molecules, including the molecule of CO2

released

A l l k t th it i id l


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Figure 9.12

Acetyl CoA

NADH

Oxaloacetate

CitrateMalate

Fumarate

Succinate

Succinyl

CoA

-Ketoglutarate

Isocitrate

Citric

acid

cycle

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 CH

COO

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 Oxidative

phosphorylation

NAD+

+ H+

ATP

Citric

acid

cycle

A closer look at the citric acid cycle


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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


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The Pathway of Electron Transport

In the electron transport chain

Electrons from NADH and FADH2 lose energy in

several steps


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At the end of the chain

Electrons are passed to

oxygen, forming water

H2O

O2

NADH

FADH2

FMN

FeS FeS

FeS

O

FAD

Cyt b

Cyt c1

Cyt c

Cyt a

Cyt a3

2 H + + 12

I

II

III

IV

Multiprotein

complexes

0

10

20

30

40

50

Free

energy(G)relativetoO

2(kcl/mo

l)


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Chemiosmosis: The Energy-Coupling Mechanism

ATP synthase

Is the enzyme thatactually makes ATP

INTERMEMBRANE SPACE

H+

H+

H+

H+

H+

H+H+

H+

P i

+

ADP

ATP

A rotorwithin themembrane spins

clockwise when

H+ flows past

it down the H+

gradient.

A statoranchored

in the membrane

holds the knobstationary.

A rod (for 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.

MITOCHONDRIAL MATRIX


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At certain steps along the electron transport chain

Electron transfer causes protein complexes to pumpH+ from the mitochondrial matrix to the intermembrane

space

The resulting H+ gradient

Stores energy 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


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Chemiosmosis and the electron transport chain

Oxidative

phosphorylation.

electron transport

and chemiosmosis

Glycolysis

ATP ATP ATP

InnerMitochondrial

membrane

H+

H+H+

H+

H+

ATPP i

Protein complex

of electron

carners

Cyt c

I

II

III

IV

(Carrying electrons

from, food)

NADH+

FADH2

NAD+FAD+ 2 H+ +1/2 O2 H

2O

ADP +

Electron transport chain

Electron transport and pumping of protons (H+),

which create an H+ gradient across the membrane

Chemiosmosis

ATP synthesis powered by the flow

Of H+ back across the membrane

ATP

synthase

Q

Oxidative phosphorylation

Intermembrane

space

Inner

mitochondrialmembrane

Mitochondrial

matrix


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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


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Electron shuttles

span membraneCYTOSOL 2 NADH

2 FADH2

2 NADH 6 NADH 2 FADH22 NADH

Glycolysis

Glucose

2

Pyruvate

2

Acetyl

CoA

Citric

acid

cycle

Oxidative

phosphorylation:

electron transport

andchemiosmosis

MITOCHONDRION

by substrate-level

phosphorylation

by substrate-level

phosphorylation

by oxidative phosphorylation, depending

on which shuttle transports electrons

from NADH in cytosol

Maximum per glucose:About

36 or 38 ATP

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

or


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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

f


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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 ofwhich releases CO2

During lactic acid fermentation

Pyruvate is reduced directly to NADH to form lactate

as a waste product


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2 ADP + 2 P1 2 ATP

GlycolysisGlucose

2 NAD+ 2 NADH

2 Pyruvate

2 Acetaldehyde2 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


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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


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Pyruvate is a key juncture in catabolism

Glucose

CYTOSOL

Pyruvate

No O2 present

Fermentation

O2 present


Ethanol

or

lactate

Acetyl CoA

MITOCHONDRION

Citric

acid

cycle


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The Evolutionary Significance of Glycolysis

Glycolysis

Occurs in nearly all organisms

Probably evolved in ancient prokaryotes before there

was oxygen in the atmosphere


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Glycolysis and the citric acid cycle connect to many

other metabolic pathways

The Versatility of Catabolism

Catabolic pathways

Funnel electrons from many kinds of organicmolecules into cellular respiration

The catabolism of various molecules from food


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Amino

acids

Sugars Glycerol Fatty

acids

Glycolysis

Glucose

Glyceraldehyde-3- P

Pyruvate

Acetyl CoA

NH3

Citric

acid

cycle

Oxidative

phosphorylation

FatsProteins Carbohydrates

Bi th i (A b li P th )


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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


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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 respiration


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The control of cellular respiration

Glucose

Glycolysis

Fructose-6-phosphate

Phosphofructokinase

Fructose-1,6-bisphosphateInhibits Inhibits

Pyruvate

ATPAcetyl CoA

Citric

acid

cycle

Citrate

Stimulates

AMP

+

chap9 cellular respiration

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