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Cellular Respiration Chapter 9

You should be able to:

1. Explain in general terms how redox reactions

are involved in energy exchanges

2. Name the three stages of cellular respiration;

for each, state the region of the eukaryotic

cell where it occurs, the products that result,

& the major intermediate steps

3. In general terms, explain the role of the

electron transport chain in cellular respiration

4. Explain where and how the respiratory

electron transport chain creates a proton

gradient

5. Distinguish between fermentation and

anaerobic respiration

6. Distinguish between obligate and facultative

anaerobes

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o Energy flows into an ecosystem as sunlight

and leaves as heat

o Photosynthesis (Chapter 10) generates O2

and organic molecules, which are used in

Cellular Respiration

o Cells use chemical energy stored in organic

molecules to regenerate ATP, which powers

work

Fig. 9-2

Light energy

ECOSYSTEM

Photosynthesis in chloroplasts

CO2 + H2O

Cellular respiration in mitochondria

Organic molecules

+ O2

ATP powers most cellular work

Heat energy

ATP

Catabolic Pathways & Production of ATP

o The breakdown of organic molecules is exergonic

o Aerobic respiration consumes organic molecules & O2 and yields ATP

o Anaerobic respiration is similar to aerobic respiration, but consumes compounds other than O2

o Fermentation is a partial degradation of sugars that occurs without O2

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o Cellular respiration includes both aerobic

and anaerobic respiration, but is most often

used to refer to aerobic respiration

o Although carbohydrates, fats, and proteins

are all consumed as fuel, it is helpful to

trace cellular respiration with the sugar

Glucose:

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

The Principle of Redox o Chemical reactions that transfer electrons

between reactants are called oxidation-reduction

reactions, or redox reactions

o In oxidation, a substance loses electrons, or is oxidized

o In reduction, a substance gains electrons, or is

reduced (the amount of positive (+) charge is

reduced)

LEO goes GER OIL RIG

Losing Electrons is Oxidation Oxidation Is Loss

Gaining Electrons is Reduction Reduction Is Gain

Fig. 9-UN1

becomes oxidized (loses electron)

becomes reduced (gains electron)

becomes oxidized

becomes reduced

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o The electron donor (Na, Xe-) is called the reducing agent (causes reduction, undergoes oxidation, loses electrons)

o The electron receptor (Cl, Y) is called the oxidizing agent (causes oxidation, undergoes reduction, gains electrons)

Redox: Electron Position

o Some redox reactions do NOT transfer electrons, but change the electron sharing in covalent bonds

oChange position in relation to the nuclei

o An example is the reaction between methane and O2

Fig. 9-3

Reactants

becomes oxidized

becomes reduced

Products

Methane (reducing

agent)

Oxygen (oxidizing

agent)

Carbon dioxide Water

When electrons are closer to electronegative nuclei, they have

less free energy

So when electrons are transferred to a more electronegative atom

(say, from C to O) energy is released-Exergonic Reaction

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Oxidation of Organic Fuel Molecules

During Cellular Respiration

During cellular respiration, the fuel (ex. the C in

glucose) is oxidized, and O2 is reduced:

becomes oxidized

becomes reduced

o In cellular respiration, glucose and other organic

molecules are broken down in a series of steps

o Electrons (with protons) from organic compounds are

usually first transferred to NAD+, a coenzyme

o As an electron acceptor, NAD+ is an oxidizing agent

during cellular respiration

o Each NADH (the reduced form of NAD+) represents stored

energy that is tapped to synthesize ATP

o NAD+ w/o extra e-, NADH w/ extra e-

e-

e-

NAD+

Stepwise Energy Harvest via NAD+ & the

Electron Transport Chain

NADH

e-

e-

NADH

NAD+

NAD+

Hi! I am NAD+

(I have 1 more

proton than

electron)

Hi! I am glucose

and I am going to

donate 2 e-s and 1 H+ to NAD+

NADH

Hi! I am NADH! I

am the reduced

form of NAD+ and

will carry my e-s to the ETC!

Dehydrogenase

The Electron Taxi Cab:

NAD+ to NADH

1 H+ drifts

away

Glucose total loss in the

presence of dehydrogenase

= 2 hydrogen atoms

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Fig. 9-4

Dehydrogenase

Reduction of NAD+

Oxidation of NADH

2 e + 2 H+ 2 e + H+

NAD+ + 2[H]

NADH

+

H+

H+

Nicotinamide (oxidized form)

Nicotinamide (reduced form)

Dehydrogenase

2 Electrons & Protons from Glucose

2 es & 1 H+ to NAD+

1 H+ drifts away

Dehydrogenase removes a pair of H

atoms (2 es and 2 protons) from glucose, oxidizing it. The enzyme

delivers the 2 es along w/ 1 proton to NAD+ and the other H+ is released as a

H+ ion

o NADH passes the electrons to the electron

transport chain

o Unlike an uncontrolled reaction, the electron

transport chain passes electrons in a series of

steps instead of one explosive reaction

o O2 pulls electrons down the chain in an energy-

yielding tumble

o The energy is used to regenerate ATP

Fig. 9-5

(a) Uncontrolled reaction

H2 + 1/2 O2

Explosive release of

heat and light energy

(b) Cellular respiration

Controlled release of energy for

synthesis of ATP

2 H+ + 2 e

2 H 1/2 O2

(from food via NADH)

1/2 O2 How we power

the space shuttle Same reaction but

occurs in stages

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Overall e- movement in cellular respiration

GlucoseNADHelectron transport chainO2

The Stages of Cellular Respiration: A Preview

o Cellular respiration has three stages:

o 1. Glycolysis (breaks down 1 glucose into 2

molecules of pyruvate)

o 2. The Citric Acid Cycle (completes the

breakdown of glucose)

o 3. Oxidative phosphorylation (accounts for

most of the ATP synthesis)

Mitochondrion

Substrate-level

phosphorylation

ATP

Cytosol

Glucose Pyruvate

Glycolysis

Electrons

carried

via NADH

Substrate-level

phosphorylation

ATP

Electrons carried

via NADH and

FADH2

Oxidative

phosphorylation

ATP

Citric

acid

cycle

Oxidative

phosphorylation:

electron transport

and

chemiosmosis

PLEASE NOTE WHERE EACH OCCURS

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o The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions

o Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration

o A smaller amount of ATP is formed in glycolysis & the citric acid cycle by substrate-level phosphorylation

oBioFlix: Cellular Respiration

Enzyme

ADP

P

Substrate

Enzyme

ATP +

Product

Substrate-Level Phosphorylation

oSome ATP is made by direct transfer of a phosphate group from

an organic substrate with more energy than ATP has to ADP, by

an enzyme

oThe ATP is more stable than the original molecule, so this is

spontaneous

oVersus oxidative phosphorylation which adds an inorganic

phosphate to ADP

Glycolysis harvests chemical energy

by oxidizing Glucose Pyruvate

o Glycolysis (splitting of sugar) breaks down Glucose into 2 molecules of Pyruvate

o Glycolysis occurs in the cytoplasm (cytosol) and

has two major phases:

1. Energy investment phase

ATP is used to phosphorylate Glucose (2x)

2. Energy payoff phase

Substrate-Level Phosphorylation (4x)

o Glycolysis occurs whether or not O2 is present

Glucose (6 C) 2 Pyruvate (3 C) + 2 ATP (net)+ 2 NADH

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Fig. 9-8

Energy investment phase

Glucose

2 ADP + 2 P 2 ATP used

formed 4 ATP

Energy payoff phase

4 ADP + 4 P

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

2 Pyruvate + 2 H2O

2 Pyruvate + 2 H2O Glucose Net

4 ATP formed 2 ATP used 2 ATP

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

Figure 9.9a

Glycolysis: Energy Investment Phase

ATP Glucose Glucose 6-phosphate

ADP

Hexokinase

1

Fructose 6-phosphate

Phosphogluco-

isomerase

2

Figure 9.9b

Glycolysis: Energy Investment Phase

ATP Fructose 6-phosphate

ADP

3

Fructose 1,6-bisphosphate

Phospho-

fructokinase

4

5

Aldolase

Dihydroxyacetone phosphate

Glyceraldehyde 3-phosphate

To step 6

Isomerase

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Figure 9.9c

Glycolysis: Energy Payoff Phase

2 NADH 2 ATP

2 ADP 2

2

2 NAD + 2 H

2 P i

3-Phospho-

glycerate

1,3-Bisphospho-

glycerate

Triose

phosphate

dehydrogenase

Phospho-

glycerokinase

6 7

Figure 9.9d

Glycolysis: Energy Payoff Phase

2 ATP

2 ADP 2 2 2 2

2 H2O

Pyruvate Phosphoenol-

pyruvate (PEP)

2-Phospho-

glycerate

3-Phospho-

glycerate

8 9

10

Phospho-

glyceromutase

Enolase Pyruvate

kinase

Preparation of Pyruvate for the

Citric Acid Cycle

CYTOSOL MITOCHONDRION

NAD+ NADH + H+

2

1 3

Pyruvate

Transport protein

CO2 Coenzyme A

Acetyl CoA

o In the presence of O2, pyruvate enters the

mitochondrion

o Before the citric acid cycle can begin, pyruvate

must be converted to acetyl CoA, which links the

cycle to glycolysis (note loss of CO2)

1. Pyruvates COO- which

is fully oxidized is

removed and given off

as CO2

2. 2C fragment is oxidized

forming acetate and e-

are transferred to

NAD+

3. CoA attaced to acetate

forming acetyl CoA

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o The citric acid cycle, (Krebs cycle), takes place within the mitochondrial matrix

o Oxidizes organic fuel derived from pyruvate

o Generates: 1 ATP, 3 NADH, and 1 FADH2 per turn (per Acetyl CoA)

Pyruvate

NAD+

NADH + H+

Acetyl CoA

CO2

CoA

CoA

CoA

Citric acid cycle

FADH2

FAD

CO2 2

3

3 NAD+

+ 3 H+

ADP +

P i

ATP

NADH

The Citric Acid Cycle completes the energy-

yielding oxidation of organic molecules

o The citric acid cycle has 8 steps, each catalyzed

by a specific enzyme

o The acetyl group of acetyl CoA joins the cycle by

combining with oxaloacetate, forming citrate

o The next 7 steps decompose the citrate back to

oxaloacetate, making the process a cycle

o The NADH and FADH2 (from FAD) produced by

the cycle relay electrons extracted from food to the

electron transport chain

Figure 9.12-1

1

Acetyl CoA

Citrate

Citric

acid

cycle

CoA-SH

Oxaloacetate

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Figure 9.12-2

1

Acetyl CoA

Citrate Isocitrate

Citric

acid

cycle

H2O

2

CoA-SH

Oxaloacetate

Figure 9.12-3

1

Acetyl CoA

Citrate Isocitrate

-Ketoglutarate

Citric

acid

cycle

NADH

+ H

NAD

H2O

3

2

CoA-SH

CO2

Oxaloacetate

Figure 9.12-4

1

Acetyl CoA

Citrate Isocitrate

-Ketoglutarate

Succinyl

CoA

Citric

acid

cycle

NADH

NADH

+ H

+ H

NAD

NAD

H2O

3

2

4

CoA-SH

CO2

CoA-SH

CO2

Oxaloacetate

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Figure 9.12-5

1

Acetyl CoA

Citrate Isocitrate

-Ketoglutarate

Succinyl

CoA

Succinate

Citric

acid

cycle

NADH

NADH

ATP

+ H

+ H

NAD

NAD

H2O

ADP

GTP GDP

P i

3

2

4

5

CoA-SH

CO2

CoA-SH

CoA-SH

CO2

Oxaloacetate

Figure 9.12-6

1

Acetyl CoA

Citrate Isocitrate

-Ketoglutarate

Succinyl

CoA

Succinate

Fumarate

Citric

acid

cycle

NADH

NADH

FADH2

ATP

+ H

+ H

NAD

NAD

H2O

ADP

GTP GDP

P i

FAD

3

2

4

5

6

CoA-SH

CO2

CoA-SH

CoA-SH

CO2

Oxaloacetate

Figure 9.12-7

1

Acetyl CoA

Citrate Isocitrate

-Ketoglutarate

Succinyl

CoA

Succinate

Fumarate

Malate

Citric

acid

cycle

NADH

NADH

FADH2

ATP

+ H

+ H

NAD

NAD

H2O

H2O

ADP

GTP GDP

P i

FAD

3

2

4

5

6

7

CoA-SH

CO2

CoA-SH

CoA-SH

CO2

Oxaloacetate

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Figure 9.12-8

NADH

1

Acetyl CoA

Citrate Isocitrate

-Ketoglutarate

Succinyl

CoA

Succinate

Fumarate

Malate

Citric

acid

cycle

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

We are through with the Glucose molecule

But.

Remember all those high-energy electrons taken by NAD+ & FAD?

During Oxidative Phosphorylation,

Chemiosmosis couples electron

transport to ATP synthesis

o NADH and FADH2 account for most of the

energy extracted from food

o These two electron carriers donate electrons

to the electron transport chain, which

powers ATP synthesis via oxidative

phosphorylation

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The Pathway of Electron

Transport

o The electron transport chain is in the cristae

of the mitochondrion

o Most of the chains components are proteins, which exist in multiprotein complexes

o The carriers alternate reduced and oxidized

states as they accept & donate electrons

o Electrons drop in free energy as they go

down the chain and are finally passed to O2,

forming H2O

o Electrons are transferred from

NADH or FADH2 (from

glycolysis and citric acid cycle)

to the electron transport chain

o Electrons are passed through a

number of proteins including

cytochromes (each with an iron

atom) to O2

o The electron transport chain

generates no ATP

oThe chains function is to break the large free-energy drop from food to O2 into smaller steps that release

energy in manageable amounts

Chemiosmosis:

The Energy-Coupling Mechanism

o Electron transfer in the electron

transport chain causes proteins to pump

H+ (protons) from the matrix intermembrane space (uphill)

o H+ then moves back across the

membrane (downhill), passing through channels in ATP synthase

o ATP synthase uses the exergonic flow

of H+ to drive phosphorylation of ATP

o This is an example of chemiosmosis,

the use of energy in a H+ gradient to

drive cellular work

INTERMEMBRANE SPACE

Rotor

H+ Stator

Internal rod

Cata- lytic knob

ADP +

P ATP i

MITOCHONDRIAL MATRIX

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o The energy stored in a H+ gradient across a

membrane couples the redox reactions of

the electron transport chain to ATP

synthesis

o The H+ gradient is referred to as a

proton-motive force, emphasizing its

capacity to do work

Protein complex of electron carriers

H+

H+ H+

Cyt c

Q

V

FADH2 FAD

NAD+ NADH

(carrying electrons from food)

Electron transport chain

2 H+ + 1/2O2 H2O

ADP + P i

Chemiosmosis

Oxidative phosphorylation

H+

H+

ATP

synthase

ATP

2 1

Notice flow of e- from NADH and FADH2 to H2O

1.Electron transport and pumping of the protons (H+) create H+ gradient across the

membrane

2.ATP synthesis powered by flow of H+ back across the membrane

Accounting of ATP Production by

Cellular Respiration

o During cellular respiration, most energy

flows in this sequence:

glucose NADH electron transport

chain proton-motive force ATP

o About 34% of the energy in a glucose

molecule is transferred to ATP during

cellular respiration, making about 32 ATP

o There are several reasons why the number

of ATP is not known exactly

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Figure 9.16

Electron shuttles span membrane

MITOCHONDRION 2 NADH

2 NADH 2 NADH 6 NADH

2 FADH2

2 FADH2

or

2 ATP 2 ATP about 26 or 28 ATP

Glycolysis

Glucose 2 Pyruvate

Pyruvate oxidation

2 Acetyl CoA

Citric acid cycle

Oxidative phosphorylation: electron transport

and chemiosmosis

CYTOSOL

Maximum per glucose: About

30 or 32 ATP

Fermentation & Anaerobic Respiration

enable ATP without oxygen

o Most cellular respiration requires O2 to

produce ATP

o Glycolysis can produce ATP with or without

O2 (in aerobic or anaerobic conditions)

o In the absence of O2, glycolysis couples with

fermentation or anaerobic respiration to

produce ATP

Anaerobic respiration uses an electron

transport chain with a different electron

acceptor (other than O2).

Example: Sulfate (SO42-)

Fermentation uses phosphorylation instead

of an electron transport chain to generate

ATP

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Types of Fermentation

There must be a place to drop off electrons from NADH, or all the NAD+ will be rapidly used up

o Fermentation = Glycolysis + reactions that regenerate NAD+, which can be reused by glycolysis

o 2 common types are alcohol fermentation and lactic acid fermentation

Animation: Fermentation Overview

o Alcohol fermentation--Pyruvate is converted to

ethanol in two steps, with the first releasing CO2

o Alcohol fermentation by yeast is used in brewing,

winemaking, and baking

o Lactic acid fermentation--Pyruvate is reduced

directly, forming lactate, with no release of CO2

oLactic acid fermentation by some fungi and bacteria is used to

make cheese and yogurt.

oHuman muscle cells use lactic acid fermentation to generate ATP

when O2 is scarce.

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Fermentation and Aerobic

Respiration Compared

o All use glycolysis (net ATP = 2) to oxidize glucose and harvest chemical energy of food

o In all three, NAD+ is the oxidizing agent that accepts electrons during glycolysis

o The processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O2 in cellular respiration

o Cellular respiration produces 32 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule

o Obligate anaerobes carry out fermentation

or anaerobic respiration and cannot survive

in the presence of O2

o Yeast and many bacteria are facultative

anaerobes--they can survive using either

fermentation or cellular respiration

Glucose

Glycolysis

Pyruvate

CYTOSOL

No O2 present:

Fermentation

O2 present:

Aerobic cellular

respiration

MITOCHONDRION

Acetyl CoA Ethanol or

lactate

Citric acid cycle

In a

facultative

anaerobe,

pyruvate is

a fork in the

metabolic

road that

leads to two

alternative

catabolic

routes

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The Evolutionary Significance of

Glycolysis

o Glycolysis occurs in nearly all organisms

o Glycolysis probably evolved in ancient

prokaryotes before there was oxygen in the

atmosphere

o Very little O2 was available in the

atmosphere until about 2.7 billion years ago,

so early prokaryotes likely used only

glycolysis to generate ATP

o Glycolysis is a very ancient process

Glycolysis & the Citric Acid Cycle

connect to many other metabolic

pathways

o Gycolysis & the Citric Acid Cycle are major intersections to various catabolic and anabolic pathways

oMany other Macromolecules can feed into these pathways

oYou dont eat only Glucose

oMany things the cell and/or body needs are originally part of these pathways (or can be made from a chemical which is part of these pathways)

oBut are shuttled off before the next bioenergetic step

The Versatility of Catabolism

o Catabolic pathways funnel electrons from

many kinds of organic molecules into cellular

respiration

o Carbs: Glycolysis accepts a wide range of

carbohydrates

o Proteins must be digested to amino acids;

amino groups can feed glycolysis or the citric

acid cycle

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o Fats are digested to glycerol (used in glycolysis)

and fatty acids (used in generating acetyl CoA)

o Fatty acids are broken down by beta oxidation

and yield acetyl CoA

o An oxidized gram of fat produces more than twice

as much ATP as an oxidized gram of carbohydrate

Carbohydrate = 4 calories/gram

Fat = 9 calories/gram

Why it is so much harder to lose weight eating a high fat so much stored energy

Fig. 9-20

Proteins Carbohydrates

Amino acids

Sugars

Fats

Glycerol Fatty acids

Glycolysis

Glucose

Glyceraldehyde-3-

Pyruvate

P

NH3

Acetyl CoA

Citric acid cycle

Oxidative phosphorylation

Biosynthesis (Anabolic

Pathways)

o The body uses small molecules to build

other substances

o These small molecules may come directly

from food, from glycolysis, or from the citric

acid cycle

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Regulation of Cellular Respiration via

Feedback Mechanisms

o Feedback inhibition is the most common

mechanism for control

o If ATP concentration begins to drop,

respiration speeds up; when there is plenty

of ATP, respiration slows down

o Control of catabolism is based mainly on

regulating the activity of enzymes at

strategic points in the catabolic pathway

Fig. 9-21 Glucose

Glycolysis

Fructose-6-phosphate

Phosphofructokinase

Fructose-1,6-bisphosphate Inhibits

AMP

Stimulates

Inhibits

Pyruvate

Citrate Acetyl CoA

Citric

acid cycle

Oxidative

phosphorylation

ATP

+

Figure 9.UN06

Inputs Outputs

Glucose

Glycolysis

2 Pyruvate 2 ATP 2 NADH

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Figure 9.UN07

Inputs Outputs

2 Pyruvate 2 Acetyl CoA

2 Oxaloacetate Citric

acid

cycle

2

2 6

8 ATP NADH

FADH2 CO2