Cellular Respiration and FermentationFigure 9.UN05 1. Glycolysis (color-coded teal throughout the...

Cellular Respiration and Fermentation

لجنة الطب البشري

Overview: Life Is Work

• Living cells require energy from outside

sources

• Some animals, such as the chimpanzee, obtain

energy by eating plants, and some animals

feed on other organisms that eat plants

© 2011 Pearson Education, Inc.

Figure 9.1

• Energy flows into an ecosystem as sunlight

and leaves as heat

• Photosynthesis generates O2 and organic

molecules, which are used in cellular

respiration

• Cells use chemical energy stored in organic

molecules to regenerate ATP, which powers

work


Figure 9.2

Lightenergy

ECOSYSTEM

Photosynthesisin chloroplasts

Cellular respirationin mitochondria

CO2 H2O O2Organic

molecules

ATP powersmost cellular work

ATP

Heatenergy

Concept 9.1: Catabolic pathways yield

energy by oxidizing organic fuels

• Several processes are central to cellular

respiration and related pathways


Catabolic Pathways and Production of ATP

• The breakdown of organic molecules is exergonic

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

• Aerobic respiration consumes organic molecules and O2 and yields ATP

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


• Cellular respiration includes both aerobic and

anaerobic respiration but is often used to refer to

aerobic respiration

• 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)


Redox Reactions: Oxidation and Reduction

• The transfer of electrons during chemical

reactions releases energy stored in organic

molecules

• This released energy is ultimately used to

synthesize ATP


The Principle of Redox

• Chemical reactions that transfer electrons

between reactants are called oxidation-reduction

reactions, or redox reactions

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

• In reduction, a substance gains electrons, or is

reduced (the amount of positive charge is

reduced)


Figure 9.UN01

becomes oxidized

(loses electron)

becomes reduced

(gains electron)

Figure 9.UN02

becomes oxidized

becomes reduced

• The electron donor is called the reducing agent

• The electron receptor is called the oxidizing agent

• Some redox reactions do not transfer electrons but change the electron sharing in covalent bonds

• An example is the reaction between methane and O2


Figure 9.3

Reactants Products

Energy

WaterCarbon dioxideMethane(reducing

agent)

Oxygen(oxidizing

agent)

becomes oxidized

becomes reduced

Oxidation of Organic Fuel Molecules During

Cellular Respiration

• During cellular respiration, the fuel (such as

glucose) is oxidized, and O2 is reduced


Figure 9.UN03

becomes oxidized

becomes reduced

Stepwise Energy Harvest via NAD+ and

the Electron Transport Chain

• In cellular respiration, glucose and other organic

molecules are broken down in a series of steps

• Electrons from organic compounds are usually

first transferred to NAD+, a coenzyme

• As an electron acceptor, NAD+ functions as an

oxidizing agent during cellular respiration

• Each NADH (the reduced form of NAD+)

represents stored energy that is tapped to

synthesize ATP


Figure 9.4

Nicotinamide(oxidized form)

NAD

(from food)

Dehydrogenase

Reduction of NAD

Oxidation of NADH

Nicotinamide(reduced form)

NADH

Figure 9.UN04

Dehydrogenase

• NADH passes the electrons to the electron

transport chain

• Unlike an uncontrolled reaction, the electron

transport chain passes electrons in a series of

steps instead of one explosive reaction

• O2 pulls electrons down the chain in an energy-

yielding tumble

• The energy yielded is used to regenerate ATP


Figure 9.5

(a) Uncontrolled reaction (b) Cellular respiration

Explosiverelease of

heat and lightenergy

Controlledrelease ofenergy for

synthesis ofATP

Fre

e e

nerg

y, G

Fre

e e

nerg

y, G

H2 1/2 O2 2 H

1/2 O2

1/2 O2

H2O H2O

2 H+ 2 e

2 e

2 H+

ATP

ATP

ATP

(from food via NADH)

The Stages of Cellular Respiration:

A Preview

• Harvesting of energy from glucose has three

stages

– Glycolysis (breaks down glucose into two

molecules of pyruvate)

– The citric acid cycle (completes the

breakdown of glucose)

– Oxidative phosphorylation (accounts for

most of the ATP synthesis)


Figure 9.UN05

Glycolysis (color-coded teal throughout the chapter)1.

Pyruvate oxidation and the citric acid cycle

(color-coded salmon)

2.

Oxidative phosphorylation: electron transport and

chemiosmosis (color-coded violet)

3.

Figure 9.6-1

Electrons

carried

via NADH

Glycolysis

Glucose Pyruvate

CYTOSOL MITOCHONDRION

ATP

Substrate-level

phosphorylation

Figure 9.6-2

Electrons

carried

via NADH

Electrons carried

via NADH and

FADH2

Citric

acid

cycle

Pyruvate

oxidation

Acetyl CoA

Glycolysis

Glucose Pyruvate


ATP ATP

Substrate-level

phosphorylationSubstrate-level

phosphorylation

Figure 9.6-3

Electrons

carried

via NADH

Electrons carried

via NADH and

FADH2

Citric

acid

cycle

Pyruvate

oxidation

Acetyl CoA

Glycolysis

Glucose Pyruvate

Oxidative

phosphorylation:

electron transport

and

chemiosmosis


ATP ATP ATP

Substrate-level

phosphorylationSubstrate-level

phosphorylation

Oxidative

phosphorylation

• The process that generates most of the ATP is

called oxidative phosphorylation because it is

powered by redox reactions


BioFlix: Cellular Respiration

09_06CellularRespiration.mpg

• Oxidative phosphorylation accounts for almost

90% of the ATP generated by cellular

respiration

• A smaller amount of ATP is formed in glycolysis

and the citric acid cycle by substrate-level

phosphorylation

• For each molecule of glucose degraded to CO2 and water by respiration, the cell makes up to

32 molecules of ATP


Figure 9.7

Substrate

Product

ADP

P

ATP

Enzyme Enzyme

Concept 9.2: Glycolysis harvests chemical

energy by oxidizing glucose to pyruvate

• Glycolysis (“splitting of sugar”) breaks down

glucose into two molecules of pyruvate

• Glycolysis occurs in the cytoplasm and has two

major phases

– Energy investment phase

– Energy payoff phase

• Glycolysis occurs whether or not O2 is present


Figure 9.8

Energy Investment Phase

Glucose

2 ADP 2 P

4 ADP 4 P

Energy Payoff Phase

2 NAD+ 4 e 4 H+

2 Pyruvate 2 H2O

2 ATP used

4 ATP formed

2 NADH 2 H+

NetGlucose 2 Pyruvate 2 H2O

2 ATP

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

4 ATP formed 2 ATP used

Figure 9.9-1

Glycolysis: Energy Investment Phase

ATPGlucose Glucose 6-phosphate

ADP

Hexokinase

1

Figure 9.9-2


ATPGlucose Glucose 6-phosphate Fructose 6-phosphate

ADP

Hexokinase Phosphogluco-

isomerase1

2

Figure 9.9-3


ATP ATPGlucose Glucose 6-phosphate Fructose 6-phosphate Fructose 1,6-bisphosphate

ADP ADP


isomerasePhospho-

fructokinase1

2 3

Figure 9.9-4


ATP ATPGlucose Glucose 6-phosphate Fructose 6-phosphate Fructose 1,6-bisphosphate

Dihydroxyacetonephosphate

Glyceraldehyde3-phosphate

Tostep 6

ADP ADP


isomerasePhospho-

fructokinase

Aldolase

Isomerase

12 3 4

5

Figure 9.9-5

Glycolysis: Energy Payoff Phase

2 NADH

2 NAD + 2 H

2 P i

1,3-Bisphospho-glycerate6

Triose

phosphate

dehydrogenase

Figure 9.9-6


2 ATP2 NADH

2 NAD + 2 H

2 P i

2 ADP

1,3-Bisphospho-glycerate

3-Phospho-glycerate

2

Phospho-

glycerokinase

67

Triose

phosphate

dehydrogenase

Figure 9.9-7


2 ATP2 NADH

2 NAD + 2 H

2 P i

2 ADP


3-Phospho-glycerate

2-Phospho-glycerate

2 2

Phospho-

glycerokinase

Phospho-

glyceromutase

67 8

Triose

phosphate

dehydrogenase

Figure 9.9-8


2 ATP2 NADH

2 NAD + 2 H

2 P i

2 ADP


3-Phospho-glycerate

2-Phospho-glycerate

Phosphoenol-pyruvate (PEP)

2 2 2

2 H2O

Phospho-

glycerokinase

Phospho-

glyceromutaseEnolase

67 8

9

Triose

phosphate

dehydrogenase

Figure 9.9-9


2 ATP 2 ATP

2 NADH

2 NAD + 2 H

2 P i

2 ADP


3-Phospho-glycerate

2-Phospho-glycerate

Phosphoenol-pyruvate (PEP)

Pyruvate

2 ADP2 2 2

2 H2O

Phospho-

glycerokinase

Phospho-

glyceromutaseEnolase Pyruvate

kinase

67 8

910

Triose

phosphate

dehydrogenase

Figure 9.9a


ATPGlucose Glucose 6-phosphate

ADP

Hexokinase

1

Fructose 6-phosphate

Phosphogluco-

isomerase

2

Figure 9.9b


ATPFructose 6-phosphate

ADP

3

Fructose 1,6-bisphosphate

Phospho-

fructokinase

4

5

Aldolase

Dihydroxyacetonephosphate

Glyceraldehyde3-phosphate

Tostep 6

Isomerase

Figure 9.9c


2 NADH2 ATP

2 ADP 2

2

2 NAD + 2 H

2 P i

3-Phospho-

glycerate1,3-Bisphospho-

glycerate

Triose

phosphate

dehydrogenase

Phospho-

glycerokinase

67

Figure 9.9d


2 ATP

2 ADP2222

2 H2O

PyruvatePhosphoenol-

pyruvate (PEP)2-Phospho-

glycerate

3-Phospho-

glycerate8

910

Phospho-

glyceromutaseEnolase Pyruvate

kinase

Concept 9.3: After pyruvate is oxidized, the

citric acid cycle completes the energy-

yielding oxidation of organic molecules

• In the presence of O2, pyruvate enters the

mitochondrion (in eukaryotic cells) where the

oxidation of glucose is completed


Oxidation of Pyruvate to Acetyl CoA

• Before the citric acid cycle can begin, pyruvate

must be converted to acetyl Coenzyme A

(acetyl CoA), which links glycolysis to the citric

acid cycle

• This step is carried out by a multienzyme

complex that catalyses three reactions


Figure 9.10

Pyruvate

Transport protein

CYTOSOL

MITOCHONDRION

CO2 Coenzyme A

NAD + HNADH Acetyl CoA

1

2

3

• The citric acid cycle, also called the Krebs

cycle, completes the break down of pyruvate

to CO2

• The cycle oxidizes organic fuel derived from

pyruvate, generating 1 ATP, 3 NADH, and 1

FADH2 per turn


The Citric Acid Cycle

Figure 9.11

Pyruvate

NAD

NADH

+ H Acetyl CoA

CO2

CoA

CoA

CoA

2 CO2

ADP + P i

FADH2

FAD

ATP

3 NADH

3 NAD

Citric

acid

cycle

+ 3 H

• The citric acid cycle has eight steps, each

catalyzed by a specific enzyme

• The acetyl group of acetyl CoA joins the cycle

by combining with oxaloacetate, forming citrate

• The next seven steps decompose the citrate

back to oxaloacetate, making the process a

cycle

• The NADH and FADH2 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

Figure 9.12-2

1

Acetyl CoA

CitrateIsocitrate

Citric

acid

cycle

H2O

2

CoA-SH

Oxaloacetate

Figure 9.12-3

1

Acetyl CoA

CitrateIsocitrate

-Ketoglutarate

Citric

acid

cycle

NADH

+ H

NAD

H2O

3

2

CoA-SH

CO2

Oxaloacetate

Figure 9.12-4

1

Acetyl CoA

CitrateIsocitrate

-Ketoglutarate

Succinyl

CoA

Citric

acid

cycle

NADH

NADH

+ H

+ H

NAD

NAD

H2O

3

2

4

CoA-SH

CO2

CoA-SH

CO2

Oxaloacetate

Figure 9.12-5

1

Acetyl CoA

CitrateIsocitrate

-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

CitrateIsocitrate

-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

CitrateIsocitrate

-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

Figure 9.12-8

NADH

1

Acetyl CoA

CitrateIsocitrate

-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

Figure 9.12a

Acetyl CoA

Oxaloacetate

CitrateIsocitrate

H2O

CoA-SH

1

2

Figure 9.12b

Isocitrate

-Ketoglutarate

Succinyl

CoA

NADH

NADH

NAD

NAD

+ H

CoA-SH

CO2

CO2

3

4

+ H

Figure 9.12c

Fumarate

FADH2

CoA-SH6

Succinate

Succinyl

CoA

FAD

ADP

GTP GDP

P i

ATP

5

Figure 9.12d

Oxaloacetate8

Malate

Fumarate

H2O

NADH

NAD

+ H

7

Concept 9.4: During oxidative

phosphorylation, chemiosmosis couples

electron transport to ATP synthesis

• Following glycolysis and the citric acid cycle,

NADH and FADH2 account for most of the

energy extracted from food

• These two electron carriers donate electrons to

the electron transport chain, which powers ATP

synthesis via oxidative phosphorylation


The Pathway of Electron Transport

• The electron transport chain is in the inner

membrane (cristae) of the mitochondrion

• 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

H2O


Figure 9.13

NADH

FADH2

2 H + 1/2 O2

2 e

2 e

2 e

H2O

NAD

Multiprotein

complexes

(originally from

NADH or FADH2)

III

III

IV

50

40

30

20

10

0

Fre

e e

ne

rgy (

G)

rela

tive

to

O2

(kc

al/

mo

l)

FMN

FeS FeS

FAD

Q

Cyt b

Cyt c1

Cyt c

Cyt a

Cyt a3

FeS

• Electrons are transferred from NADH or FADH2to the electron transport chain

• Electrons are passed through a number of

proteins including cytochromes (each with an

iron atom) to O2

• The electron transport chain generates no ATP

directly

• It breaks the large free-energy drop from food

to O2 into smaller steps that release energy in

manageable amounts


Chemiosmosis: The Energy-Coupling

Mechanism

• Electron transfer in the electron transport chain

causes proteins to pump H+ from the

mitochondrial matrix to the intermembrane space

• H+ then moves back across the membrane,

passing through the proton, 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


Figure 9.14

INTERMEMBRANE SPACE

Rotor

StatorH

Internal

rod

Catalytic

knob

ADP

+

P i ATP

MITOCHONDRIAL MATRIX

Figure 9.15

Proteincomplexof electroncarriers

(carrying electronsfrom food)

Electron transport chain

Oxidative phosphorylation

Chemiosmosis

ATPsynth-ase

I

II

III

IVQ

Cyt c

FADFADH2

NADH ADP P iNAD

H

2 H + 1/2O2

H

HH

21

H

H2O

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


An Accounting of ATP Production by

Cellular Respiration

• During cellular respiration, most energy flows

in this sequence:

glucose NADH electron transport chain

proton-motive force ATP

• About 34% of the energy in a glucose molecule

is transferred to ATP during cellular respiration,

making about 32 ATP

• There are several reasons why the number of

ATP is not known exactly


Figure 9.16

Electron shuttlesspan membrane

MITOCHONDRION2 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

Citricacidcycle

Oxidativephosphorylation:electron transport

andchemiosmosis

CYTOSOL

Maximum per glucose:About

30 or 32 ATP

Concept 9.5: Fermentation and anaerobic

respiration enable cells to produce ATP

without the use of oxygen

• Most cellular respiration requires O2 to produce

ATP

• Without O2, the electron transport chain will

cease to operate

• In that case, glycolysis couples with

fermentation or anaerobic respiration to

produce ATP


• Anaerobic respiration uses an electron

transport chain with a final electron acceptor

other than O2, for example sulfate

• Fermentation uses substrate-level

phosphorylation instead of an electron

transport chain to generate ATP


Types of Fermentation

• Fermentation consists of glycolysis plus

reactions that regenerate NAD+, which can be

reused by glycolysis

• Two common types are alcohol fermentation

and lactic acid fermentation


• In alcohol fermentation, pyruvate is converted

to ethanol in two steps, with the first releasing

CO2

• Alcohol fermentation by yeast is used in

brewing, winemaking, and baking


Animation: Fermentation Overview

09_18FermentationOverview_A.html

Figure 9.17

2 ADP 2 ATP

Glucose Glycolysis

2 Pyruvate

2 CO22

2 NADH

2 Ethanol 2 Acetaldehyde

(a) Alcohol fermentation (b) Lactic acid fermentation

2 Lactate

2 Pyruvate

2 NADH

Glucose Glycolysis

2 ATP2 ADP 2 Pi

NAD

2 H

2 Pi

2 NAD

2 H

2 ADP 2 P i 2 ATP

Glucose Glycolysis

2 Pyruvate

2 CO22 NAD

2 NADH

2 Ethanol 2 Acetaldehyde

(a) Alcohol fermentation

2 H

Figure 9.17a

• In lactic acid fermentation, pyruvate is reduced

to NADH, forming lactate as an end product,

with no release of CO2

• Lactic acid fermentation by some fungi and

bacteria is used to make cheese and yogurt

• Human muscle cells use lactic acid

fermentation to generate ATP when O2 is

scarce


(b) Lactic acid fermentation

2 Lactate

2 Pyruvate

2 NADH

Glucose Glycolysis

2 ADP 2 P i 2 ATP

2 NAD

2 H

Figure 9.17b

Comparing Fermentation with Anaerobic

and Aerobic Respiration

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

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

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

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


• Obligate anaerobes carry out fermentation or

anaerobic respiration and cannot survive in the

presence of O2

• Yeast and many bacteria are facultative

anaerobes, meaning that they can survive

using either fermentation or cellular respiration

• In a facultative anaerobe, pyruvate is a fork in

the metabolic road that leads to two alternative

catabolic routes


Figure 9.18

Glucose

CYTOSOLGlycolysis

Pyruvate

No O2 present:

Fermentation

O2 present:

Aerobic cellular

respiration

Ethanol,

lactate, or

other products

Acetyl CoA

MITOCHONDRION

Citric

acid

cycle

The Evolutionary Significance of Glycolysis

• Ancient prokaryotes are thought to have used

glycolysis long before there was oxygen in the

atmosphere

• 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

• Glycolysis is a very ancient process


Concept 9.6: Glycolysis and the citric acid

cycle connect to many other metabolic

pathways

• Gycolysis and the citric acid cycle are major

intersections to various catabolic and anabolic

pathways


The Versatility of Catabolism

• Catabolic pathways funnel electrons from many

kinds of organic molecules into cellular

respiration

• Glycolysis accepts a wide range of

carbohydrates

• Proteins must be digested to amino acids;

amino groups can feed glycolysis or the citric

acid cycle


• Fats are digested to glycerol (used in

glycolysis) and fatty acids (used in generating

acetyl CoA)

• Fatty acids are broken down by beta oxidation

and yield acetyl CoA

• An oxidized gram of fat produces more than

twice as much ATP as an oxidized gram of

carbohydrate


Figure 9.19

CarbohydratesProteins

Fatty

acids

Amino

acids

Sugars

Fats

Glycerol

Glycolysis

Glucose

Glyceraldehyde 3- P

NH3 Pyruvate

Acetyl CoA

Citric

acid

cycle

Oxidative

phosphorylation

Biosynthesis (Anabolic Pathways)

• The body uses small molecules to build other

substances

• These small molecules may come directly

from food, from glycolysis, or from the citric

acid cycle


Regulation of Cellular Respiration via

Feedback Mechanisms

• Feedback inhibition is the most common

mechanism for control

• If ATP concentration begins to drop,

respiration speeds up; when there is plenty

of ATP, respiration slows down

• Control of catabolism is based mainly on

regulating the activity of enzymes at

strategic points in the catabolic pathway


Figure 9.20

Phosphofructokinase

Glucose

GlycolysisAMP

Stimulates

Fructose 6-phosphate

Fructose 1,6-bisphosphate

Pyruvate

Inhibits Inhibits

ATP Citrate

Citric

acid

cycle

Oxidative

phosphorylation

Acetyl CoA

Figure 9.UN06

Inputs Outputs

Glucose

Glycolysis

2 Pyruvate 2 ATP 2 NADH

Figure 9.UN07

Inputs Outputs

2 Pyruvate 2 Acetyl CoA

2 OxaloacetateCitric

acid

cycle

2

26

8ATP NADH

FADH2CO2

Figure 9.UN08

Protein complex

of electron

carriers

(carrying electrons from food)

INTERMEMBRANE

SPACE

MITOCHONDRIAL MATRIX

H

HH

2 H + 1/2 O2 H2O

NAD

FADH2 FAD

Q

NADH

I

II

III

IV

Cyt c

Figure 9.UN09

INTER-

MEMBRANE

SPACE

HADP + P i

MITO-

CHONDRIAL

MATRIX

ATP

synthase

H

ATP

Figure 9.UN10

Time

pH

dif

fere

nce

acro

ss m

em

bra

ne

Figure 9.UN11

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