Chapter 9 Cellular Respiration: Harvesting Chemical Energy Updated October 2008

Chapter 9

Cellular Respiration: Harvesting Chemical Energy

Updated October 2008

Overview: Life Is Work

Living cells require transfusions of energy from outside sources to perform their many tasks

The giant panda...

Obtains energy for its cells by eating plants

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


has three key pathways

1. glycolysis

2. the citric acid cycle (Kreb cycle)

3. oxidative phosphorylation

Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels

Catabolic metabolic pathways release the energy stored in complex

organic molecules The breakdown of organic molecules is

exergonic One catabolic process, fermentation

is a partial degradation of sugars that occurs without O2


A more efficient and widespread catabolic process 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 analogy

Combustion of gasoline in an automobile engine after O2 is mixed with a hydrocarbon fuel Food is the fuel for respiration. The

exhaust is carbon dioxide and water The overall process is:

organic compounds + O2 CO2 + H2O + energy (ATP + heat)


Carbohydrates, fats, and proteins can all be used as the fuel, but it is most useful to consider glucose

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

The catabolism of glucose is exergonic with a G of −686 kcal per mole of glucose

Some of this energy is used to produce ATP, which can perform cellular work

Redox Reactions: Oxidation and Reduction

Catabolic pathways transfer the electrons stored in food molecules releasing energy that is used to make

ATP An electron loses potential energy

when it shifts from a less electronegative atom toward

a more electronegative one

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

Example of a redox reaction

Na + Cl Na+ + Cl–

becomes oxidized(loses electron)

becomes reduced(gains electron)

The formation of table salt from sodium and chloride is a redox reaction

Generally...

Xe− + Y X + Ye−

X, the electron donor is the reducing agent and reduces Y

Y, the electron recipient is the oxidizing agent and oxidizes X

Redox reactions require both a donor and acceptor

Some redox reactions

Do not completely exchange electrons but change the degree of electron

sharing in covalent bonds

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

Oxidizing... O is more electronegative than C CH4

has lost potential energy & has been oxidized into CO2

CH4

H

H

HH

C O O O O OC

H H





becomes oxidized

becomes reduced

Reactants Products

Figure 9.3

CH4

H

H

HH

C O O O O OC

H H





becomes oxidized

becomes reduced

Reactants Products

Figure 9.3

Reducing...

O2 is sharing the electrons, but electrons fall towards O in H2O

Oxygen has been reduced

Oxygen is very electronegative & is one of the most potent of all oxidizing agents

Redox energy transfer

Energy must be added to pull an electron away from an atom The more electronegative the atom, the

more energy is required to take an electron away from it

An electron loses potential energy when it shifts from a less electronegative atom toward a more electronegative one

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

Stepwise energy harvest via NAD+ and the electron transport chain (ETC)

Cellular respiration Oxidizes glucose in a series of steps,

each catalyzed by a specific enzyme. At key steps, electrons are stripped from

the glucose. In many oxidation reactions, the

electron is transferred with a proton, as a hydrogen atom.

Electrons from organic compounds

Are usually first transferred to NAD+, a coenzyme

NAD+

HO

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

NADH, the reduced form of NAD+

Passes the electrons to the electron transport chain

As an electron acceptor, NAD+ functions as an oxidizing agent during cellular respiration

Each NADH molecule formed during respiration represents stored energy This energy is tapped to synthesize ATP

as electrons “fall” from NADH to 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 (ETC)

The ETC consists of several molecules (primarily proteins) built into the inner membrane of a mitochondrion Electrons released from food are shuttled

by NADH to the “top” higher-energy end of the chain

At the “bottom” lower-energy end, oxygen captures the electrons along with H+ to form water

ETC 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

F

ree

ener

gy,

G

(b) Cellular respiration

+

Figure 9.5 B

Electrons are passed to increasingly electronegative molecules in the chain until they reduce O2, the most electronegative receptor

The stages of cellular respiration: A Preview

Respiration occurs in three metabolic stages Glycolysis The citric acid cycle Oxidative phosphorylation

Three Metabolic Stages

Glycolysis occurs in the cytoplasm Breaks down glucose into two molecules

of pyruvate The citric acid cycle in the mitochondrial

matrix Completes the breakdown of glucose to

CO2

Oxidative phosphorylation Is driven by the ETC Generates ATP

Overview of Cellular Respiration

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

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

BioFlix: Cellular RespirationBioFlix: Cellular Respiration

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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


Substrate-level phosphorylation

Both glycolysis & the citric acid cycle Can generate ATP by substrate-level

phosphorylation

Figure 9.7

Enzyme Enzyme

ATP

ADP

Product

SubstrateP

+

Concept 9.2: 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 Can occur whether O2 is present or not

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: Two major phases

Energy investment phase

Energy payoff phase

A closer look at the energy investment phase

Make sure you can describe what’s happening at each phase in glycolysis, page 166-167

Do not need to memorize compounds or enzymes

What are the important inputs & outputs?

A closer look at the energy payoff phase

In the energy payoff phase ATP is produced by substrate-level

phosphorylation & NAD+ is reduced to NADH by electrons released by the oxidation of glucose

The net yield from glycolysis is 2 ATP and 2 NADH per glucose

No CO2 is produced during glycolysis

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

Before the citric acid cycle can begin Pyruvate must first be converted to acetyl

CoA, which links the cycle to glycolysis

Conversion of pyruvate to acetyl CoA

An overview of the citric acid cycle

A closer look at the citric acid cycle

Need to know: All the compounds & where they are in

the cycle All the inputs & outputs What is happening at each phase Be able to answer questions like “How

many times does the cycle have to run to use1 glucose molecule?”

The citric acid cycle oxidizes organic fuel derived from pyruvate

The citric acid cycle has eight steps each catalyzed by a specific enzyme.

The next seven steps decompose the citrate back to oxaloacetate It is the regeneration of oxaloacetate that

makes this process a cycle The cycle generates one ATP per turn

by substrate-level phosphorylation

Citric acid cycle: Energy

Most of the chemical energy is transferred to NAD+ & FAD during the redox reactions

The reduced coenzymes NADH & FADH2 then transfer high-energy electrons to the ETC

Each cycle produces one ATP by substrate-level phosphorylation, three NADH, and one FADH2 per acetyl CoA

Figure 9.12

Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis

• Following glycolysis & 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 electron transport chain is in the 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

The Pathway of Electron Transport

At the end of the chain...

Electrons are passed to oxygen, forming water

Generating energy

The ETC generates no ATP directly Its function is to break the large free

energy drop from food to oxygen into a series of smaller steps that release

energy in manageable amounts How does the mitochondrion couple

electron transport & energy release to ATP synthesis? The answer is a mechanism called

chemiosmosis

Chemiosmosis: The Energy-Coupling Mechanism

ATP synthase Is the enzyme

that actually makes ATP

INTERMEMBRANE SPACE

Rotor

H+

Stator

Internalrod

Cata-lyticknob

ADP+P ATP

i

MITOCHONDRIAL MATRIX

ATP synthesis

ATP uses the energy of an existing proton gradient to power ATP synthesis

The proton gradient develops between the intermembrane space & the matrix The proton gradient is produced by the

movement of electrons along the ETC

The resulting H+ gradient

Stores energy Drives chemiosmosis in ATP synthase Is referred to as a proton-motive force

The gradient has the capacity to do work

Fig. 9-16

Protein complexof electroncarriers

H+

H+H+

Cyt c

Q

V

FADH2 FAD

NAD+NADH

(carrying electronsfrom food)

Electron transport chain

2 H+ + 1/2O2H2O

ADP + P i

Chemiosmosis

Oxidative phosphorylation

H+

H+

ATP synthase

ATP

21

Chemiosmosis – Plants & Prokaryotes

Chemiosmosis in chloroplasts also generates ATP but light drives the electron flow down an

ETC & H+ gradient formation. Prokaryotes generate H+ gradients

across their plasma membrane They can use this proton-motive force not

only to generate ATP, but also to pump nutrients & waste products across the membrane & to rotate their flagella

An Accounting of ATP Production by Cellular Respiration

During respiration, most energy flows in this sequence glucose NADH electron transport

chain proton-motive force 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

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

Most cellular respiration requires O2 to produce ATP

Glycolysis can produce ATP with or without O2 (in aerobic or anaerobic conditions)

In the absence of O2, glycolysis couples with fermentation or anaerobic respiration to produce ATP

Anaerobic respiration vs. fermentation

Anaerobic respiration uses an electron transport chain with an electron acceptor other than O2, for example sulfate

Fermentation uses 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 OverviewAnimation: Fermentation Overview


Alcohol fermentation

Fig. 9-18a

2 ADP + 2 P i 2 ATP

Glucose Glycolysis

2 Pyruvate

2 NADH2 NAD+

+ 2 H+CO2

2 Acetaldehyde2 Ethanol

(a) Alcohol fermentation

2

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


Lactic acid fermentation

Fig. 9-18b

Glucose

2 ADP + 2 P i 2 ATP

Glycolysis

2 NAD+ 2 NADH+ 2 H+

2 Pyruvate

2 Lactate

(b) Lactic acid fermentation

Fermentation & Cellular Respiration Compared

Both fermentation & cellular respiration Use glycolysis to oxidize glucose and

other organic fuels to pyruvate Fermentation and cellular respiration

Differ in their final electron acceptor Cellular respiration

Produces more ATP

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


Fig. 9-19

Glucose

Glycolysis

Pyruvate

CYTOSOL

No O2 present:Fermentation

O2 present:

Aerobic cellular respiration

MITOCHONDRION

Acetyl CoAEthanolor

lactateCitricacidcycle

Facultative anaerobes

Yeast and many bacteria are facultative anaerobes that can survive using either fermentation or respiration At a cellular level, human muscle cells

can behave as facultative anaerobes Methane can also be a product of

bacterial fermentation Produces methane in wetlands and

ruminant stomachs

Famous anaerobes in history The oldest bacterial fossils are more than 3.5

billion years old, appearing long before appreciable quantities of O2 accumulated in the atmosphere Therefore, the first prokaryotes may have generated

ATP exclusively from glycolysis The fact that glycolysis is a ubiquitous

metabolic pathway & occurs in the cytosol without membrane-enclosed organelles suggests that glycolysis evolved early in the history

of life

Concept 9.6: Glycolysis & the citric acid cycle connect to many other metabolic pathway

The Versatility of Catabolism Catabolic pathways funnel electrons from

many kinds of organic molecules into cellular respiration

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

Fig. 9-20

Proteins

Carbohydrates

Aminoacids

Sugars

Fats

Glycerol Fattyacids

Glycolysis

Glucose

Glyceraldehyde-3-

Pyruvate

P

NH3

Acetyl CoA

Citricacidcycle


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

Anabolic, or biosynthetic, pathways consume ATP.

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

The control of cellular respiration

Glucose

Glycolysis

Fructose-6-phosphate

Phosphofructokinase

Fructose-1,6-bisphosphateInhibits Inhibits

Pyruvate

ATPAcetyl CoA

Citricacidcycle

Citrate


Stimulates

AMP

+

– –

Figure 9.20

You should now 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 and the products that result

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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Chapter 9 Cellular Respiration: Harvesting Chemical Energy Updated October 2008