Ch. 9 Cellular Respiration Living cells require energy from outside sources Heterotrophs and...

Ch. 9 Cellular Respiration

• Living cells require energy from outside sources• Heterotrophs and autotrophs

• 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

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Figure 9.2

Lightenergy

ECOSYSTEM

Photosynthesisin chloroplasts

Cellular respirationin mitochondria

CO2 H2O O2

Organicmolecules

ATP powersmost cellular workATP

Heatenergy

• Cellular respiration is often used to refer to aerobic respiration

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

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Redox Reactions

• During cellular respiration, the fuel (such as glucose) is oxidized, and O2 is reduced

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Figure 9.UN03

becomes oxidized

becomes reduced

NAD+

• 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 will eventually synthesize ATP

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Figure 9.4

Nicotinamide(oxidized form)

NAD

(from food)

Dehydrogenase

Reduction of NAD

Oxidation of NADH

Nicotinamide(reduced form)

NADH

• NADH passes the electrons to the electron transport chain

• O2 pulls electrons down the chain in an energy-yielding tumble

• The energy yielded is used to regenerate ATP

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Three stages of respiration

• Harvesting of energy from glucose has three stages– Glycolysis (breaks down glucose into two

molecules of pyruvate)– The citric acid cycle and oxidation of pyruvate

(completes the breakdown of glucose)– Oxidative phosphorylation – electron transport

(accounts for most of the ATP synthesis)

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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 andchemiosmosis (color-coded violet)

3.

Figure 9.6-1

Electronscarried

via NADH

Glycolysis

Glucose Pyruvate

CYTOSOL MITOCHONDRION

ATP

Substrate-levelphosphorylation

Figure 9.6-2

Electronscarried

via NADH

Electrons carriedvia NADH and

FADH2

Citricacidcycle

Pyruvateoxidation

Acetyl CoA

Glycolysis

Glucose Pyruvate

CYTOSOL MITOCHONDRION

ATP ATP

Substrate-levelphosphorylation

Substrate-levelphosphorylation

Figure 9.6-3

Electronscarried

via NADH

Electrons carriedvia NADH and

FADH2

Citricacidcycle

Pyruvateoxidation

Acetyl CoA

Glycolysis

Glucose Pyruvate

Oxidativephosphorylation:electron transport

andchemiosmosis

CYTOSOL MITOCHONDRION

ATP ATP ATP

Substrate-levelphosphorylation

Substrate-levelphosphorylation

Oxidative phosphorylation

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

• 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

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BioFlix: Cellular Respiration

Glycolysis

• 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

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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 9.9a

Glycolysis: Energy Investment Phase

ATPGlucose Glucose 6-phosphate

ADP

Hexokinase

1

Fructose 6-phosphate

Phosphogluco-isomerase

2

Figure 9.9b

Glycolysis: Energy Investment Phase

ATPFructose 6-phosphate

ADP

3

Fructose 1,6-bisphosphate

Phospho-fructokinase

4

5

Aldolase

Dihydroxyacetonephosphate

Glyceraldehyde3-phosphate

Tostep 6Isomerase

Figure 9.9c

Glycolysis: Energy Payoff Phase

2 NADH2 ATP

2 ADP 2

2

2 NAD + 2 H

2 P i

3-Phospho-glycerate

1,3-Bisphospho-glycerate

Triosephosphate

dehydrogenase

Phospho-glycerokinase

67

Figure 9.9d

Glycolysis: Energy Payoff Phase

2 ATP

2 ADP2222

2 H2O

PyruvatePhosphoenol-pyruvate (PEP)

2-Phospho-glycerate

3-Phospho-glycerate

89

10

Phospho-glyceromutase

Enolase Pyruvatekinase

• In the presence of O2, pyruvate enters the mitochondrion (in eukaryotic cells) where the oxidation of glucose is completed

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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

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Figure 9.10

Pyruvate

Transport protein

CYTOSOL

MITOCHONDRION

CO2 Coenzyme A

NAD + HNADH Acetyl CoA

1

2

3

• The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn

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The Citric Acid Cycle - Krebs

Figure 9.11Pyruvate

NAD

NADH

+ HAcetyl CoA

CO2

CoA

CoA

CoA

2 CO2

ADP + P i

FADH2

FAD

ATP

3 NADH

3 NAD

Citricacidcycle

+ 3 H

• 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

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Figure 9.12-1

1

Acetyl CoA

Citrate

Citricacidcycle

CoA-SH

Oxaloacetate

Figure 9.12-2

1

Acetyl CoA

CitrateIsocitrate

Citricacidcycle

H2O

2

CoA-SH

Oxaloacetate

Figure 9.12-3

1

Acetyl CoA

CitrateIsocitrate

-Ketoglutarate

Citricacidcycle

NADH+ H

NAD

H2O

3

2

CoA-SH

CO2

Oxaloacetate

Figure 9.12-4

1

Acetyl CoA

CitrateIsocitrate

-Ketoglutarate

SuccinylCoA

Citricacidcycle

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

SuccinylCoA

Succinate

Citricacidcycle

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

SuccinylCoA

Succinate

Fumarate

Citricacidcycle

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

SuccinylCoA

Succinate

Fumarate

Malate

Citricacidcycle

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

SuccinylCoA

Succinate

Fumarate

Malate

Citricacidcycle

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

SuccinylCoA

NADH

NADH

NAD

NAD

+ H

CoA-SH

CO2

CO2

3

4

+ H

Figure 9.12c

Fumarate

FADH2

CoA-SH6

SuccinateSuccinyl

CoA

FAD

ADP

GTP GDP

P i

ATP

5

Figure 9.12d

Oxaloacetate8

Malate

Fumarate

H2O

NADH

NAD

+ H

7

Electron Transport Chain

• The electron transport chain is in the inner membrane (cristae) of the mitochondrion

• 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

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The Pathway of Electron Transport

• Most of the chain’s components are proteins, • 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

• Electrons are passed through a number of proteins including cytochromes (each with an iron atom) to O2

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Figure 9.13

NADH

FADH2

2 H + 1/2 O2

2 e

2 e

2 e

H2O

NAD

Multiproteincomplexes

(originally from NADH or FADH2)

III

III

IV

50

40

30

20

10

0

Fre

e e

ner

gy

(G)

rela

tiv

e to

O2 (

kcal

/mo

l)

FMN

FeS FeS

FAD

Q

Cyt b

Cyt c1

Cyt c

Cyt a

Cyt a3

FeS

Chemiosmosis: ATP production through H+

• Electron transfer 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

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Figure 9.14INTERMEMBRANE SPACE

Rotor

StatorH

Internalrod

Catalyticknob

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 i

NAD

H

2 H + 1/2O2

H

HH

21

H

H2O

ATP

An Accounting of ATP Production by Cellular Respiration

• glucose NADH electron transport chain ATP

About 30-32 total ATP are made (26/28 via ETC and 4 via substrate level)

• Takes energy to move ATP into cytosol after it is made, takes E to move pyruvate in

• Depends on shuttle systems that bring the NADH electrons into the mitochondria

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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 CoACitricacidcycle

Oxidativephosphorylation:electron transport

andchemiosmosis

CYTOSOL

Maximum per glucose:About

30 or 32 ATP

Anaerobic respiration - Fermentation

• Most cellular respiration requires O2 to produce ATP

• Without O2, no electron transport chain

• Fermentation uses substrate-level phosphorylation instead of an electron transport chain to generate ATP

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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

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• 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

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Animation: Fermentation Overview Right-click slide / select “Play”

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

• 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

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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

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• 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

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Figure 9.18Glucose

CYTOSOLGlycolysis

Pyruvate

No O2 present:Fermentation

O2 present: Aerobic cellular respiration

Ethanol,lactate, or

other products

Acetyl CoA

MITOCHONDRION

Citricacidcycle

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

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Getting E from other sources

• electrons from many kinds of organic molecules funnel into cellular respiration

• Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle

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• 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

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Figure 9.19CarbohydratesProteins

Fattyacids

Aminoacids

Sugars

Fats

Glycerol

Glycolysis

Glucose

Glyceraldehyde 3- P

NH3 Pyruvate

Acetyl CoA

Citricacidcycle

Oxidativephosphorylation

Regulation of Cellular Respiration via Feedback Mechanisms

• If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down

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Figure 9.20

Phosphofructokinase

Glucose

GlycolysisAMP

Stimulates

Fructose 6-phosphate

Fructose 1,6-bisphosphate

Pyruvate

Inhibits Inhibits

ATP Citrate

Citricacidcycle

Oxidativephosphorylation

Acetyl CoA