Biology 201 Chapter 7

Chapter 07

Lecture and Animation Outline

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2

Respiration• Organisms can be classified based on how they

obtain energy:• Autotrophs

– Able to produce their own organic molecules through photosynthesis

• Heterotrophs– Live on organic compounds produced by other

organisms• All organisms use cellular respiration to extract

energy from organic molecules

3

Cellular respiration

• Cellular respiration is a series of reactions• Oxidized – loss of electrons• Reduced – gain of electron• Dehydrogenation – lost electrons are

accompanied by protons– A hydrogen atom is lost (1 electron, 1 proton)

4

Redox• During redox reactions, electrons carry

energy from one molecule to another• Nicotinamide adenosine dinucleotide

(NAD+)

– An electron carrier– NAD+ accepts 2 electrons and 1 proton to

become NADH– Reaction is reversible

5

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Product

Enzyme

NAD+

NAD+ NAD NAD

2e–

H+

+H+

Energy-richmolecule

NAD+

1. Enzymes that use NAD+

as a cofactor for oxidationreactions bind NAD+ and the substrate.

2. In an oxidation–reductionreaction, 2 electrons anda proton are transferredto NAD+, forming NADH.A second proton isdonated to the solution.

3. NADH diffuses awayand can then donateelectrons to othermolecules.

Reduction

Oxidation

HH

HH

HH

H H

• In overall cellular energy harvest– Dozens of redox reactions take place– Number of electron acceptors including NAD+

• In the end, high-energy electrons from initial chemical bonds have lost much of their energy

• Transferred to a final electron acceptor

6

7

• Aerobic respiration– Final electron receptor is oxygen (O2)

• Anaerobic respiration– Final electron acceptor is an inorganic

molecule (not O2)• Fermentation

– Final electron acceptor is an organic molecule

8

Aerobic respiration

C6H12O6 + 6O2 6CO2 + 6H2O

Free energy = – 686 kcal/mol of glucose Free energy can be even higher than this in

a cell• This large amount of energy must be

released in small steps rather than all at once.

9

2e–

Electrons from food

High energy

Low energy

2H+ 1/2O2

Energy releasedfor ATP synthesis

H2O


Electron carriers• Many types of carriers used

– Soluble, membrane-bound, move within membrane

• All carriers can be easily oxidized and reduced

• Some carry just electrons, some electrons and protons

• NAD+ acquires 2 electrons and a proton to become NADH

10

11


H H

O

NH2 + 2H

CH2O

O

O

H

H

NH2

H

H

N

N

N

CH2O

O

O

H

H

NH2

O–

H

H

NN

N

H H O

N

N

H H

H

C C

O P

O PO–O P

O–O P

N N

OH

CH2

NADH: Reduced form of nicotinamide

O

O O

O

NAD+: Oxidized form of nicotinamide

Reduction

Oxidation

Adenine Adenine

OH OH

OH OHOH

OHOH

NH2 + H+

CH2

O–

12

ATP• Cells use ATP to drive endergonic

reactions– ΔG (free energy) = –7.3 kcal/mol

• 2 mechanisms for synthesis1. Substrate-level phosphorylation

• Transfer phosphate group directly to ADP• During glycolysis

2. Oxidative phosphorylation• ATP synthase uses energy from a proton gradient

13

PEP

– ADP

Enzyme Enzyme

– ATPAdenosine

Pyruvate

P

PP

P

P

Adenosine

P


14

Oxidation of Glucose

The complete oxidation of glucose proceeds in stages:

1. Glycolysis2. Pyruvate oxidation3. Krebs cycle4. Electron transport chain & chemiosmosis

15

Outermitochondrial

membrane

Intermembranespace

Mitochondrialmatrix

FAD O2

Innermitochondrial

membrane

ElectronTransport Chain

ChemiosmosisATP Synthase

NAD+

Glycolysis

Pyruvate

Glucose

PyruvateOxidation

Acetyl-CoA

KrebsCycle

CO2

ATPH2O

ATP

e–

e–

NADH

NADH

CO2

ATP

NADH

FADH2


H+

e–

16

Glycolysis

• Converts 1 glucose (6 carbons) to 2 pyruvate (3 carbons)

• 10-step biochemical pathway• Occurs in the cytoplasm• Net production of 2 ATP molecules by

substrate-level phosphorylation• 2 NADH produced by the reduction of

NAD+

17


Glycolysis

NADH

Pyruvate Oxidation

ATP

Electron Transport ChainChemiosmosis

KrebsCycle

6-carbon sugar diphosphate

NAD+ NAD+

Prim

ing

Rea

ctio

nsC

leav

age

Oxi

datio

n an

d A

TP F

orm

atio

n

NADH NADH

P P

P P

ATP ATP

ADP ADP

3-carbon sugarphosphate

3-carbon sugarphosphate

PiPi

ADP ADP

ATP ATP

ATP

ADP ADP

ATP

3-carbonpyruvate

3-carbonpyruvate

6-carbon glucose(Starting material)

Glycolysis beginswith the addition ofenergy. Two high-energy phosphates(P) from twomolecules of ATPare added to the6-carbon moleculeglucose, producinga 6-carbonmolecule with twophosphates.

Then, the 6-carbonmolecule with twophosphates is split in two, forming two3-carbon sugarphosphates.

An additionalInorganic phosphate ( Pi ) is incorporated into each 3-carbon sugar phosphate. Anoxidation reactionconverts the twosugar phosphatesinto intermediatesthat can transfer aphosphate to ADP to form ATP. Theoxidation reactionsalso yield NADHgiving a net energyyield of 2 ATP and 2NADH.

18


NADH

NAD+

NADH

PiNAD+

Glucose

Hexokinase

Phosphofructokinase

Glucose 6-phosphate

Fructose 6-phosphate

Fructose 1,6-bisphosphate

IsomeraseAldolase

Pyruvate Pyruvate

Enolase

Pyruvate kinase

ADP

10

Glu

cose

Gly

cera

ldeh

yde

3-ph

osph

ate

Pyru

vate

Glycolysis: The ReactionsGlycolysis

NADH

Pyruvate Oxidation

H2O

ATP

ADP


KrebsCycle

ATP

ATP

Phosphoglucoseisomerase

Glyceraldehyde 3-phosphate (G3P)

Dihydroxyacetonephosphate

1. Phosphorylation ofglucose by ATP.

2–3. Rearrangement,followed by a secondATP phosphorylation.

4–5. The 6-carbon moleculeis split into two 3-carbonmolecules—one G3P,another that is convertedinto G3P in anotherreaction.

6. Oxidation followed byphosphorylation producestwo NADH molecules andtwo molecules of BPG,each with onehigh-energy phosphatebond.

7. Removal of high-energyphosphate by two ADPmolecules produces twoATP molecules and leavestwo 3PG molecules.

8–9. Removal of water yieldstwo PEP molecules, eachwith a high-energyphosphate bond.

10. Removal of high-energyphosphate by two ADPmolecules produces twoATP molecules and two pyruvate molecules.

1,3-Bisphosphoglycerate (BPG) 1,3-Bisphosphoglycerate

(BPG)

Glyceraldehyde3-phosphate

dehydrogenase

Pi

ADP

Phosphoglyceratekinase

ADP

ATP

3-Phosphoglycerate(3PG)




H2O

ATP

Phosphoenolpyruvate(PEP)

Phosphoenolpyruvate(PEP)

ADP ADP

ATP ATP

Phos

phoe

nol-

pyru

vate

3-Ph

osph

o-gl

ycer

ate

1,3-

Bis

phos

pho-

glyc

erat

eG

luco

se6-

phos

phat

eFr

ucto

se6-

phos

phat

eFr

ucto

se1,

6-bi

spho

spha

teD

ihyd

roxy

acet

one

Phos

phat

e2-

Phos

pho-

glyc

erat

e

CH2OHO

CH2 O

O

P

CH2 O

O

P

CH2OH

O CH2 CH2 O

O

P P

CHOH

H

C O

CH2 O P

C O

O CH2P

CH2OH

CHOH

O C O

CH2 O

P

P

CHOH

O–

C O

CH2 O P

H C O

O–

C O

CH2OH

P

C O

O–

C O

CH2

P

C O

O–

C O

CH3

8

9

10

7

4 5

3

2

1

6

Phosphoglyceromutase

19

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20

NADH must be recycled

• For glycolysis to continue, NADH must be recycled to NAD+ by either:

1. Aerobic respiration– Oxygen is available as the final electron

acceptor– Produces significant amount of ATP

2. Fermentation– Occurs when oxygen is not available– Organic molecule is the final electron acceptor

21

Fate of pyruvate

• Depends on oxygen availability.– When oxygen is present, pyruvate is oxidized

to acetyl-CoA which enters the Krebs cycle• Aerobic respiration

– Without oxygen, pyruvate is reduced in order to oxidize NADH back to NAD+

• Fermentation

22


Without oxygen

NAD+

O2 NADH

ETC in mitochondria

Acetyl-CoA

Ethanol

NAD+

CO2

NAD+

H2O

Lactate

Pyruvate

AcetaldehydeNADH

NADH

With oxygen

KrebsCycle

23

Pyruvate Oxidation

• In the presence of oxygen, pyruvate is oxidized– Occurs in the mitochondria in eukaryotes

• multienzyme complex called pyruvate dehydrogenase catalyzes the reaction

– Occurs at the plasma membrane in prokaryotes

24

• For each 3-carbon pyruvate molecule:– 1 CO2

• Decarboxylation by pyruvate dehydrogenase– 1 NADH– 1 acetyl-CoA which consists of 2 carbons

from pyruvate attached to coenzyme A• Acetyl-CoA proceeds to the Krebs cycle

Products of pyruvate oxidation

25


Glycolysis

Pyruvate Oxidation

NADH

KrebsCycle


Pyruvate Oxidation: The Reaction

NAD+

CO2

CoA

Acetyl Coenzyme A

Pyruvate

Pyru

vate

Ace

tyl C

oenz

yme

A

O

CH 3

C

O–

C O

S CoA

CH3

OC

NADH

26

Krebs Cycle

• Oxidizes the acetyl group from pyruvate• Occurs in the matrix of the mitochondria• Biochemical pathway of 9 steps in three

segments 1. Acetyl-CoA + oxaloacetate → citrate2. Citrate rearrangement and decarboxylation3. Regeneration of oxaloacetate

27


CoA-(Acetyl-CoA)

CoA

4-carbonmolecule

(oxaloacetate) 6-carbon molecule(citrate)

NAD +

NADH

CO2

5-carbonmolecule

NAD+

NADH

CO2

4-carbonmolecule

ADP + P

Krebs Cycle

FAD

FADH2

4-carbonmolecule

NAD+

NADH

ATP

Glycolysis

Pyruvate Oxidation


ATP

4-carbonmolecule

Pyruvate from glycolysis is oxidized Krebs Cycle into an acetyl group that feeds into the Krebs cycle. The 2-C acetyl group combines with 4-C oxaloacetate to produce the 6-C compound citrate (thus this is also called the citric acid cycle). Oxidation reactions are combined with two decarboxylations to produce NADH, CO2, and a new 4-carbon molecule. Two additional oxidations generate another NADH and an FADH2 and regenerate the original 4-C oxaloacetate.

NADH

FADH2

KrebsCycle

28

Krebs Cycle

• For each Acetyl-CoA entering:– Release 2 molecules of CO2 – Reduce 3 NAD+ to 3 NADH– Reduce 1 FAD (electron carrier) to FADH2 – Produce 1 ATP– Regenerate oxaloacetate

29


Glycolysis

NADH

FADH2

Pyruvate Oxidation

ATP

Krebs Cycle: The Reactions

Citratesynthetase

NAD+

NADH

H2O

NAD+

NADH

CO2

Isocitratedehydrogenase

Fumarase

CoA-SH1

2

Aconitase3

4

8

9

7

CoA-SH

NAD+

CO2

5

6

NADH

CoA-SH

GDP + Pi

Acetyl-CoA

═

CH3— C— S

O CoA—

KrebsCycle

Malatedehydrogenase

-Ketoglutaratedehydrogenase

Succinyl-CoAsynthetase

GTP

ATP

ADP

Succinatedehydrogenase

FADH2

8–9. Reactions 8 and 9: Regeneration of oxaloacetate and the fourth oxidation

7. Reaction 7: The third oxidation

6. Reaction 6: Substrate-level phosphorylation

5. Reaction 5: The second oxidation

4. Reaction 4: The first oxidation

2–3. Reactions 2 and 3: Isomerization

1. Reaction 1: Condensation

Electron T ransport ChainChemiosmosis

Oxaloacetate (4C)

CH2

O ═ C

COO—

COO—

——

—

Citrate (6C)

HO—C—COO—

COO—

COO—

CH2

CH2

——

——

Isocitrate (6C)

HC—COO—

COO—

COO—

CH2

HO—CH

——

——

-Ketoglutarate (5C)

CH2

COO—

COO—

CH2

C—O—

——

—

Succinyl-CoA (4C)

CH2

COO—

S—CoA

CH2

C═ O

——

——

Succinate (4C)COO—

CH2

COO—

CH2

——

—

Fumarate (4C)

HC

CH

═

COO—

COO—

——

Malate (4C)

HO— CH

COO—

CH2

COO—

——

—

FAD

30

At this point

• Glucose has been oxidized to:– 6 CO2

– 4 ATP– 10 NADH– 2 FADH2

• Electron transfer has released 53 kcal/mol of energy by gradual energy extraction

• Energy will be put to use to manufacture ATP

These electron carriers proceedto the electron transport chain

31

Electron Transport Chain (ETC)

• ETC is a series of membrane-bound electron carriers

• Embedded in the inner mitochondrial membrane

• Electrons from NADH and FADH2 are transferred to complexes of the ETC

• Each complex– A proton pump creating proton gradient– Transfers electrons to next carrier

32


Mitochondrial matrix

NADH + H+ADP + PiH2O

H+ H+

2H+ + 1/2O2

Glycolysis

Pyruvate Oxidation

2

KrebsCycle ATP


NADH dehydrogenase bc1 complexCytochrome

oxidase complex

Innermitochondrial membrane

Intermembrane space

a. The electron transport chain

ATPsynthase

b. Chemiosmosis

NAD+

QC

e–

FADH2

H+H+

H+H+

e–22 e–22

ATP

FAD

33

Chemiosmosis

• Accumulation of protons in the intermembrane space drives protons into the matrix via diffusion

• Membrane relatively impermeable to ions• Most protons can only reenter matrix

through ATP synthase– Uses energy of gradient to make ATP from

ADP + Pi

34

ADP + Pi

Catalytic head

Stalk

Rotor

H+

H+

Mitochondrialmatrix

Intermembranespace

H+ H+

H+

H+H+

ATP


35


H2O

CO2

CO2

H+

H+

2H+

+1/2O2

H+

e–

H+

32 ATPKrebsCycle

2 ATP

NADH

NADH

FADH2

NADH

PyruvateOxidationAcetyl-CoA

e–

QC

e–

Glycolysis

Glucose

Pyruvate

36

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37

Energy Yield of Respiration

• Theoretical energy yield– 38 ATP per glucose for bacteria– 36 ATP per glucose for eukaryotes

• Actual energy yield– 30 ATP per glucose for eukaryotes– Reduced yield is due to

• “Leaky” inner membrane• Use of the proton gradient for purposes other than

ATP synthesis

38

Chemiosmosis

Chemiosmosis

2 5

2 3

6 15

2

2

2 5NADH

NADH

NADH

Total net ATP yield = 32(30 in eukaryotes)

ATP

ATP

ATP

ATP

ATP

ATP

KrebsCycle

Pyruvate oxidation

FADH2

Glycolysis2

Glucose

Pyruvate

ATP


39

Regulation of Respiration

• Example of feedback inhibition• 2 key control points

1. In glycolysis• Phosphofructokinase is allosterically inhibited by

ATP and/or citrate

2. In pyruvate oxidation• Pyruvate dehydrogenase inhibited by high levels of

NADH• Citrate synthetase inhibited by high levels of ATP

40

Glucose

Acetyl-CoA

Fructose 6-phosphate

Fructose 1,6-bisphosphate

Pyruvate

Pyruvate Oxidation

KrebsCycle

Electron Transport Chainand

Chemiosmosis

Citrate

ATP

NADH

Inhibits

InhibitsInhibits

Phosphofructokinase

Pyruvate dehydrogenase

GlycolysisCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

ADP

Activates

41

Oxidation Without O2

1. Anaerobic respiration– Use of inorganic molecules (other than O2) as

final electron acceptor– Many prokaryotes use sulfur, nitrate, carbon

dioxide or even inorganic metals

2. Fermentation– Use of organic molecules as final electron

acceptor

42

Anaerobic respiration

• Methanogens– CO2 is reduced to CH4 (methane)– Found in diverse organisms including cows

• Sulfur bacteria– Inorganic sulphate (SO4) is reduced to

hydrogen sulfide (H2S)– Early sulfate reducers set the stage for

evolution of photosynthesis

43

a: © Wolfgang Baumeister/Photo Researchers, Inc.; b: NPS Photo


b.a. 0.625 µm

44

Fermentation

• Reduces organic molecules in order to regenerate NAD+

1. Ethanol fermentation occurs in yeast– CO2, ethanol, and NAD+ are produced

2. Lactic acid fermentation– Occurs in animal cells (especially muscles)– Electrons are transferred from NADH to

pyruvate to produce lactic acid

45

CO2

2 Acetaldehyde

2 ADP

2 Lactate

Alcohol Fermentation in Yeast

2 ADP

Lactic Acid Fermentation in Muscle Cells

2 NAD+

2 NAD+

2 NADH

2 NADH

2 ATP

2 ATP

C O

C O

O–

CH3

C O

H

CH3

C O

C O

CH3

O–

CH3

H C OH

C O

O–

H

2 Ethanol

H C OH

CH3

2 Pyruvate

2 Pyruvate

Glucose

Glucose

GLYCOLYSIS

GLYCOLYSIS


46

Catabolism of Protein• Amino acids undergo deamination to

remove the amino group• Remainder of the amino acid is converted

to a molecule that enters glycolysis or the Krebs cycle– Alanine is converted to pyruvate– Aspartate is converted to oxaloacetate

47

C

HO O

HO O

C

C

HO O

O

C

C O

CH H

CH HCH H

CH H

CH2N HNH3

HO

Urea

Glutamate α-Ketoglutarate


48

Catabolism of Fat

• Fats are broken down to fatty acids and glycerol– Fatty acids are converted to acetyl groups by

-oxidation – Oxygen-dependent process

• The respiration of a 6-carbon fatty acid yields 20% more energy than 6-carbon glucose.

49


CoA

Fatty acid

OO

—C —C—C—

═ H

H

— ═—

ATP

NADH

H2O

KrebsCycle

PPiAMP +

FADH2

Fatty acid2C shorter

CoA

CoA

Fatty acid

OH

H

—C — C—C—

——

H

H

— ═—

Fatty acidOH

OH

H

—C —C— C═

—

——

H

H

——

CoA

FAD

Fatty acid

OH

—C ═ C —C—

— H— ═

CoA

Fatty acid

OHO

H

—C —C—C—

——

H

H— ═

— CoA

NAD+

Acetyl-CoA

50

Cell building blocks

Deamination -oxidationGlycolysis

Oxidative respiration

Ultimate metabolic products

Acetyl-CoA

Pyruvate

Macromoleculedegradation

KrebsCycle

Nucleic acids Polysaccharides

Nucleotides Amino acids Fatty acidsSugars

Proteins Lipids and fats

NH3 H2O CO2


51

Evolution of Metabolism

• Hypothetical timeline1. Ability to store chemical energy in ATP2. Evolution of glycolysis

• Pathway found in all living organisms

3. Anaerobic photosynthesis (using H2S) 4. Use of H2O in photosynthesis (not H2S)

• Begins permanent change in Earth’s atmosphere5. Evolution of nitrogen fixation6. Aerobic respiration evolved most recently

Education

Biology 201 Chapter 7