How Cells make ATP: Energy-Releasing Pathways

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How Cells make ATPEnergy-Releasing Pathways

• Chapter 8

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Learning Objective 1

• In aerobic respiration, which reactant is
  oxidized and which is reduced?

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Aerobic Respiration

• A catabolic process
• fuel (glucose) broken down to carbon dioxide and
  water
• Redox reactions
• transfer electrons from glucose (oxidized)
• to oxygen (reduced)
• Energy released
• produces 36 to 38 ATP per glucose

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KEY CONCEPTS

• Aerobic respiration is an exergonic redox process
  in which glucose becomes oxidized, oxygen becomes
  reduced, and energy is captured to make ATP

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Learning Objective 2

• What are the four stages of aerobic respiration?

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4 Stages of Aerobic Respiration

1. Glycolysis
2. Formation of acetyl CoA
3. Citric acid cycle
4. Electron transport chain and chemiosmosis

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Glycolysis

• 1 molecule of glucose degraded
• to 2 molecules pyruvate
• 2 ATP molecules (net) produced
• by substrate-level phosphorylation
• 4 hydrogen atoms removed
• to produce 2 NADH

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Glycolysis
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Electron transport and chemiosmosis
Glycolysis
Formation of acetyl coenzyme A
Citric acid cycle
Glucose
Pyruvate
32 ATP
2 ATP
2 ATP
Fig. 8-3, p. 175
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GLYCOLYSIS
Energy investment phase and splitting of
glucose Two ATPs invested per glucose
Glucose
2 ATP
3 steps
2 ADP
Fructose-1,6-bisphosphate
P
P
Glyceraldehyde phosphate (G3P)
Glyceraldehyde phosphate (G3P)
P
P
Fig. 8-3, p. 175
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Energy capture phase Four ATPs and two
NADH produced per glucose
P
P
(G3P)
(G3P)
NAD
NAD
NADH
NADH
5 steps
2 ADP
2 ADP
2 ATP
2 ATP
Pyruvate
Pyruvate
Net yield per glucose Two ATPs and two NADH
Fig. 8-3, p. 175
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Formation of Acetyl CoA

• 1 pyruvate molecule
• loses 1 molecule of carbon dioxide
• Acetyl group coenzyme A
• produce acetyl CoA
• 1 NADH produced per pyruvate

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Formation of Acetyl CoA
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Electron transport and chemiosmosis
Glycolysis
Formation of acetyl coenzyme A
Citric acid cycle
Glucose
Pyruvate
32 ATP
2 ATP
2 ATP
Fig. 8-5, p. 178
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Carbon dioxide
CO2
Pyruvate
NAD
Coenzyme A
NADH
Acetyl coenzyme A
Fig. 8-5, p. 178
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Citric Acid Cycle

• 1 acetyl CoA enters cycle
• combines with 4-C oxaloacetate
• forms 6-C citrate
• 2 C enter as acetyl CoA
• 2 leave as CO2
• 1 acetyl CoA
• transfers H atoms to 3 NAD , 1 FAD
• 1 ATP produced

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Citric Acid Cycle
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Electron transport and chemiosmosis
Formation of acetyl coenzyme A
Citric acid cycle
Glycolysis
Glucose
Pyruvate
2 ATP
2 ATP
32 ATP
Fig. 8-6, p. 179
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Coenzyme A
Acetyl coenzyme A
Citrate
Oxaloacetate
NADH
NAD
NAD
C I T R I C A C I D C Y C L E
H2O
NADH
CO2
FADH2
5-carbon compound
FAD
NADH
GTP
GDP
CO2
4-carbon compound
ADP
ATP
Fig. 8-6, p. 179
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Electron Transport Chain

• H atoms (or electrons) transfer
• from one electron acceptor to another
• in mitochondrial inner membrane
• Electrons reduce molecular oxygen
• forming water

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Electron Transport Chain
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Cytosol
Outer mitochondrial membrane
Intermembrane space
Complex IV Cytochrome c oxidase
Complex I NADHubiquinone oxidoreductase
Complex III Ubiquinone cytochrome c
oxidoreductase
Complex II Succinate ubiquinone reductase
Inner mitochondrial membrane
Matrix of mitochondrion
FADH2
FAD
2 H
H2O
1/2 O2
NAD
NADH
Fig. 8-8, p. 181
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Oxidative Phosphorylation

• Redox reactions in ETC are coupled to ATP
  synthesis through chemiosmosis

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KEY CONCEPTS

• Aerobic respiration consists of four stages
  glycolysis, formation of acetyl coenzyme A, the
  citric acid cycle, and the electron transport
  chain and chemiosmosis

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Learning Objective 3

• Where in a eukaryotic cell does each stage of
  aerobic respiration take place?

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Aerobic Respiration

• Glycolysis occurs in the cytosol
• All other stages in the mitochondria

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1
2
3
4
Glycolysis
Formation of acetyl coenzyme A
Citric acid cycle
Electron transport and chemiosmosis
Glucose
Mitochondrion
Electron transport and chemiosmosis
Acetyl coenzyme A
Citric acid cycle
Pyruvate
2 ATP
2 ATP
32 ATP
Fig. 8-2, p. 173
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Learning Objective 4

• Add up the energy captured (as ATP, NADH, and
  FADH2) in each stage of aerobic respiration

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Energy Capture

• Glycolysis
• 1 glucose 2 NADH, 2 ATP (net)
• Conversion of 2 pyruvates to acetyl CoA
• 2 NADH
• Citric acid cycle
• 2 acetyl CoA 6 NADH, 2 FADH2, 2 ATP
• Total 4 ATP, 10 NADH, 2 FADH2

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Energy Transfer

• Electron transport chain (ETC)
• 10 NADH and 2 FADH2 produce 32 to 34 ATP by
  chemiosmosis
• 1 glucose molecule yields 36 to 38 ATP

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Energy from Glucose
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Substrate-level phosphorylation
Oxidative phosphorylation
Glycolysis
Glucose
Pyruvate
Acetyl coenzyme A
Citric acid cycle
Total ATP from oxidative phosphorylation
Total ATP from substrate-level phosphorylation
Fig. 8-11, p. 185
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Learning Objective 5

• Define chemiosmosis
• How is a gradient of protons established across
  the inner mitochondrial membrane?

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Chemiosmosis

• Energy of electrons in ETC
• pumps H across inner mitochondrial membrane
• into intermembrane space
• Protons (H) accumulate in intermembrane space
• lowering pH

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Proton Gradient
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Outer mitochondrial membrane
Cytosol
Inner mitochondrial membrane
Intermembrane space low pH
Matrix higher pH
Fig. 8-9, p. 183
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Learning Objective 6

• How does the proton gradient drive ATP synthesis
  in chemiosmosis?

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ATP Synthase

• Enzyme ATP synthase
• forms channels through inner mitochondrial
  membrane
• Diffusion of protons through channels provides
  energy to synthesize ATP

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ETC and Chemiosmosis
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Cytosol
Outer mitochondrial membrane
Intermembrane space
Complex V ATP synthase
Complex III
Complex IV
Complex I
Inner mitochondrial membrane
Complex II
Matrix of mitochondrion
FADH2
NAD
1
2
NADH
Pi
ADP
ATP
Fig. 8-10a, p. 184
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Projections of ATP synthase
250 nm
(b) This TEM shows hundreds of projections of ATP
synthase complexes along the surface of the inner
mitochondrial membrane.
Fig. 8-10b, p. 184
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Learning Objective 7

• How do the products of protein and lipid
  catabolism enter the same metabolic pathway that
  oxidizes glucose?

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Amino Acids

• Undergo deamination
• Carbon skeletons converted
• to intermediates of aerobic respiration

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Lipids

• Glycerol and fatty acids
• both oxidized as fuel
• Fatty acids
• converted to acetyl CoA by ß-oxidation

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Catabolic Pathways
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PROTEINS
CARBOHYDRATES
FATS
Amino acids
Fatty acids
Glycerol
Glycolysis
Glucose
G3P
Pyruvate
CO2
Acetyl coenzyme A
Citric acid cycle
Electron transport and chemiosmosis
End products
H2O
CO2
NH3
Fig. 8-12, p. 186
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Stepped Art
End products
Fig. 8-12, p. 186
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KEY CONCEPTS

• Nutrients other than glucose, including many
  carbohydrates, lipids, and amino acids, can be
  oxidized by aerobic respiration

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Learning Objective 8

• Compare the mechanism of ATP formation, final
  electron acceptor, and end products of anaerobic
  respiration and fermentation

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Anaerobic Respiration

• Electrons transferred
• from fuel molecules to ETC
• coupled to ATP synthesis (chemiosmosis)
• Final electron acceptor
• inorganic substance
• nitrate or sulfate (not molecular oxygen)

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KEY CONCEPTS

• In anaerobic respiration carried out by some
  bacteria, ATP is formed during a redox process in
  which glucose becomes oxidized and an inorganic
  substance becomes reduced

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Fermentation

• Anaerobic process
• no ETC
• Net energy gain only 2 ATP per glucose
• produced by substrate-level phosphorylation
  during glycolysis
• NAD
• produced by transferring H from NADH to organic
  compound from nutrient

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Fermentation

• Alcohol fermentation
• in yeast cells
• waste products ethyl alcohol, CO2
• Lactate (lactic acid) fermentation
• some fungi, prokaryotes, animal cells
• H atoms added to pyruvate
• waste product lactate

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KEY CONCEPTS

• Fermentation is an inefficient anaerobic redox
  process in which glucose becomes oxidized and an
  organic substance becomes reduced
• Some fungi and bacteria, as well as muscle cells
  under conditions of low oxygen, obtain low yields
  of ATP through fermentation

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Fermentation
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Fig. 8-13, p. 187
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25 µm
Fig. 8-13a, p. 187
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Glycolysis
Glucose
2 NAD
2 NADH
2 ATP
2 Pyruvate
CO2
2 Ethyl alcohol
(b) Alcohol fermentation
Fig. 8-13b, p. 187
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Glycolysis
Glucose
2 NAD
2 NADH
2 ATP
2 Pyruvate
2 Lactate
(c) Lactate fermentation
Fig. 8-13c, p. 187
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Summary Reaction

• Complete oxidation of glucose
• C6H12O6 6 O2 6 H2O ?
• 6 CO2 12 H2O energy (36 to 38 ATP)

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Summary Reaction

• Glycolysis
• C6H12O6 2 ATP 2 ADP 2 Pi 2 NAD
• ? 2 pyruvate 4 ATP 2 NADH H2O

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Glycolysis in Detail
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Energy investment phase and splitting of
glucose Two ATPs invested per glucose
Glucose
Glycolysis begins with preparation reaction in
which glucose receives phosphate group from ATP
molecule. ATP serves as source of both phosphate
and energy needed to attach phosphate to glucose
molecule. (Once ATP is spent, it becomes ADP and
joins ADP pool of cell until turned into ATP
again.) Phosphorylated glucose is known as
glucose-6-phosphate. (Note phosphate attached to
its carbon atom 6.) Phosphorylation of glucose
makes it more chemically reactive.
1
ATP
Hexokinase
ADP
Glucose-6-phosphate
Phosphoglucoisomerase
Fig. 8-4a, p. 176
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Glucose-6-phosphate undergoes another preparation
reaction, rearrangement of its hydrogen and
oxygen atoms. In this reaction glucose-6-phosphate
is converted to its isomer, fructose-6-phosphate.

2
Fructose-6-phosphate
ATP
Phosphofructokinase
ADP
Next, another ATP donates phosphate to molecule,
forming fructose-1,6-bisphosphate. So far, two
ATP molecules have been invested in process
without any being produced. Phosphate groups are
now bound at carbons 1 and 6, and molecule is
ready to be split.
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Fructose-1,6-bisphosphate
Aldolase
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Fructose-1,6-bisphosphate is then split into two
3-carbon sugars, glyceraldehyde-3- phosphate
(G3P) and dihydroxyacetone phosphate. Dihydroxyac
etone phosphate is enzymatically converted to its
isomer, glyceraldehyde-3- phosphate, for further
metabolism in glycolysis.
Isomerase
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Dihydroxyacetone phosphate
Glyceraldehyde- 3-phosphate (G3P)
Fig. 8-4a, p. 176
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Energy capture phase Four ATPs and two NADH
produced per glucose
Two glyceraldehyde-3-phosphate (G3P) from bottom
of previous page
2 NAD
Glyceraldehyde-3-phosphate dehydrogenase
2 NADH
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Each glyceraldehyde-3-phosphate undergoes
dehydrogenation with NAD as hydrogen acceptor.
Product of this very exergonic reaction is
phosphoglycerate, which reacts with inorganic
phosphate present in cytosol to yield
1,3-bisphosphoglycerate.
Two 1,3-bisphosphoglycerate
2 ADP
Phosphoglycerokinase
2 ATP
One of phosphates of 1,3-bisphosphoglycerate
reacts with ADP to form ATP. This transfer of
phosphate from phosphorylated intermediate to ATP
is referred to as substrate-level phosphorylation.
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Two 3-phosphoglycerate
Phosphoglyceromutase
Fig. 8-4b, p. 177
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3-phosphoglycerate is rearranged to
2-phosphoglycerate by enzymatic shift of position
of phosphate group. This is a preparation
reaction.
Two 2-phosphoglycerate
Enolase
2 H2O
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Next, molecule of water is removed, which results
in formation of double bond. The product,
phosphoenolpyruvate (PEP), has phosphate group
attached by an unstable bond (wavy line).
Two phosphoenolpyruvate
2 ADP
Pyruvate kinase
2 ATP
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Each of two PEP molecules transfers its phosphate
group to ADP to yield ATP and pyruvate. This is
substrate-level phosphorylation reaction.
Two pyruvate
Fig. 8-4b, p. 177
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Summary Reaction

• Conversion of pyruvate to acetyl CoA
• 2 pyruvate 2 coenzyme A 2 NAD ?
• 2 acetyl CoA 2 CO2 2 NADH

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Summary Reaction

• Citric acid cycle
• 2 acetyl CoA 6 NAD 2 FAD 2 ADP
• 2 Pi 2 H2O ? 4 CO2 6 NADH
• 2 FADH2 2 ATP 2 CoA

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Citric Acid Cycle in Detail
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Summary Reactions

• Hydrogen atoms in ETC
• NADH 3 ADP 3 Pi 12 O2 ? NAD 3 ATP H2O
  
• FADH2 2 ADP 2 Pi 12 O2 ? FAD 2 ATP H2O

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Summary Reaction

• Lactate fermentation
• C6H12O6 ? 2 lactate energy (2 ATP)

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Summary Reaction

• Alcohol fermentation
• C6H12O6 ? 2 CO2 2 ethyl alcohol energy (2
  ATP)

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The Overall Reactions of Glycolysis
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