How Cells Harvest Energy
1
Outline
• Cellular Energy Harvest
• Cellular Respiration
– Glycolysis
– Oxidation of Pyruvate
– Krebs Cycle
– Electron Transport Chain
• Catabolism of Protein and Fat
• Fermentation
• Evolution of Metabolism
2
Chemical Energy to Drive Metabolism
• Autotrophs harvest sunlight and convert
radiant energy into chemical energy.
• Heterotrophs live off the energy produced by
autotrophs.
– extract energy from food via digestion and
catabolism
3
Cellular Respiration
• Cells harvest energy by breaking bonds and
shifting electrons from one molecule to
another.
– aerobic respiration - final electron acceptor
is oxygen
– anaerobic respiration - final electron
acceptor is inorganic molecule other than
oxygen
– fermentation - final electron acceptor is an
organic molecule
4
ATP
• Adenosine Triphosphate (ATP) is the energy
currency of the cell.
– used to drive movement
5
Glucose Catabolism
• Cells catabolize organic molecules and
produce ATP in two ways:
– substrate-level phosphorylation
– aerobic respiration
in most organisms, both are combined
glycolysis
pyruvate oxidation
Krebs cycle
electron transport chain
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Aerobic Respiration
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Stage One - Glycolysis
• For each molecule of glucose that passes
through glycolysis, the cell nets two ATP
molecules.
• Priming
– glucose priming
– cleavage and rearrangement
• Substrate-level phosphorylation
– oxidation
– ATP generation
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Fig. 9.7a (TEArt)
CH2OH
1. Phosphorylation Glucose O
Glucose of glucose by ATP. 1
ATP
Hexokinase
Glycolysis ADP CH2 O P
O
Glucose 6-phosphate
Pyruvate 2-3. Rearrangement, 2
oxidation followed by a second Phosphoglucose
ATP phosphorylation. CH2 O P
isomerase CH2OH
O
Fructose 6-phosphate
Krebs
cycle 3
ATP
Phosphofructokinase
P O CH2 CH2 O P
Electron transport ADP O
chain Fructose 1,6-bisphosphate
4–5. The six-carbon molecule 4,5 Aldolase
is split into two three-carbon H
P O CH2 Isomerase
molecules—one G3P, another
C O
that is converted into G3P in C O Dihydroxyacetone Glyceraldehyde 3
another reaction. CHOH
CH2OH phosphate -phosphate (G3P)
CH2 O P
6. Oxidation followed by NAD+ Pi 6 Pi NAD+
phosphorylation produces
NADH Glyceraldehyde
two NADH molecules and 3-phosphate NADH P O O
two molecules of BPG, each dehydrogenase
with one high-energy CHOH
phosphate bond. 1,3-Bisphosphoglycerate 1,3-Bisphosphoglycerate
(BPG) (BPG) CH2 O P
9
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Fig. 9.7b (TEArt)
ADP 7 ADP
7. Removal of high-energy Phosphoglycerate O-
phosphate by two ADP kinase
molecules produces two ATP ATP C
ATP molecules and leaves
3-Phosphoglycerate 3-Phosphoglycerate CHOH
two 3PG molecules.
(3PG) (3PG) CH2 O P
8
Phosphogly- O-
ceromutase C O
8–9. Removal of water yields
two PEP molecules, each H C O P
2-Phosphoglycerate 2-Phosphoglycerate
with a high-energy
phosphate bond. (2PG) (2PG) CH2OH
9 O-
H2O Enolase H2O
C O
C O P
Phosphoenolpyruvate Phosphoenolpyruvate
(PEP) (PEP) CH2
10. Removal of high-energy O-
phosphate by two ADP ADP 10 ADP
molecules produces two Pyruvate kinase C O
ATP molecules and two ATP ATP
pyruvate molecules. C O
Pyruvate Pyruvate CH3
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Recycling NADH
• As long as food molecules are available to
be converted into glucose, a cell can
produce ATP.
– Continual production creates NADH
accumulation and NAD+ depletion.
NADH must be recycled into NAD .
+
aerobic respiration
fermentation
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Recycling NADH
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Stage Two - Oxidation of Pyruvate
• Within mitochondria, pyruvate is
decarboxylated, yielding acetyl-CoA, NADH,
and CO2.
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Stage Three - Krebs Cycle
• Acetyl-CoA is oxidized in a series of nine
reactions.
– two steps:
priming
energy extraction
14
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Oxidation of pyruvate
Fig. 9.12a (TEArt)
Pyruvate
NAD+ CO2 Glucose
The cycle begins when
a 2C unit from acetyl-
CoA reacts with a 4C Glycolysis
NADH S CoA
molecule (oxaloacetate)
to produce citrate (6C). C O
Pyruvate
CH3 oxidation
Acetyl-CoA (2C)
Krebs cycle
COO– CoA-SH Krebs
The COO– cycle
dehydrogenation of O C
malate produces a CH2
third NADH, and the CH2 Electron transport
Citrate HO C COO–
cycle returns to its COO– chain
1 synthetase CH 2
starting point. Oxaloacetate (4C) Citrate (6C)
COO–
NADH COO–
2
Aconitase CH2
COO– Malate
NAD+ 9 dehydrogenase 3 HC COO–
HO CH
Isocitrate (6C) HO CH
CH2
Malate (4C) COO–
COO –
15
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Fig. 9.12b (TEArt)
Krebs cycle Isocitrate
8 Fumarase 4 dehydrogenase Oxidative
NAD+ decarboxylation
H2 O produces NADH
COO– CO2 with the release
CH Fumarate (4C) NADH of CO2.
HC a-Ketoglutarate (5C) COO–
Succinate
COO FADH 7
2 dehydrogenase 5 CO2 NAD + CH2
CoA-SH a-Ketoglutarate CH2
6 dehydrogenase NADH C O
FAD Succinyl-CoA CoA-SH
Succinate (4C) synthetase COO–
The oxidation of Succinyl-CoA (4C)
succinate COO–
COO–
produces FADH2. CH2 A second oxidative
CH decarboxylation
CH2 GTP GDP + Pi 2
CH2 produces a second NADH
– with the release of a
COO
C O second CO2.
S CoA
ADP
ATP
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Krebs Cycle
• 1: Condensation
• 2-3: Isomerization
• 4: First oxidation
• 5: Second oxidation
• 6: Substrate-level phosphorylation
• 7: Third oxidation
• 8-9: Regeneration and oxaloacetate
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Harvesting Energy by Extracting Electrons
• Glucose catabolism involves a series of
oxidation-reduction reactions that release
energy by repositioning electrons closer to
oxygen atoms.
– Energy is harvested from glucose
molecules in gradual steps, using NAD+ as
an electron carrier.
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Electron Transport
20
Stage Four: The Electron Transport Chain
• NADH molecules carry electrons to the inner
mitochondrial membrane, where they
transfer electrons to a series of membrane-
associated proteins.
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Electron Transport Chain
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Chemiosmosis
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Theoretical ATP Yield of Aerobic Respiration
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Regulating Aerobic Respiration
• Control of glucose catabolism occurs at two
key points in the catabolic pathway.
– glycolysis - phosphofructokinase
– Krebs cycle - citrate synthetase
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Control of Glucose Catabolism
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Catabolism of Proteins and Fats
• Proteins are utilized by deaminating their
amino acids, and then metabolizing the
product.
• Fats are utilized by beta-oxidation.
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Cellular Extraction of Chemical Energy
29
Fermentation
• Electrons that result from the glycolytic
breakdown of glucose are donated to an
organic molecule.
+
– regenerates NAD from NADH
ethanol fermentation
lactic acid fermentation
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Evolution of Cellular Respiration
• degradation
• glycolysis
• anaerobic photosynthesis
• oxygen-forming photosynthesis
• nitrogen fixation
• aerobic respiration
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Summary
• Cellular Energy Harvest
• Cellular Respiration
– Glycolysis
– Oxidation of Pyruvate
– Krebs Cycle
– Electron Transport Chain
• Catabolism of Protein and Fat
• Fermentation
• Evolution of Metabolism
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