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The Citric Acid Cycle
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.
This step is carried out by a multienzyme complex that catalyses
three reactions (blue numbers).
Figure 9.10
MITOCHONDRION
CYTOSOL
CO2
Coenzyme A
3
1
2
Pyruvate
Transport protein
NAD
NADH + H
Acetyl CoA
What is getting oxidized?
• After pyruvate is oxidized, the citric acid
cycle completes the energy-yielding
oxidation of organic molecules
• This process happens in the mitochondria
of eukaryotic cells only when O2 is
present.
The Citric Acid Cycle
• The citric acid cycle (also called the Krebs
cycle) completes the break down of
pyruvate to CO2.
• The cycle oxidizes organic fuel (acetyl
coA).
• And generates 1 ATP, 3 NADH, and 1
FADH2 per turn.
• How many of each does it make per
glucose molecule?
Figure 9.11
Pyruvate
CO2
NAD
CoA
Summary Picture
(the important stuff!)
NADH
+ H
Acetyl CoA
CoA
CoA
Citric
acid
cycle
2 CO2
3 NAD
FADH2
3 NADH
FAD
+ 3 H
ADP + P i
ATP
The citric acid cycle has eight steps,
each catalyzed by a specific enzyme.
Figure 9.12-1
Acetyl CoA
CoA-SH
1
Oxaloacetate
Citrate
Citric
acid
cycle
The acetyl group of acetyl CoA joins the cycle by
combining with oxaloacetate, forming citrate.
Figure 9.12-2
Acetyl CoA
CoA-SH
H2O
1
Oxaloacetate
2
Citrate
Isocitrate
Citric
acid
cycle
The next seven steps decompose the citrate back to
oxaloacetate, making the process a cycle.
Figure 9.12-3
Acetyl CoA
CoA-SH
H2O
1
Oxaloacetate
2
Citrate
Isocitrate
NAD
Citric
acid
cycle
3
NADH
+ H
CO2
-Ketoglutarate
Figure 9.12-4
Acetyl CoA
CoA-SH
H2O
1
Oxaloacetate
2
Citrate
Isocitrate
NAD
Citric
acid
cycle
NADH
3
+ H
CO2
CoA-SH
-Ketoglutarate
4
NAD
NADH
Succinyl
CoA
+ H
CO2
Figure 9.12-5
Acetyl CoA
CoA-SH
H2O
1
Oxaloacetate
2
Citrate
Isocitrate
NAD
Citric
acid
cycle
NADH
3
+ H
CO2
CoA-SH
-Ketoglutarate
4
CoA-SH
5
NAD
Succinate
GTP GDP
ADP
ATP
Pi
Succinyl
CoA
NADH
+ H
CO2
Figure 9.12-6
Acetyl CoA
CoA-SH
H2O
1
Oxaloacetate
2
Citrate
Isocitrate
NAD
Citric
acid
cycle
Fumarate
NADH
3
+ H
CO2
CoA-SH
-Ketoglutarate
4
6
CoA-SH
5
FADH2
NAD
FAD
Succinate
GTP GDP
ADP
ATP
Pi
Succinyl
CoA
NADH
+ H
CO2
Figure 9.12-7
Acetyl CoA
CoA-SH
H2O
1
Oxaloacetate
2
Malate
Citrate
Isocitrate
NAD
Citric
acid
cycle
7
H2O
Fumarate
NADH
3
+ H
CO2
CoA-SH
-Ketoglutarate
4
6
CoA-SH
5
FADH2
NAD
FAD
Succinate
GTP GDP
ADP
ATP
Pi
Succinyl
CoA
NADH
+ H
CO2
Figure 9.12-8
Acetyl CoA
CoA-SH
NADH
The NADH and
FADH2
produced by the
cycle relay
electrons
extracted from
food to the
electron
transport chain.
+ H
H2O
1
NAD
8
Oxaloacetate
2
Malate
Citrate
Isocitrate
NAD
Citric
acid
cycle
7
H2O
Fumarate
NADH
3
+ H
CO2
CoA-SH
-Ketoglutarate
4
6
CoA-SH
5
FADH2
NAD
FAD
Succinate
GTP GDP
ADP
ATP
Pi
Succinyl
CoA
NADH
+ H
CO2
Concept Map
–
–
–
–
–
–
–
–
–
–
–
Cellular Respiration
Aerobic
Anaerobic
Glycolysis
The Citric Acid Cycle
Oxidative
Phosphorylation
The Electron
Transport Chain
Fermentation
ATP
NADH
NAD+
–
–
–
–
–
–
–
–
–
–
Carbon dioxide
Oxygen
Glucose
Water
H+ ions
Membrane
proteins
Mitochondria
Cytosol
Eukaryotes
Prokaryotes