The energy in the pyruvate molecules
produced in glycolysis can only be
released using oxygen in a process
called the Krebs Cycle.
• Before pyruvate can
enter the Krebs
Cycle, it is oxidised in
the Link Reaction.
• Pyruvate molecules
are actively
transported into the
mitochondrial matrix.
The Link Reaction
• Carbon dioxide is removed
from the pyruvate
• Hydrogen atoms are removed
and transferred to NAD.
• The reactions are carried out
by a collection of enzymes
known as the Pyruvate
Dehydrogenase Complex.
• A 2 carbon compound called
an acetyl group is formed
which is attached to a cofactor
called coenzyme A.
• The resulting acetyl CoA can
enter the Krebs Cycle.
Pyruvate (3C)
NAD
NADH/H+
CO2
Acetyl CoA (2C)
Extension – the Pyruvate
Dehydrogenase Complex
The Krebs Cycle
• Acetyl CoA from the Link Reaction is
combined with a 4C compound called
oxaloacetate, forming 6C citrate.
• Citrate undergoes a series of
decarboxylation and dehydrogenation
reactions which result in the regeneration
of oxaloacetate.
The Krebs Cycle
Krebs
Cycle
animation
Products of the Krebs Cycle
• Each turn of the Krebs Cycle produces:
3 molecules of reduced NAD
1 molecule of reduced FAD
1 molecule of ATP
2 molecules of CO2
• The Krebs Cycle turns twice for every glucose
molecule broken down, so per glucose molecule
the yield is doubled: 6 NAD; 2 FAD; 2ATP and 4
CO2