Krebs Cycle
• state that the Krebs cycle takes place in the mitochondrial matrix;
• outline the Krebs cycle, with reference to the formation of citrate
from acetate and oxaloacetate and the reconversion of citrate to
oxaloacetate (names of intermediate compounds are not
required);
• explain that acetate is combined with coenzyme A to be carried to
the next stage;
• explain that during the Krebs cycle, decarboxylation and
dehydrogenation occur, NAD and FAD are reduced and substrate
level phosphorylation occurs;
Respiration: The 4 Parts
Respiration consists of 4 parts:
• Glycolysis
• Link Reaction
• Krebs Cycle
• Oxidative Phosphorylation (the electron
transport chain)
The Krebs Cycle is the Third Stage of
Aerobic Respiration
• The krebs cycle takes place in the matrix of
the mitochondria.
• It happens once for each pyruvate molecule
made in glycolysis, and so it goes round twice
for every glucose molecule that enters the
respiration pathway
Acetyl CoA
ATP
4- carbon
compound
ADP + Pi
oxaloacetate
NADH
4C
2C
CoA
6- carbon compound
NAD
citrate
FADH
6C
FAD
CO2
5- carbon
compound
NADH
NAD
5C
CO2
1C
NAD
NADH
1C
The Krebs Cycle
• Acetyl co enzyme A from the link reaction combines
with oxaloacetate to form citrate
• Coenzyme A is released back to the link reaction to
be used again
• The 6-carbon citrate molecule is decarboxylated
(loses CO2) to give a 5-carbon molecule
• Both citrate and the 5-carbon molecule formed from
it are also dehydrogenated (lose hydrogen) in the
cycle, to reduce the coenzymes NAD and FAD (flavin
adenine dinucleotide)
The Krebs Cycle
• Overall, 3 reduced NAD+ and 1 reduced FAD
are produced. These coenzymes can now be
used to carry the hydrogen to the electron
transport chain
• The 5 carbon compound is decarboxylated,
bringing you back to 4-carbon oxaloacetate.
ATP and CO2 are released
Products of the Krebs Cycle enter the final Stage
of Aerobic Respiration
• Some products are reused, some are released
and others are used in the final stage,
oxidative phosphorylation: the electron
transport chain…
• There is one turn of the Krebs cycle for each
molecule of acetate, which was made from
one molecule of pyruvate. Therefore there are
two turns of the cycle for each molecule of
glucose
Products of the Krebs Cycle enter the final Stage
of Aerobic Respiration
• One coA is reused in the next link reaction
• Oxaloacetate is regenerated so it can be reused in the next krebs cycle
• Two carbon dioxide molecules are released as a waste product of
respiration
• One molecule of ATP is made per turn of the cycle- by substrate level
phosphorylation
• Three reduced NAD and one reduced FAD co-enzymes are made and enter
the electron transport chain
Substrate-level phosphorylation is a type of chemical reaction that results in the formation and creation of
adenosine triphosphate (ATP) by the direct transfer and donation of a phosphoryl (PO3) group to adenosine
diphosphate (ADP)
Krebs Questions
1. Why do living organisms only have a small
amount of oxaloacetate in their cells?
2. Explain why each stage of the Krebs cycle
needs to be catalysed by its own specific
enzyme
3. State the role of pyruvate dehydrogenase
Krebs Questions
1. Why do living organisms only have a small
amount of oxaloacetate in their cells? It is
constantly being regenerated
2. Explain why each stage of the Krebs cycle needs
to be catalysed by its own specific enzyme each
substrate needs it’s own enzyme with a
specifically shaped active site that is
complementary to the substrate
3. State the role of pyruvate dehydrogenase it
removes hydrogen from pyruvate
Task
• Now try to draw glycolysis, the link reaction
and the Krebs Cycle on A3 with no notes 