3. link and krebs cycle

A level

1. The Link Reaction

2. If oxygen is available
energy can be released
from pyruvate via
the Krebs Cycle
and
oxidative phosphorylation

3. Pyruvate from glycolysis can move into the matrix of the
mitochondrion
Pyruvate must cross the outer and inner membranes
This is done by active transport which requires energy

4. Pyruvate must be converted to Acetyl
Coenzyme A (Acetyl CoA)
This used Coenzyme A

Pyruvate + NAD + CoA Acetyl CoA +
NADreduced + CO
2

NAD is reduced and CO
2
is made
Remember this happens twice for each
glucose molecule

5. 3Carbon pyruvate
Pyruvate + NAD + CoA Acetyl CoA + NADreduced
+ CO
2

Coenzyme A
NAD
CO
2 Acetyl CoA
In effect this is a 2Carbon
acetyl molecule
These 2 carbons will enter the
Krebs Cycle
H
NADreduced

6. Remember this
process
happens twice
per glucose

7. The Krebs Cycle
Also called
Citric Acid Cycle and
the Tricarboxylic Acid Cycle
Described by Hans Krebs in 1937

8. The cycle is a series of enzyme controlled reactions:
In Step 1 - Citrate (6C) is formed from acetyl CoA +
oxaloacetate(4C)
In Steps 2-8Citrate is
decarboxylated to yield CO
2
gas
and dehydrogenated to release Hydrogen ions
these will reduce NAD and FAD
Oxaloacetate is regenerated
One ATP is formed by phosphorylation
The Krebs Cycle will go around twice for each
molecule of glucose

9. This is a cycle but for convenience we will start with 4-carbon oxaloacetate
This combines with the 2C acetyl group to form 6-carbon Citrate

10. This is what
oxaloacetate looks
like
There are 4
carbons

11. This is the structure of
citric acid (citrate)
Note it has 6 carbons
There are 3 carboxylic
groups so it used to be
called tricarboxyilic acid
TCA
This is the first step in the Krebs cycle so that is why it is also called
the citric acid or tricarboxylic acid cycle
Citric acid is what makes citrus fruit acidic
It is also used as a food additive

12. In a series of steps citric
acid is dehydrogenated
(hydrogen is removed
and transferred to NAD)
And decarboxylated
(A carbon and 2 oxygens
are removed)
This leaves a 5C cpd

13. This happens again to
reduce another NAD
and release another
CO2
Leaving a 4C cpd

14. In a further series
of reactions more
hydrogens are
removed and used
to reduce NAD
ATP is made by
substrate level
phosphorylation

15. (6C)

16. Products of the Krebs
cycle per glucose molecule
4CO
2
2ATP
2FAD red
6NAD red

17. NAD reduced is a hydrogen carrier molecule
It will carry hydrogen to the electron transfer
chain where ATP will be made
The more hydrogens available the more energy in
the form of ATP can be made

18. FAD is also simply a hydrogen carrier molecule
Each molecule of glucose will only make 2 FAD reduced

19. ATP NAD
red
FAD CO
2
Glycolysi
s
2 2 - -
Link - 2(1) - 2(1)
Krebs
cycle
2(1) 2(3) 2(1) 2(2)
Total 4 10 2 6
Where it goes
next
used To ETC To ETC Diffuses
out
Summary so far…(per glucose molecule)