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Module 4
Respiration
1.4.5. LINK REACTION AND KREBS CYCLE
Starter

Complete Glycolysis summary sheet
Learning Outcomes

Outline the link reaction, with reference to
decarboxylation of pyruvate to acetate and the
reduction of NAD

Explain that coenzyme A carries acetate from the
link reaction to Krebs cycle

Describe the conversion of citrate to oxaloacetate

Outline the Krebs cycle,
including the roles of NAD
and FAD, and substratelevel phosphorylation
(Grade A-B)
Respiration
The process of respiration can be split
into 4 stages.
1.
Glycolysis
2.
Link reaction
3.
Krebs Cycle
4.
Electron Transport Chain
Products of Glycolysis

2 reduced NAD (NADH + H+)

2 Pyruvate

2 ATP
The Fate of Pyruvate?
THIS DEPENDS ON THE AVAILABILITY
OF OXYGEN!!!
18.2 Summary questions

1 Hydrogen is also removed (1); removal of
hydrogen oxidises pyruvate (1).

2 Acetyl group (1); carbon dioxide (1)

3 Pyruvate (1); acetyl CoA (1); reduced NAD (1);

4 Enzymes required are in cytoplasm/ORA (1);
glucose molecule too large to move into
mitochondrion (1); no transport proteins for
pyruvate (1); mitochondria not originally present in
(eukaryotic) cells (1).
The fate of pyruvate
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Aerobic Respiration

If O2 is present 3C pyruvate passes into
mitochondria.

Here it is completely oxidised forming CO2 and H2O.

The second stage of aerobic respiration is the link
reaction.
The Link Reaction
Cytoplasm
Mitochondria
The Link reaction
Glycolysis
The Krebs Cycle
 Links Glycolysis to the Krebs Cycle.
 The end product of the link reaction can enter
the Krebs Cycle.
Link Reaction sequence
NAD+
NADH + H+
Acetate
(2C)
Pyruvate
(3C)
Coenzyme A
Acetyl coenzyme A
(2C)
CO2
Coenzyme
A now
combines
with the
The Pyruvate
is decarboxylated
(CO2) to form
Start
with
Pyruvate
produced
in Acetate
Glycolysis
This
produces
Acetate
Acetyl coenzyme
It is also dehydrogenated
(H+Aremoved)
The Acetyl coenzyme A now enters Krebs cycle
Link Reaction




Pyruvate
decarboxylase
removes a carboxyl group
from pyruvate in the form of
CO2.
Pyruvate
removes
pyruvate
Pyruvate(3C)
dehydrogenase
H
atoms
from
The
remaining
2-carbon
molecule
combines
with
coenzyme A to produce
acetylcoenzyme A (acetyl
CoA).
Another oxidation reaction
occurs when NAD+ collects
hydrogen ions. This forms
reduced NAD (NADH + H+)
Decarboxylation
CO2 (1C)
Oxidation
Coenzyme
(CoA)
NAD+
Reduction
NADH + H+
Acetyl CoA (2C)
Task

Write a summary equation for the link reaction
Overall equation for one link reaction and Krebs cycle:
Pyruvate + NAD + CoA
acetyl CoA + reduced NAD + CO2
The Products of the Link Reaction go to
the Krebs Cycle (takes place in matrix)
So for each glucose molecule: (6C was split into 2x3C)

2 acetylcoenzyme A (go into the Krebs cycle)

2 Carbon dioxide (released as a waste products)

2 Reduced NAD (go to the electron transport chain)
Krebs cycle – metabolic
Each step in the cycle
requires enzymes to
reduce the activation
energy.
The reactions all take
place in the matrix of
the mitochondria and
are usually
represented as a
circular diagram.
cycle
Oxaloacetate
Acetate
2C
Citrate
The Krebs Cycle
Acetyl Coenzyme A (2C)
Coenzyme A
Acetate 2C
Oxaloacetate
(4C)
FAD
(4C)
(4C)
Citrate (6C)
(6C)
NAD
CO2
Reduced
FAD
(4C)
Reduced
NAD
(5C)
(4C)
DeNa DeNa A Fa... Na

This little tune will help you to remember the Krebs
Cycle.

Remember, respiration is all about releasing energy
from your food.

Oxidation releases energy.

When a carbon compound is oxidised, coenzymes
are reduced.

The coenzymes involved are: NAD and FAD.

Carboxylation is the removal of CO2.

Remember this: 665 and five 4’s.
DENA DENA A FA... NA

DENA: Decarboxylation
and production of reduced NAD

DENA: Decarboxylation
and production of reduced NAD

A: Production

FA... : Production

NA: Production
of ATP
of reduced FAD (The ‘...’ means a gap)
of reduced NAD
Oxaloacetate
citrate
DENA
NA
FA
DENA
A
Krebs cycle
DENA

Acetyl CoA joins with the C4(oxaloacetate) group

CoA is released to transport more pyruvate into the
matrix

A C6 fragment is formed (citrate)

C6 (citrate) is oxidatively decarboxylated.

A C5 group is formed.

The carbon is given off as carbon dioxide

NAD+ is reduced to NADH + H+
Krebs cycle

The C5 fragment is oxidised and
decarboxylated further to a C4
compound.

Again the carbon removed forms carbon
dioxide.

A further NAD+ is reduced to NADH + H+.
DENA
Krebs cycle

The final stage in the cycle has the C4
(oxaloacetate) acceptor regenerated.

ADP is reduced to ATP
A
(by phosphate from an intermediate compound – so SLP)

FAD (Coenzyme)is reduced to FADH2 .
FA

There is a reduction of NAD+ to NADH + H+.
NA
Krebs Cycle Summary
Each Acetyl CoA entering the cycle results in:
1.
2 CO2 molecules
2.
1 ATP molecule (S.L.P)
3.
8 pairs of H atoms (Later used in the E.T.C to produce
ATP)
Used to reduce NAD and FAD. Three reduced NAD are
produced and 1 reduced FAD per cycle.
4.
NAD = Nicotinamide adenine dinucleotide
FAD = Flavine adenine dinucleotide
Importance of the Krebs cycle
1)
Breaks down macromolecules into smaller ones
2)
Produces H atoms that are carried by NAD to ETC
for oxidative phosphorylation, to produce ATP.
3)
Regenerates 4 –carbon molecule that combines
with acetylcoenzyme A, which would otherwise
accumulate.
4)
Source of intermediate compounds used by cells
in the manufactures of other important
substances such as fatty acids, amino acids and
chlorophyll
18.3 Summary questions

1 ATP – three phosphate groups (1); one ribose (1); one
nitrogenous base (1); NAD – two phosphate groups (1);
two riboses (1); two nitrogenous bases (1) (max 3
comparisons)

2 Idea that it is used to link reactions (1); idea that
energy is released as a result of the activity of one
enzyme and used by another enzyme (1).

3 Students answers may vary but must include: glucose
to triose phosphate (1); triose phosphate to pyruvate
(1); addition of two ATP (1); production of four ATP and
two reduced NAD (1) (2 max).

4 One per turn (1) two in total (1)
18.3 Summary questions

5 Hydrogen needs to be removed for cycle to
continue (1); hydrogen removed using NAD/FAD
and reduced (1), then NAD/FAD are oxidised at
electron transport chain (1); oxygen required for
electron transport (1).

6 Enzymes are specific (1); active site
complementary to substrate (1); different steps
have different substrates (1); different steps require
different enzymes (1); different enzymes (may)
require different coenzymes (1); only one step in
cycle has enzyme which requires FAD coenzyme (1).
Plenary - Summary Table
Product from one Krebs
cycle
Where it goes
1 coenzyme A
Reused in next link reaction
Oxaloacetate
Regenerated for next Krebs
cycle
2 carbon dioxide
Released as waste product
1 ATP
Used for energy
3 reduced NAD
To oxidative phosphorylation
1 reduced FAD
To oxidative phosphorylation
Remember – the Krebs Cycle happens TWICE per glucose molecule.
If 6 glucose molecules were respired – how much carbon dioxide
would be produced?
24
Krebs cycle
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Counting carbons
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