1. 4.2 Link Reaction and Krebs Cycle
RESPIRATION
4.2 Link Reaction and Krebs Cycle
RESOURCES:
MITOCHONDRIA DIAGRAM HANDOUT
MOLYMODS

2. Learning Objectives Success Criteria
Produce accurate notes on the link reaction
Summarise in a diagram the key events in the Krebs cycle
Carry out an experiment and analyse the results to demonstrate the role of coenzymes in respiration
Describe what the link reaction is
Describe what happens during the Krebs cycle
Explain the role of hydrogen carrier molecules in the Krebs cycle

3. Starter Write down the word equation for Glycolysis
What is used up in glycolysis?
What else is produced in glycolysis?

4. Glycolysis You finish with Pyruvate You start with glycogen.
NAD+ + 2H  NADH + H+ (oxidised form ) (reduced form) NB Rather then write NADH, examiners often simply refer to it as reduced NAD or reduced coenzyme
Glycogen
You start with glycogen.
2 ATP
4 ADP + 4Pi
4 ATP
Glyceraldehyde
3-phosphate
(3C x 2)
Pyruvate
(3C x2)
Glucose
(6C)
2H x 2
NAD
Reduced NAD
2 ADP
Nicotinamide adenine dinucleotide = So it gains electrons
You finish with Pyruvate

5. Respiration The process of respiration can be split into 4 stages.
Glycolysis
Link reaction
Krebs Cycle
Electron Transport Chain

6. What Happens Where? Glycolysis = Cytoplasm of the cell.
Link reaction = Matrix of the mitochondria.
Krebs cycle = Also in the matrix.
Electron transfer chain Utilises proteins found in the membrane of the christa.

7. The Fate of Pyruvate? Products of Glycolysis
2 reduced NAD (NADH + H+)
2 Pyruvate
2 ATP
The Fate of Pyruvate?
THIS DEPENDS ON THE AVAILABILITY OF OXYGEN!!!

8. 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.

9. The Link Reaction Links Glycolysis to the Krebs Cycle.
Cytoplasm
Mitochondria
The Link reaction
The Krebs Cycle
Glycolysis
Links Glycolysis to the Krebs Cycle.
The end product of the link reaction can enter the Krebs Cycle.

10. Coenzyme A now combines with the Acetate to form
NAD+
NADH + H+
Coenzyme A
Acetyl coenzyme A
(2C)
Pyruvate
(3C)
Acetate
(2C)
CO2
Coenzyme A now combines with the Acetate to form
Acetyl coenzyme A
Start with Pyruvate produced in Glycolysis
The Pyruvate is decarboxylated (CO2)
It is also dehydrogenated (H+ removed)
This produces Acetate
The Acetyl coenzyme A now enters Krebs cycle

11. Link Reaction Pyruvate(3C) CO2 (1C) NAD+ NADH + H+ Coenzyme (CoA)
One carbon atom is removed from pyruvate in the form of CO2.
The remaining 2-carbon molecule combines with coenzyme A to produce acetylcoenzyme A (acetyl CoA).
Another oxidation reaction occurs when NAD+ collects more hydrogen ions. This forms reduced NAD (NADH + H+)
No ATP is produced in this reaction.
Pyruvate(3C)
Decarboxylation
CO2 (1C)
NAD+
Oxidation
Reduction
NADH + H+
Coenzyme (CoA)
Acetyl CoA (2C)

12. The Link Reaction occurs Twice for every Glucose Molecule
For each glucose molecule used in glycolysis, two pyruvate molecules are made.
But the link reaction uses only one pyruvate molecule, so the link reaction and the Krebs cycle happen twice for every glucose molecule which goes through glycolysis.
Overall equation for one link reaction and Kreb cycle:
Pyruvate + NAD + CoA acetyl CoA +
reduced NAD + CO2

13. The Products of the Link Reaction go to the Krebs Cycle and the ETC
So for each glucose molecule:
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)

14. Krebs Cycle Sir Hans Krebs in 1937
AKA Citric acid cycle or the Tricarboxylic acid cycle (TCA cycle)
Takes place in the matrix.

15. Krebs Cycle From one glucose molecules you get 2 pyruvate molecules.
So Krebs cycle go around twice per glucose molecule.

16. The Krebs Cycle Acetyl Coenzyme A (2C) NAD Coenzyme A 2C Reduced NAD
Oxaloacetate (4C)
Citrate (6C)
(6C)
(4C)
NAD
(4C)
CO2
FAD
Reduced
NAD
Reduced
FAD
CO2
(5C)
(4C)
NAD
ADP + Pi
ATP
(4C)
Reduced
NAD

17. 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.

18. DENA DENA A FA... NA
DENA: Decarboxylation and production of reduced NAD
A: Production of ATP
FA... : Production of reduced FAD (The ‘...’ means a gap)
NA: Production of reduced NAD

19. The Krebs Cycle: DENA DENA A FA... NA
Acetyl Coenzyme A (2C)
NAD
Coenzyme A 2C NA
Reduced
NAD
Oxaloacetate (4C)
Citrate (6C)
(6C)
(4C)
NAD
(4C)
DENA
CO2
FAD
Reduced
NAD FA
Reduced
FAD
CO2
(5C)
(4C)
DENA
NAD
ADP + Pi
ATP A
(4C)
Reduced
NAD

20. Krebs Cycle Summary 2 CO2 molecules 1 ATP molecules (S.L.P)
Each Acetyl CoA entering the cycle results in:
2 CO2 molecules
1 ATP molecules (S.L.P)
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.
NAD = Nicotinamide adenine dinucleotide
FAD = Flavine adenine dinucleotide

21. Coenzymes in Respiration
Coenzymes are complex organic molecules that are used by enzymes to accept or donate molecules involved in a reaction.
They are often referred to as ‘Helper’ molecules as they carry chemical groups or ions about, e.g. NAD removes H+ and carries it to other molecules.
Now carry out the practical about Coenzymes in respiration

22. Homework Complete the questions on the practical sheet
Read ‘coenzymes in respiration’ on page 55 of the textbook then answer questions 1-5