1. (8) Energy systems in muscle cells
(A)
Creatine Phosphate
(B) Lactic Acid Metabolism
(C)
Types of skeletal muscle fibres

2. (b) Energy systems in muscle cells
State the alternative route of pyruvate and when this occurs
Explain how muscle fatigue develops
Explain what is meant by the term ‘oxygen debt’ and how it can be ‘repaid’

3. What can you remember about these words?
Lactic acid
Muscle fatigue
Oxygen debt

4. Glucose Pyruvate Lactic Acid
LACTIC ACID METABOLISM
(occurs in the absence of oxygen)
In the absence of oxygen, neither the citric acid cycle nor the electron transport chain can occur!
Therefore a cells only source of ATP is through glycolysis.
When glucose is broken down into pyruvate, 2 ATP and 2NADH are made.
This is followed by the conversion of pyruvate into lactic acid AND THE TRANSFER OF HYDROGEN from NADH.
Since the hydrogen has been removed, therefore regenerating NAD. (NAD must be present for glycolysis to continue).
But as more and more lactic acid builds up, muscle fatigue is caused and an oxygen debt is created.
NAD
NADH
Oxygen debt builds up
Glucose Pyruvate Lactic Acid
Oxygen debt repaid
2 ADP+Pi
2 ATP
(through aerobic respiration)

5. Lactic acid accumulates in muscle causing fatigue.
During vigorous exercise, the muscle cells do not get sufficient oxygen to support the electron transport chain.
Under these conditions, pyruvate is converted to lactic acid. This conversion involves the transfer of hydrogen from the NADH produced during glycolysis to pyruvic acid to produce lactic acid. This regenerates the NAD needed to maintain ATP production through glycolysis.
Lactic acid accumulates in muscle causing fatigue.
The oxygen debt is repaid when exercise is complete, as this allows respiration to provide the energy to convert lactic acid back to pyruvic acid and glucose in the liver. 1 3 2 3 4 5 6 7 8
This is a handout 9 10

6. (b) Energy systems in muscle cells
State the alternative route of pyruvate and when this occurs
Explain how muscle fatigue develops
Explain what is meant by the term ‘oxygen debt’ and how it can be ‘repaid’