1. Describe the differences between mRNA and tRNA
Lesson starter
Describe the differences between mRNA and tRNA

2. Oxygen transport in the body
describe and explain the oxygen dissociation curve for haemoglobin;
describe and explain the significance of the oxygen dissociation curves of adult oxyhaemoglobin at different levels of carbon dioxide (Bohr effect);
describe and explain the difference in affinity for oxygen between haemoglobin and myoglobin;
describe and explain the build up of an oxygen deficit and oxygen debt/EPOC (Excess Post-exercise Oxygen Consumption);

3. Haemoglobin and myoglobin

4. Haemoglobin
Majority of blood’s oxygen is combined with haemoglobin as oxyhaemoglobin
4 molecules of oxygen can be carried by one haemoglobin
Oxygen is tightly bound to the haem groups
When all haem groups are occupied, the molecule is said to be saturated.
One of 4 prosthetic haem groups where oxygen combines

5. Myoglobin Only has one haem group and only one polypeptide chain
Acts as an oxygen reserve in the muscles
Only gives up oxygen when concentration is very low e.g. intense exercise
Much higher affinity for oxygen than haemoglobin

6. Question Compare the structure of haemoglobin and myoglobin.
Write you answer out in full sentences

7. Oxygen association/dissociation
Where would you get lots of oxygen combining with haemoglobin?
Disassociation
Where would you get lots of oxygen leaving haemoglobin?

8. Oxygen association/dissociation
pO2 refers to the partial pressure of oxygen
At low pO2 oxygen finds it difficult to join up with haemoglobin  this means there is not a lot of percentage saturation of haemoglobin
As the pO2 builds up, it gets easier for oxygen to combine with haemoglobin 
If the pO2 gets too high however, haemoglobin cannot get fully saturated, so the curve starts to level off (plateau).
This gives an S-shaped, or sigmoid curve. 3 2 1

9. Dissociation curves for haemoglobin
The S-shaped dissociation curve is of physiological importance to the body
Haemoglobin becomes saturated in the lungs where the pO2 is high. The opposite is true for respiring tissues such as muscle.
This means that when RBCs in capillaries get to muscle tissue, oxygen quickly unloads from haemoglobin.
This is seen in the steep part of the curve
% saturation of haemoglobin
Partial pressure of oxygen (Kpa)

11. Questions
Why do you think haemoglobin is used to transport oxygen around the body?
Why is the dissociation curve for oxygen described as S-shaped?
Remember to write your answers in sentences

12. The Bohr shift and effect of CO2 on the oxygen dissociation curve
Carbon dioxide can be dissolved in blood plasma to form carbonic acid
When this acid dissociates, it forms H+ ions and bicarbonate ions.
The H+ ions are picked up by haemoglobin to form haemoglobinic acid
Haemoglobin acts as a buffer to prevent a drop in blood pH levels
The effect this causes is to lower the affinity of haemoglobin for oxygen. This means that oxygen is given up to the surrounding tissues easier, even when the pO2 is high

13. The Bohr shift and effect of CO2 on the oxygen dissociation curve
The effect of increasing CO2 levels on % saturation of haemoglobin is called the Bohr effect.
The effect of shifting along the S-shaped curve to the right is known as the Bohr shift.
It is significant to the human body because it leads to greater unloading of oxygen from haemoglobin when the concentration of CO2 is high i.e. during exercise

14. The oxygen debt and excessive post-exercise oxygen consumption (EPOC)
After exercise, oxyhaemoglobin and oxymyoglobin stores need to be replenished.
This occurs during the recovery period.

15. The graph eventually levels out when the oxygen usage is equal to oxygen delivery and uptake by cells. At this point, the Bohr effect is shifting the S-shaped curve to the right, allowing oxygen to leave haemoglobin easily.
O2 uptake rises over the first few minutes of exercise.
Eventually, ATP demand cannot be met therefore anaerobic respiration takes place
Creatine phosphate (energy store in muscles) can be used when ATP is in short supply.
Myoglobin unloads oxygen to release energy, but some lactate will be produced as ATP is made anaerobically

16. The oxygen debt and excessive post-exercise oxygen consumption (EPOC)
As exercise stops, O2 uptake in cells decreases, but is still higher than at rest.
This phase is known as the oxygen debt. The purpose of this is to:
Reload stocks of oxyhaemoglobin and oxymyoglobin
Replace stocks of ATP, creatine phosphate and glycogen
Convert lactate, made during aerobic respiration, to glucose or glycogen
Meet the demands of increased metabolic rate, heart and respiratory functions as a result of the temperature rise during exercise
EPOC = total oxygen consumed after exercise – pre-exercise level of oxygen consumption

19. Question
Explain why most exercise will result in an oxygen debt being formed even if the person does not push themselves so hard that they go above their aerobic threshold.