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3.4.4Exchange and Transport

The Oxyhaemoglobin Dissociation Curve — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 30 of 48 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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3.4.4 The Oxyhaemoglobin Dissociation Curve — AQA A Level Biology 7402 notes: Describe and interpret the oxyhaemoglobin dissociation curve and explain its S shape using cooperative binding.The oxyhaemoglobin dissociation curve plots the percentage saturation of haemoglobin with o…

  • Low pO2 (flat start)
  • Steep middle
  • Plateau at high pO2
  • curve to the left
  • curve to the right
  • Exam-style question 1

Part 1 of 7What the curve shows

The oxyhaemoglobin dissociation curve plots the percentage saturation of haemoglobin with oxygen (y axis) against the partial pressure of oxygen (pO2, in kPa, x axis). It is S-shaped (sigmoid), not a straight line. It shows how haemoglobin responds to different oxygen concentrations in different parts of the body.

Part 2 of 7Explaining the shape

Low pO2 (flat start)
Low pO2 (flat start) haemoglobin is in its original shape and the haem groups are hidden within the molecule, so the first oxygen molecule binds with difficulty, and the saturation rises only slowly.
Steep middle
Steep middle when the first oxygen binds, the quaternary structure changes, which exposes the other haem groups and makes it easier for the second and third to bind.
  • A small increase in pO2 gives a large rise in saturation.
  • This is cooperative binding.
Plateau at high pO2
Plateau at high pO2 it is harder to bind the fourth oxygen because few sites remain unoccupied, so the curve flattens as haemoglobin approaches saturation.

Part 3 of 7Reading off the curve

At a pO2 of about 13 kPa (alveolar), saturation is roughly 97%.

1
At about 5 kPa (resting tissue), saturation is roughly 75%.
2
The steep part of the curve lies in the range of pO2 found in the tissues, so a small fall in pO2 results in a large release of oxygen.
3
This is exactly what is needed when tissues become active and use more oxygen.
4
The plateau in the lungs means that even if the alveolar pO2 falls a little, as at moderate altitude, the haemoglobin still loads almost fully.

To compare curves: curve to the left means a higher affinity for oxygen (it loads more at a given pO2 but releases less); curve to the right means lower affinity, so oxygen is released more readily.

Part 4 of 7Calculations and data

Calculations and data
To find oxygen delivered, read the saturation at the lungs and at the tissues and subtract.
The pO2 at which haemoglobin is 50% saturated is called the P50. A lower P50 means a higher affinity, shown as a shift of the curve to the left.
Percentage of oxygen unloaded = (saturation at the lungs − saturation at the tissues) ÷ saturation at the lungs × 100.

Exampleif the saturation is 97% in the lungs and 22% in exercising muscle, then oxygen unloaded is (97 − 22) ÷ 97 × 100 = 77%.

Part 5 of 7Link to the next topics

The position of the curve is not fixed. In the presence of high carbon dioxide the curve shifts right (the Bohr effect, 3.4.5), whereas for animals living at high altitude, or for a fetus, the curve may be to the left (3.4.6). Be careful when the question gives two curves and asks you which animal or condition each represents.

Part 6 of 7Drawing and using the curve in an exam

Many marks are lost through poor graph work.

Sketch
When asked to sketch a curve, draw axes labelled "partial pressure of oxygen / kPa" and "percentage saturation of haemoglobin with oxygen".
The curve should start at the origin, rise slowly, rise steeply in the middle region and flatten near 100% saturation at about 10–13 kPa.
A second curve for a different condition should be drawn on the same axes and labelled.

Typical values for adult human haemoglobin

  • at 2 kPa about 25%, at 4 kPa about 60%, at 6 kPa about 80%, at 8 kPa about 90%, at 13 kPa about 97%.
  • These are approximate and examination curves may differ slightly, so always read the values from the graph provided.

For comparison with myoglobin, the myoglobin curve is to the left of the haemoglobin curve and is hyperbolic, which means it is saturated at a very low pO2 and will only release oxygen when pO2 falls to very low levels. The difference of the two curves represents the transfer of oxygen from haemoglobin to myoglobin in muscle.

Answer structure for a six-mark questiondescribe the shape, explain it with cooperative binding in terms of changes in shape, relate each section of the curve to its location in the body, and add the effect of a shift.

Part 7 of 7Exam-style questions

  1. Explain why the oxyhaemoglobin dissociation curve is S-shaped. [4]4 marks

  2. The P50 of haemoglobin from animal A is 3 kPa and that from animal B is 6 kPa. State which has the higher affinity for oxygen and justify your answer. [2]2 marks

  3. Explain the advantage of the steep section of the curve occurring at the pO2 found in respiring tissues. [2]2 marks

Key terms

TermMeaning
Dissociation curveGraph of percentage saturation of haemoglobin against pO2.
SigmoidS-shaped.
P50pO2 at which haemoglobin is half saturated.
Cooperative bindingBinding of one oxygen makes the next easier.
SaturationThe proportion of haem groups carrying oxygen.
AffinityTendency to bind to oxygen.

Avoid these mistakes

1
Drawing the curve as a straight line or an exponential curve without the S shape.
2
Saying that a curve to the right means higher affinity.
3
Using concentration of haemoglobin instead of percentage saturation on the axis.
4
Forgetting to name cooperative binding when explaining the shape.
5
Not subtracting values when asked for the amount of oxygen released.

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