The big idea: An oxygen dissociation curve shows how saturated haemoglobin is at each partial pressure of oxygen: a picture of its affinity for oxygen.
The points to remember
- y axis: saturation of haemoglobin with oxygen (%); x axis: partial pressure of oxygen (kPa or mmHg).
- To read a value: go up from the partial pressure to the curve, then across: at 5 kPa, about 71%.
- Describe it in three points: saturation rises with partial pressure, the curve is S-shaped (sigmoid), and it levels off at high partial pressure.
Two traps: Give the unit: 71%, not 71.
'It reaches 100%' is not a description: the curve levels off just below full saturation.
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The big idea: The curve is S-shaped because of cooperative binding: each oxygen that binds makes the next one bind more easily.
The points to remember
- Flat start: at low partial pressure the first oxygen binds with difficulty (low affinity).
- Steep middle: binding changes haemoglobin's shape, so the affinity rises and oxygen binds fast.
- Plateau: almost every haem group is full, so little more can bind.
- The steep part matches the partial pressures in respiring tissues: a small fall in partial pressure releases a lot of oxygen.
- In the lungs (high partial pressure) haemoglobin loads; in the tissues (low) it unloads.
Remember it as: Slow start, fast middle, full at the top.
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How it is asked: Expect a curve to read and describe, or a 4-mark Outline of how haemoglobin's binding makes oxygen transport efficient.
Describe the relationship between the partial pressure of oxygen and the percentage saturation of haemoglobin.
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A longer question on the same idea:
Outline how reversible changes to haemoglobin molecules increase the efficiency of oxygen transport in the body.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.