Read the adult haemoglobin oxygen curve
Describe and explain the sigmoid oxygen dissociation curve of adult haemoglobin.
Section 8.2 · Transport of oxygen and carbon dioxide
AS Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; microscope recognition, biological drawing and supervised practical competence require practical work, while human subject-teacher review remains pending.
Before you begin
- Know that mass transport moves substances between exchange surfaces and cells, and that diffusion occurs down a concentration or partial-pressure gradient.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does the vertical axis of an oxygen dissociation curve show?
What you will learn
- Define the variables on an oxygen dissociation curve
- Explain the sigmoid shape using cooperative binding
Activity and recovery records
| Trial | Rest | After activity | After 2 min |
|---|---|---|---|
| 1 | 70 | 118 | 82 |
| 2 | 72 | 120 | 84 |
| 3 | 74 | 122 | 86 |
| Mean | 72 | 120 | 84 |
Build the explanation
An oxygen dissociation curve plots percentage saturation of haemoglobin with oxygen against oxygen partial pressure. Adult haemoglobin's curve is sigmoid: at low partial pressure, little oxygen is bound; through the steep middle region, a small rise in partial pressure produces a large increase in saturation; at high partial pressure, the curve plateaus as binding sites become occupied. Binding of one oxygen molecule increases haemoglobin's affinity for the next oxygen molecules; this is cooperative binding.
Apply it carefully
Read values from the axes before interpreting the curve. A movement along one unchanged curve means oxygen partial pressure changed while the curve conditions stayed the same. A shift of the whole curve means haemoglobin affinity changed. The sigmoid is not a straight-line relation, and percentage saturation is not the same quantity as oxygen partial pressure.
Pause and explain
What does movement along one unchanged curve usually indicate?
Worked example
Why does a small increase in oxygen partial pressure produce a large increase in saturation in the steep region?
Show the worked solution
- Identify that many haemoglobin molecules are in the transition region.
- Cooperative binding increases affinity as oxygen molecules bind.
Answer In the steep region, cooperative binding means a small rise in oxygen partial pressure can cause many more haemoglobin binding sites to become occupied, so saturation rises sharply.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the shape of the adult haemoglobin oxygen dissociation curve.
Give me a hint
Mention the low, middle and high partial-pressure regions.
Compare my reasoning
- It is sigmoid.
- It is relatively shallow at low and high partial pressure and steep through the middle.
The curve is sigmoid: shallow at low partial pressure, steep through the middle and then flattening at high partial pressure as haemoglobin approaches full saturation.
Look for these in your work
- Sigmoid shape named
- Three regions described
Distinguish movement along a curve from a shift of the whole curve.
Give me a hint
Ask whether oxygen partial pressure or haemoglobin affinity conditions changed.
Compare my reasoning
- Moving along one curve reflects a change in oxygen partial pressure.
- A shift indicates a change in haemoglobin affinity under different conditions.
Movement along an unchanged curve occurs when oxygen partial pressure changes. A shift of the whole curve indicates altered haemoglobin affinity due to changed conditions, such as pH or carbon dioxide concentration.
Look for these in your work
- Movement along linked to partial pressure
- Shift linked to affinity conditions
A graph shows saturation rising from 25% to 70% across a small oxygen partial-pressure interval in the middle of the curve. Explain the pattern without claiming that the curve is linear.
Give me a hint
Use the steep section and cooperative binding.
Compare my reasoning
- The interval lies on the steep section.
- Cooperative oxygen binding makes saturation rise rapidly over that range.
The interval lies in the steep region, where a small increase in oxygen partial pressure is associated with a large increase in saturation. Cooperative binding explains this responsiveness; it does not make the entire curve linear.
Look for these in your work
- Steep region identified
- Cooperative binding applied
- Non-linearity retained
Common mistakes
- Calling the whole curve linear
- Confusing oxygen partial pressure with haemoglobin saturation
- Calling movement along a curve a shift
What can you explain now?
What does the vertical axis of an oxygen dissociation curve show?
Compare with the explanation
Percentage saturation of haemoglobin with oxygen
The curve relates haemoglobin saturation to oxygen partial pressure.
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Make it stick
What do the axes show, and why is the adult haemoglobin curve sigmoid?
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