8.2.48 · Transport in Mammals

The Oxygen Dissociation Curve — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 10 parts Lesson 14 of 21 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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8.2.4 The Oxygen Dissociation Curve — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe and explain the oxygen dissociation curve of haemoglobin and compare curves for different pigments.Axes — percentage saturation of haemoglobin with oxygen (y-axis) against p…

Conformational change
Amount unloaded
Left shift
Right shift
Typical values for adult haemoglobin
Reliable loading

Part 1 of 8What the curve shows

Axes

Axes percentage saturation of haemoglobin with oxygen (y-axis) against partial pressure of oxygen, pO₂, in kPa (x-axis).

Saturation

Saturation it is the proportion of haem groups combined with oxygen;
- haemoglobin is saturated when all four sites hold O₂.

Affinity

Affinity how readily haemoglobin binds and releases oxygen.
High affinity means it binds easily and dissociates slowly.
Low affinity means it binds slowly and dissociates easily.

Shape

Shape the curve for adult human haemoglobin is S-shaped (sigmoid).
In water oxygen would dissolve at a constant rate, giving a straight line.

Part 2 of 8Explaining the S-shape: cooperative binding

Low pO2

Low pO2 the curve is shallow because the first O₂ binds with difficulty, as the haem groups are buried inside the molecule.

Conformational change

Conformational change once the first O₂ binds, the molecule changes shape and opens up.
This makes the second and third O₂ bind more easily.
The curve rises steeply; this is positive cooperativity.

High pO2

High pO2 the curve flattens because most haem groups are occupied and a free site for the fourth O₂ is hard to find.

Part 3 of 8Reading the curve

Lungs

Lungs at about 13 kPa haemoglobin is about 97 per cent saturated, and a large rise in pO₂ changes saturation little.

Tissues

Tissues the steep part matches respiring tissue pO₂, so a small fall releases a lot of oxygen.
- (about 3 to 6 kPa)

Low pO2

Low pO2 few O₂ molecules remain and release of the last one is harder.

Amount unloaded

Amount unloaded saturation in the lungs minus saturation at the tissue pO₂.
At rest, arterial saturation of 97 per cent and tissue saturation of 75 per cent mean 22 per cent is unloaded.
In exercise tissue pO₂ falls to about 2 kPa (saturation about 30 per cent), so unloading may reach 67 per cent.

Part 4 of 8Affinity and curve position

Pigment	Curve position	Reason and advantage
Adult haemoglobin	Reference S-shaped curve	Loads in lungs, unloads in tissues
Fetal haemoglobin	To the left	Higher affinity, takes oxygen from maternal haemoglobin across the placenta
Myoglobin	Far to the left, hyperbolic	Single chain, no cooperativity; stores oxygen in muscle and releases it only at very low pO₂
Llama or other high-altitude animals	To the left	Loads efficiently where pO₂ is low
Small active mammals such as the shrew	To the right	Unloads readily to meet high oxygen demand
Lugworm in low-oxygen burrows	Far to the left	Gathers oxygen when little is available

Left shift

Left shift higher affinity: haemoglobin takes up oxygen at a lower pO₂ and unloads less easily.

Right shift

Right shift lower affinity: oxygen is released more readily, which suits active tissues;
- carbon dioxide causes this (Bohr shift).

Part 5 of 8Plotting the curve

1Axes

Axes pO₂ on the horizontal axis and saturation on the vertical axis, both labelled with units.

2Line

Line join points with a smooth curve, not straight lines.

3Typical values for adult haemoglobin

0 per cent at 0 kPa, 20 per cent at 2 kPa, 50 per cent at 3.5 kPa, 75 per cent at 5 kPa, 90 per cent at 8 kPa and 97 per cent at 13 kPa.

4P50

P50 the pO₂ at 50 per cent saturation.
A lower P₅₀ means a higher affinity (curve to the left).
A higher P₅₀ means a lower affinity (curve to the right).

Part 6 of 8Biological significance

Biological significance

Reliable loading

Reliable loading haemoglobin is nearly fully loaded over a wide range of lung pO₂, even at moderate altitude.

Efficient unloading

Efficient unloading a small fall in tissue pO₂ gives a large fall in saturation.

Comparison with myoglobin

Comparison with myoglobin a hyperbolic curve loads well but unloads poorly.

Structure and function

Structure and function cooperativity is a feature of quaternary structure.
Allosteric enzymes also show cooperative behaviour.
Haemoglobin S has a lower affinity for oxygen.

Part 7 of 8More from the specification

Part 8 of 8More on this topic

Purpose
- the Bohr effect gives the largest oxygen supply to the tissues making the most CO₂.

Key terms

Term	Meaning
Percentage saturation	The proportion of haem groups combined with oxygen.
Affinity	How readily haemoglobin binds oxygen.
Cooperative binding	Binding of one O_{2} changes shape and makes binding of the next easier.
Sigmoid	S-shaped.
P_{50}	The pO_{2} at which haemoglobin is 50 per cent saturated.
Conformational change	A change in the shape of a protein molecule.

Avoid these mistakes

Saying the curve is a straight line — it is S-shaped because of cooperative binding.

Saying the curve to the left unloads more oxygen — a left shift means higher affinity and less unloading.

Saying haemoglobin is only 50 per cent saturated in the lungs — it is about 97 per cent.

Drawing the curve with straight lines between points — it must be a smooth curve.

Last updated: 15 October 2026