The oxygen dissociation curve, the Bohr shift and carbon dioxide transport
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 1 worked example, 4 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Load in the lungs, unload in the muscle
- The curve
- Why S-shaped: cooperative binding
Read the transcript of the teaching part
Last time
Before we start: last time, in Blood vessels, tissue fluid and lymph, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Load in the lungs, unload in the muscle
Haemoglobin has a hard job. It must pick up oxygen where there is plenty, in the lungs, and let go of it where it is needed, in a working muscle. It manages this because its affinity for oxygen changes with conditions. The graph that shows this is the oxygen dissociation curve.
The curve
The oxygen dissociation curve plots percentage saturation of haemoglobin against the partial pressure of oxygen, measured in kilopascals. The curve is S shaped, or sigmoid. At low partial pressure, saturation rises slowly. In the middle, it rises very steeply. At high partial pressure, it levels off near one hundred percent. In the lungs, where the partial pressure is high, haemoglobin is almost fully saturated. In the tissues, where it is low, haemoglobin releases much of its oxygen.
Why S-shaped: cooperative binding
Haemoglobin has four haem groups. The first oxygen molecule binds with difficulty, because the molecule is tightly packed. Binding changes the shape of the molecule. This makes it easier for the second and third oxygen molecules to bind. This is cooperative binding. The fourth oxygen binds slowly again, because few binding sites are left. This gives the plateau at the top of the curve.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Carbon dioxide transport and the chloride shift · Haemoglobin and the oxygen dissociation curve · The Bohr shift.
The whole chapter: Transport in Mammals, Transport in Mammals.
Chapter 8: Transport in mammals
- The heart, double circulation and the heartbeat 5 min
- The cardiac cycle and blood cells 5 min
- Blood vessels, tissue fluid and lymph 6 min
- The oxygen dissociation curve, the Bohr shift and carbon dioxide transport 6 min
Also for A Level Biology 9700
Which topics come up most in A Level Biology 9700 past papers, counted from every paper since 2004: Paper 1 topic frequency, Paper 2 topic frequency, Paper 4 topic frequency.