IB Biology HLGas Exchange SystemsPaper 1 & 2~12 min read
The Oxygen Dissociation Curve
One S-shaped line carries most of the marks in this topic. If oxygen simply dissolved in blood the graph would be a straight line, and your tissues would be in trouble. The curve is the shape it is because of one property of haemoglobin – and once you see that, every region of the graph explains itself.
📚 What you need to know
The curve shows the percentage saturation of haemoglobin against the partial pressure of oxygen (pO2).
Haemoglobin is saturated when all four binding sites are full.
In a simple liquid such as water, oxygen dissolves at a constant rate and the graph would be a straight line. Haemoglobin binds at different rates as pO2 changes, which is why the graph is a curve.
Low pO2: shallow start – the first oxygen is difficult to bind, so affinity and saturation are low.
Medium pO2: steep middle – cooperative binding means a small change in pO2 gives a large change in saturation.
High pO2: plateau – nearly all sites are already full, so a big rise in pO2 changes saturation very little.
Read left to right for how haemoglobin associates with oxygen; read right to left for how it dissociates.
What the axes mean
x-axis: partial pressure of oxygen, in kPa. This is the pressure oxygen exerts within a mixture of gases, and it is really a measure of oxygen concentration. High in the alveoli, low in respiring tissue.
y-axis: percentage saturation of haemoglobin. 100% means every haem group in the sample is holding an oxygen molecule; 50% means half of them are.
The line answers one question at every point: if blood sits in this much oxygen, how full does its haemoglobin get?
Why it is not a straight line
Put oxygen next to water and it dissolves steadily – twice the pressure, roughly twice the amount dissolved. A straight line. Blood does not behave like that, because the oxygen is not dissolving, it is binding to a protein that changes shape as it fills up.
The dashed line is what a simple solution would do. The real curve starts slower, overtakes in the middle, and then flattens off.
Reading it left to right: binding
Region
What the curve does
Why
Low pO2 (bottom left)
Rises slowly; saturation stays low
The shape of the molecule makes the first oxygen difficult to bind, so haemoglobin has a low affinity here and cannot pick much up
Medium pO2 (middle)
Rises steeply; saturation climbs fast
Once the first oxygen is bound the protein changes conformation, so the next ones bind easily. A small rise in pO2 gives a large rise in saturation
High pO2 (top right)
Levels off near 100%
Most binding sites are already occupied, so there is little left to fill. Even a large rise in pO2 barely changes saturation
Reading it right to left: releasing
The same line read backwards tells you about dissociation – haemoglobin letting go.
In the lungs, where pO2 is high, there is very little dissociation. Blood leaves almost fully saturated.
At medium pO2, on the steep part, oxygen dissociates readily. A small drop in pO2 causes a large drop in saturation, so a lot of oxygen is handed over.
At very low pO2, dissociation slows again. Few oxygen molecules are left on the binding sites, and the last one is as reluctant to leave as the first was to arrive.
Here is the part worth understanding rather than memorising: the steep middle of the curve sits at exactly the partial pressures found in respiring tissue. That is not a coincidence. It means a small drop in tissue oxygen triggers a big release, so the busier a tissue gets, the more oxygen it is given.
Steep means sensitive. Flat means stable. The body uses both: stable loading in the lungs, sensitive unloading in the tissues.
Putting the whole shape together
The explanation in one chain
first O2 binds slowly → protein changes shape → next ones bind easily → sites run out → curve flattens
Shallow bottom left – the shape of the molecule makes it difficult for the first oxygen to reach a haem group, so binding is slow.
Steep middle – after the first oxygen binds, the protein changes conformation and the remaining sites become easier to fill. This cooperative binding speeds everything up.
Plateau top right – as haemoglobin approaches saturation there is a shortage of empty sites, so the fourth oxygen takes longer to bind and the line levels off.
🧠
Hard, easy, easy, hard
Four words for the four oxygen molecules. They give you the three regions of the curve and the reason for each one, which is most of what any “explain the shape” question is asking for.
Do not forget the curve can move. Everything on this page describes one set of conditions. Raise the carbon dioxide level and the whole curve shifts right (the Bohr shift); foetal haemoglobin sits to the left. The shape stays the same – only the position changes.
Worked examples
WE 1
Explaining the shape
Explain the shape of the oxygen dissociation curve for haemoglobin. (4 marks)
Point 1: the shallow start
Because of the shape of the molecule, the first oxygen binds slowly, so the curve is shallow at low pO₂.
Point 2: the change in shape
Once one oxygen has bound, the haemoglobin changes conformation, making the next binding sites easier to reach.
Point 3: the steep middle
This cooperative binding speeds up binding of the next oxygen molecules, so a small increase in pO₂ causes a large increase in saturation.
Point 4: the plateau
Near saturation there are few empty sites left, so the fourth oxygen binds slowly and the curve levels off.
Hard, easy, easy, hard – and the curve followsthe words conformation and cooperative binding are both worth marks; use them by name
WE 2
Using the steep region
Using the graph, explain the importance of the steep region of the curve to a respiring muscle. (3 marks)
Step 1: read the values
Between 6 and 4 kPa the saturation falls from about 82% to about 59%, a drop of roughly 23 percentage points.
Step 2: what that means
A small decrease in pO₂ causes a large decrease in saturation, so a lot of oxygen dissociates from haemoglobin.
Step 3: why it matters
Respiring tissues have partial pressures in exactly this range, so oxygen is released readily where it is needed for aerobic respiration.
Steep region = large release for a small change in pO₂quote two readings from the graph; “it releases more oxygen” alone will not get the data mark
WE 3
Why the plateau is useful
Explain why the percentage saturation of haemoglobin changes very little at partial pressures above about 10 kPa, and suggest why this is an advantage. (3 marks)
Point 1: the reason
At high pO₂ nearly all the binding sites are already occupied, so there is little capacity left and extra oxygen makes almost no difference.
Point 2: the reading
From 12 to 14 kPa the saturation only rises from about 97% to about 98%.
Point 3: the advantage
Blood leaves the lungs almost fully saturated even if the partial pressure in the alveoli falls, for example at altitude or during illness, so oxygen delivery stays reliable.
The plateau makes loading in the lungs safe and stable“suggest” invites a sensible biological reason; the altitude point is a strong one
💡 Exam tips
Always name the axes properly: percentage saturation of haemoglobin against partial pressure of oxygen.
Quote figures from the graph in any data question, with the units.
Describe the regions as shallow, steep, plateau, then explain each with cooperative binding.
Link the steep region to the partial pressures found in respiring tissue. That is the mark most people miss.
Remember that reading right to left describes dissociation, which is what tissue questions are about.
Check whether the question is about the shape of the curve or its position – position questions are Bohr or foetal haemoglobin questions.
⚠ Common mistakes
Saying the y-axis shows the amount of oxygen in the blood. It shows the percentage saturation of the haemoglobin.
Describing the curve without explaining it. “It goes up steeply” earns nothing on its own.
Forgetting cooperative binding when asked to explain the shape.
Saying haemoglobin has a fixed affinity. Affinity changes across the curve and with conditions.
Reading the graph carelessly. Use a ruler and read to the nearest gridline.
Mixing shape with position. Cooperative binding explains the shape; carbon dioxide and foetal haemoglobin explain a shift.
That completes Gas Exchange Systems. The whole topic is one idea told six ways: get a big, thin, moist surface, keep fresh supplies moving on both sides of it, and let diffusion do the rest – in an alveolus, in a leaf, and on the surface of a haemoglobin molecule.
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