OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
(j) the oxygen dissociation curve for fetal and adult human haemoglobin.
Full specification wording
Transport in animals: (j) the oxygen dissociation curve for fetal and adult human haemoglobin.
You must interpret and compare the oxygen dissociation curves of adult and fetal human haemoglobin.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 10 of 10
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
You must interpret and compare the oxygen dissociation curves of adult and fetal human haemoglobin. The curve plots percentage oxygen saturation of haemoglobin against partial pressure of oxygen (pO₂). Its sigmoid (S) shape arises because binding the first oxygen molecule changes the haemoglobin's shape, making further binding easier (cooperative binding). At low pO₂ the curve is steep; at high pO₂ it plateaus as haemoglobin becomes fully saturated. Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, so its curve lies to the left of the adult curve. This means that at any given pO₂, fetal haemoglobin is more saturated. In the placenta, where maternal and fetal blood are close, fetal haemoglobin can therefore take up oxygen from maternal blood.
What earns the marks
- Describes the sigmoid shape of the oxygen dissociation curve and explains cooperative binding.
- States the axes: percentage saturation of haemoglobin against partial pressure of oxygen.
- Explains that fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin.
- States that the fetal curve lies to the left of the adult curve.
- Explains that at a given pO₂ fetal haemoglobin has a higher saturation, allowing oxygen uptake across the placenta.
- Interprets the plateau region as full saturation and the steep region as where small pO₂ changes cause large saturation changes.
Common mistakes and how to put them right
Thinking the fetal curve lies to the right.
Put it right by stating higher affinity shifts the curve to the left.
Confusing affinity with capacity.
Put it right by noting affinity is how readily oxygen binds at a given pO₂, not the maximum amount carried.
Describing the curve as linear.
Put it right by identifying the sigmoid shape caused by cooperative binding.
Exam tips
- Always read values from the correct curve and quote the axis units when interpreting a graph.
- When comparing curves, state the direction of the shift (left or right) and what it means for affinity.
- Link the fetal curve to the placenta as the site where oxygen transfer occurs.
Try the lesson
The start of the Transport in animals: (j) the oxygen dissociation curve for fetal and adult human haemoglobin. lesson, made from checked teaching notes for this specification point.
Have a go first
Compared with adult haemoglobin at the same partial pressure of oxygen, the oxygen dissociation curve of fetal haemoglobin is shifted in which direction?
Listen
You must interpret and compare the oxygen dissociation curves of adult and fetal human haemoglobin. The curve plots percentage oxygen saturation of haemoglobin against partial pressure of oxygen (pO₂). Its sigmoid (S) shape arises because binding the first oxygen molecule changes the haemoglobin's shape, making further binding easier (cooperative binding). At low pO₂ the curve is steep; at high pO₂ it plateaus as haemoglobin becomes fully saturated. Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, so its curve lies to the left of the adult curve. This means that at any given pO₂, fetal haemoglobin is more saturated. In the placenta, where maternal and fetal blood are close, fetal haemoglobin can therefore take up oxygen from maternal blood. A strong answer earns marks for these points. Describes the sigmoid shape of the oxygen dissociation curve and explains cooperative binding. States the axes: percentage saturation of haemoglobin against partial pressure of oxygen. Explains that fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin. States that the fetal curve lies to the left of the adult curve. A common slip: Thinking the fetal curve lies to the right. Instead, stating higher affinity shifts the curve to the left. Exam tip: Always read values from the correct curve and quote the axis units when interpreting a graph.
Key words
Say what each word means before you turn the card.
Oxygen dissociation curve
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Cooperative binding
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Haemoglobin affinity
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See every definition
- Oxygen dissociation curve
- A curve plotting percentage oxygen saturation of haemoglobin against partial pressure of oxygen (pO₂), with a sigmoid (S) shape.
- Cooperative binding
- Binding the first oxygen molecule changes the haemoglobin's shape, making further binding easier, which gives the curve its sigmoid shape.
- Haemoglobin affinity
- How readily oxygen binds at a given pO₂; fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin.
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