IB Biology HLGas Exchange SystemsPaper 1 & 2~12 min read
Haemoglobin & Oxygen
Oxygen barely dissolves in water, so blood plasma on its own could never carry enough of it. Haemoglobin fixes that – but the clever part is not that it grabs oxygen. It is that it grabs oxygen in the lungs and then lets go of it again in the tissues, without anyone telling it to.
📚 What you need to know
Haemoglobin is a globular protein in red blood cells made of four polypeptide subunits.
Each subunit holds an iron-containing haem group that binds one oxygen molecule, so one haemoglobin carries four.
Partial pressure (symbol p) is the pressure one gas exerts in a mixture; for oxygen it is written pO2 and it is a measure of oxygen concentration.
The first oxygen binds with difficulty; once it does, the protein changes shape and the next ones bind more easily. This is cooperative binding.
Affinity is how strongly haemoglobin holds oxygen. High pO2 → high affinity → binds.Low pO2 → low affinity → releases.
Foetal haemoglobin has a higher affinity than adult haemoglobin, so its dissociation curve is shifted to the left.
Haemoglobin is an allosteric protein: it exists in more than one conformation. Carbon dioxide is an allosteric inhibitor, and it affects foetal haemoglobin less.
What haemoglobin is
Haemoglobin is a globular protein found in huge numbers inside red blood cells. It is built from four polypeptide subunits, and at the centre of each subunit is a haem group containing an atom of iron. Oxygen combines with the iron in the haem group.
One haem group binds one oxygen molecule.
One haemoglobin molecule therefore carries four oxygen molecules when it is full.
Haemoglobin is called saturated when all four binding sites are occupied.
Partial pressure, in plain words
Air is a mixture of gases, and each one pushes with its own share of the total pressure. That share is the partial pressure of that gas, written with a lower-case p. So pO2 just means “how much oxygen is around”, measured in kilopascals.
High pO2: in the alveoli of the lungs, where fresh air keeps arriving.
Low pO2: in respiring tissue such as working muscle, where oxygen is being used up.
Cooperative binding
Because of the shape of the molecule, the first oxygen has a hard time getting into a haem group. But the moment it binds, the whole protein changes shape, or conformation. The remaining sites become easier to reach, so the second and third oxygens bind quickly. The fourth is slow again, simply because there is only one site left to find.
Slow, fast, fast, slow. Hold on to that pattern – it is the reason the oxygen dissociation curve has the shape it does.
Affinity: binding and letting go
Affinity for oxygen means how tightly haemoglobin holds on. It is not fixed – it depends on how much oxygen is around.
Where
pO2
Affinity
What happens
Alveoli in the lungs
High
High
Haemoglobin and oxygen bind easily, so the blood leaves the lungs almost fully saturated
Respiring muscle
Low
Low
Haemoglobin and oxygen dissociate easily, so oxygen is released to the cells that need it
That single property is what makes a transport system possible. A carrier that only picked oxygen up would be useless; a carrier that only dropped it would be worse. Haemoglobin does both, and which one it does is decided by the local pO2.
🧠
Loads where there is lots, unloads where there is little
High pO2 in the lungs → loads up. Low pO2 in the tissues → unloads. The busier the tissue, the lower its pO2, and the more oxygen it gets.
Foetal haemoglobin
A foetus has a problem. It cannot breathe – it has to take oxygen from its mother’s blood at the placenta. But the mother’s haemoglobin is holding that oxygen already.
The solution is a different protein. Foetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, so at the placenta, where pO2 is fairly low, the mother’s haemoglobin is releasing oxygen while the foetal haemoglobin is still binding it. Oxygen therefore moves from mother to foetus.
On a graph, higher affinity means the whole curve shifts to the left. At any given partial pressure of oxygen, foetal haemoglobin has a higher percentage saturation than adult haemoglobin.
The dashed lines show the same partial pressure giving two very different saturations – which is exactly how oxygen ends up moving from mother to foetus.
After birth the baby starts producing adult haemoglobin, which gradually replaces the foetal form. That matters because a growing, active child needs haemoglobin that releases oxygen easily in the tissues, and a high-affinity protein is bad at letting go.
Allosteric proteins. Haemoglobin can exist in more than one conformation, which is what “allosteric” means. Carbon dioxide is an allosteric inhibitor: when it binds, the protein cannot change shape as freely, so its affinity for oxygen falls. Carbon dioxide has less of this effect on foetal haemoglobin, which helps the foetus keep a high affinity even in blood that is rich in carbon dioxide.
Worked examples
WE 1
Structure and capacity
Explain how the structure of haemoglobin allows it to transport four oxygen molecules. (3 marks)
Point 1: the subunits
Haemoglobin is a globular protein made of four polypeptide subunits.
Point 2: the haem groups
Each subunit contains an iron-containing haem group, and oxygen combines with the iron.
Point 3: the maths
One haem group binds one oxygen molecule, so four subunits bind four oxygen molecules in total; haemoglobin is then described as saturated.
Four subunits, four haem groups, four oxygen moleculessay iron-containing haem group, not just “iron” – the haem is the binding site
WE 2
Why oxygen moves from mother to foetus
Explain how the properties of foetal haemoglobin allow a foetus to obtain oxygen from its mother’s blood. (4 marks)
Point 1: the difference
Foetal haemoglobin has a higher affinity for oxygen than adult haemoglobin.
Point 2: the curve
Its dissociation curve is shifted to the left, so at any given pO₂ it has a higher percentage saturation.
Point 3: at the placenta
The pO₂ there is low, so the mother’s haemoglobin is dissociating from oxygen while foetal haemoglobin is still binding it.
Point 4: the result
Oxygen therefore diffuses from the mother’s blood into the foetal blood and is carried away by foetal haemoglobin.
Higher affinity, curve to the left, so the foetus takes the oxygenthe phrase “at any given partial pressure” is worth a mark on its own
WE 3
Reading the graph
Using the curves above, state the difference in percentage saturation between foetal and adult haemoglobin at a pO2 of 4 kPa, and explain the biological importance of this difference. (3 marks)
Step 1: read both values
Foetal is about 77% saturated and adult is about 59% saturated.
Step 2: subtract77 − 59 = 18 percentage points higher for foetal haemoglobin.
Step 3: explain
At the low partial pressures found at the placenta, foetal haemoglobin holds far more oxygen than adult haemoglobin can, so oxygen transfers from mother to foetus.
About 18 percentage points higher for foetal haemoglobinuse a ruler on the graph and always quote the units of the axis you read from
💡 Exam tips
Write affinity for oxygen, not “likes oxygen”. The examiner is looking for the technical word.
Link affinity to a place: high pO2 in the lungs, low pO2 in respiring tissue.
For any left or right shift, always add “at any given partial pressure of oxygen“.
Use percentage saturation for the y-axis, not “amount of oxygen”.
Learn cooperative binding and be able to say why it makes the curve S-shaped.
Remember the two allosteric points: haemoglobin has more than one conformation, and carbon dioxide is an inhibitor.
⚠ Common mistakes
Saying haemoglobin carries four oxygen atoms. It carries four oxygen molecules, which is eight atoms.
Writing that the foetus takes oxygen because its blood mixes with the mother’s. The two blood supplies never mix; oxygen diffuses across the placenta.
Saying foetal haemoglobin releases oxygen more easily. It holds on more tightly – that is the point.
Confusing left and right shifts. Left is higher affinity; right is lower affinity.
Treating affinity as fixed. It changes with pO2, with carbon dioxide and with pH.
Saying oxygen binds to the protein. It binds to the iron in the haem group.
Up next: The Bohr Shift – what happens to that curve when a muscle starts producing carbon dioxide, and why it is exactly the change the muscle needs.
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