The big idea: Haemoglobin in red blood cells carries oxygen. It loads oxygen in the lungs and unloads it in respiring tissues, because the binding is reversible.
The points to remember
- Four subunits, each with a haem group whose iron binds one oxygen molecule: up to four O₂ per haemoglobin.
- Cooperative binding: when one oxygen binds, the molecule changes shape and the other subunits' affinity for oxygen rises.
- So the more oxygen is bound, the easier the next one binds; losing one makes the rest easier to lose.
- Allosteric binding of carbon dioxide: it binds at a site other than the haem and lowers the affinity, so oxygen is released.
Remember it as: Four haems, four oxygens; each one makes the next easier.
Trap: Carbon dioxide does not take oxygen's place on the haem: it binds at a different site and changes the shape.
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The big idea: Foetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, so it can take oxygen from the mother's blood in the placenta.
The points to remember
- Foetal haemoglobin has different subunits, which give it a higher affinity for oxygen.
- Its dissociation curve is S-shaped and lies to the left of the adult curve.
- At the low partial pressure of oxygen in the placenta, the mother's haemoglobin unloads oxygen and the foetus's haemoglobin loads it.
- To sketch the foetal curve: the same S shape, drawn to the left of the adult one.
Two traps: Draw the foetal curve to the left, never to the right: to the right would mean a lower affinity.
Keep the S shape: a straight line or a curve that starts steeply scores nothing.
Real example: newborn babies. After birth a baby breathes for itself, and over its first months its foetal haemoglobin is replaced by adult haemoglobin.
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How it is asked: Expect an adult dissociation curve to sketch the foetal one on, a one-mark adaptation of haemoglobin, or naming the molecule that carries oxygen.
On a graph showing the oxygen dissociation curve of adult haemoglobin, sketch an oxygen dissociation curve for foetal haemoglobin.
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The same question goes on to ask about the molecule itself:
Describe an adaptation of haemoglobin for the transport of oxygen.
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