Describe the structure of an adult haemoglobin molecule.

It is a globular protein with a quaternary structure of four polypeptide chains, two alpha and two beta, each folded around a haem prosthetic group containing an iron ion.

Why does haemoglobin load oxygen in the lungs but release it in respiring tissue?

The partial pressure of oxygen is high in the alveolar capillaries, where haemoglobin has a high affinity and becomes almost fully saturated, and low in respiring tissue, where its affinity is much lower so oxygen dissociates.

Explain why the oxygen dissociation curve of haemoglobin is S-shaped.

Binding of the first oxygen molecule alters the shape of the molecule so that the remaining binding sites become more accessible, making each of the next two easier to bind, while the fourth is harder because so few sites remain free.

OCR A-Level Biology B H422

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Metabolism and Exercise

Haemoglobin and the transport of oxygen

3 things to know
Components 01, 02 and 03

Overview

Recall questions on the structure of haemoglobin, loading and unloading of oxygen, the S-shaped dissociation curve, the Bohr effect, carbonic anhydrase and the chloride shift, the effects of pH and temperature on oxygen release, and carbon monoxide poisoning.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Which part of a haemoglobin molecule binds oxygen, and how many oxygen molecules can one haemoglobin carry?

Key term

Write in words what happens when haemoglobin loads oxygen in the lungs.

Key term

Define the terms loading and unloading as applied to haemoglobin.

Questions from this subtopic

Each one with the model answer underneath.

1
Describe the structure of an adult haemoglobin molecule.
Model answer
It is a globular protein with a quaternary structure of four polypeptide chains, two alpha and two beta, each folded around a haem prosthetic group containing an iron ion.
2
Which part of a haemoglobin molecule binds oxygen, and how many oxygen molecules can one haemoglobin carry?
Model answer
Oxygen binds to the iron ion at the centre of each haem group, so one haemoglobin molecule can carry up to four molecules of oxygen.
3
Write in words what happens when haemoglobin loads oxygen in the lungs.
Model answer
Haemoglobin combines reversibly with four molecules of oxygen to form oxyhaemoglobin.
4
Define the terms loading and unloading as applied to haemoglobin.
Model answer
Loading, or association, is the binding of oxygen to haemoglobin at the gas exchange surface, and unloading, or dissociation, is its release at respiring tissue.
5
Why does haemoglobin load oxygen in the lungs but release it in respiring tissue?
Model answer
The partial pressure of oxygen is high in the alveolar capillaries, where haemoglobin has a high affinity and becomes almost fully saturated, and low in respiring tissue, where its affinity is much lower so oxygen dissociates.
6
Explain why the oxygen dissociation curve of haemoglobin is S-shaped.
Model answer
Binding of the first oxygen molecule alters the shape of the molecule so that the remaining binding sites become more accessible, making each of the next two easier to bind, while the fourth is harder because so few sites remain free.

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