12U4 L12 Haemoglobin

Multiple Choice

Haemoglobin is a globular protein, which consists of four polypeptide chains joined together. What level of structure does this represent?

• Primary

• Secondary

• Tertiary

• Quaternary

​Haemoglobin

Multiple Choice

How many oxygen molecules (O₂) can bind to one haemoglobin molecule?

• 1

• 2

• 3

• 4

​Cooperative binding

After the first oxygen molecule binds to a haem group, it is easier for the next oxygen molecule to bind to the next haem group on the same haemoglobin molecule. We can say that the affinity of haemoglobin for oxygen increases after the first molecule of oxygen binds. This increase in affinity is called 'cooperative binding'.

Multiple Choice

What happens to the affinity of haemoglobin for oxygen after the first oxygen molecule has bound?

• It increases

• It decreases

• It stays the same

• I don't know what this means

Multiple Choice

What is cooperative binding?

• When the binding of one oxygen molecule increases the likelihood that another will bind

• When the binding of one oxygen molecule decreases the likelihood that another will bind

• When all four of the haem groups are bound to oxygen and no more can attach

• When two oxygen molecules bind to a haem group

​The oxygen dissociation curve

An oxygen dissociation curve shows the affinity of haemoglobin for oxygen at different partial pressures of oxygen. (Partial pressure is a way of measuring oxygen concentration).

When haemoglobin has a high affinity for oxygen, it binds quickly and dissociates slowly. In other words it picks up oxygen easily and won't let it go.

When haemoglobin has a low affinity for oxygen, it binds slowly and dissociates quickly. In other words, it will throw it away easily and won't pick up more.

Multiple Select

When haemoglobin has a high affinity for oxygen it...

• Binds quickly

• Binds slowly

• Dissociates quickly

• Dissociates slowly

Multiple Select

When haemoglobin has a low affinity for oxygen it...

• Binds quickly

• Binds slowly

• Dissociates quickly

• Dissociates slowly

​The oxygen dissociation curve

At low partial pressures of oxygen, the affinity of haemoglobin for oxygen is low. This means that where there is little oxygen (e.g. at the tissues), haemoglobin does not bind to oxygen.

​The oxygen dissociation curve

Conversely, at high partial pressures of oxygen, affinity of haemoglobin for oxygen is high.

This means that where there is lots of oxygen (e.g. at the lungs), haemoglobin will bind to oxygen.

Multiple Choice

When is the affinity of haemoglobin for oxygen the highest?

• At low partial pressures of oxygen

• At high partial pressures of oxygen

• At around 50% partial pressure of oxygen

​The oxygen dissociation curve

As you can see, the relationship between partial pressure and affinity is not linear.

This is because the first and last oxygen molecules are more difficult to bind than the second and third.

​The oxygen dissociation curve

You learned earlier that, as a result of cooperative binding, once the first molecule of oxygen has bound, it is easier for the following molecules.

This explains why a small increase in partial pressure causes a big increase in affinity in the middle part of the graph.

​The oxygen dissociation curve

In addition, it is more difficult for haemoglobin to bind to oxygen once it is close to saturation.

This means it is harder for the fourth oxygen molecule to bind, so at high partial pressures of oxygen, a large increase in partial pressure is needed to create a small change in saturation.

Multiple Choice

Which of the following is true?

• At low partial pressures of oxygen, a small increase in partial pressure causes a big increase in affinity

• At high partial pressures of oxygen, a small increase in partial pressure causes a big increase in affinity

• At 30-50% partial pressure of oxygen, a small increase in partial pressure causes a big increase in affinity

Multiple Choice

Why is the oxygen dissociation curve a curve, not linear?

• Because it is more difficult for the second and third oxygen molecules to bind than the first and last.

• Because it is easier for the second and third oxygen molecules to bind than the first and last.