Oxygen Dissociation Curves

Text p.152

Objectives

• What is an oxygen dissociation curve?• What is the effect of carbon dioxide

concentration on the curve and why?

What is an oxygen dissociation curve?

• Why don’t we just use oxygen concentration?

How is oxygen concentration measured?Oxygen binds to haemoglobin when oxygen is at a high concentration, and dissociates from haemoglobin when oxygen is at a low concentration.

Partial pressure is measured in kilopascals (kPa) and is written as P(O2) or pO2, P(CO2), etc.

Write down pp definition from blue box

The concentration of a gas in a mixture of gases can be quantified in terms of its partial pressure. This is the amount of pressure exerted by the gas relative to the total pressure exerted by all the gases in the mixture.

The oxygen dissociation curve

Oxygen dissociation curves• Using your diagram of adult human oxygen dissociation

curve:1. Add the title2. Annotate the axes so you really understand what is on each

of the x and y3. What does it show?4. Why isn’t it a straight line?5. What is the name for this shape of curve?6. What 2 facts are important to remember about dissociation

curves?

A schematic visual model of oxygen binding process, showing all four monomers and haems, and protein chains.

Oxygen is not shown in this model, but it binds to the iron (red sphere) in the flat haem.

This causes the iron atom to move backward into the haem which holds it, tugging the histidine residue (modeled as a red pentagon on the right of the iron) closer, as it does. This, in turn, pulls on the protein chain holding the histidine.

Factors affecting oxygen dissociation

Effects of carbon dioxide concentration – the Bohr effect.

• Using your diagram of the effect of carbon dioxide concentration on the oxygen dissociation curve:– Add the title– Describe to your partner:

• what effect carbon dioxide has on oxygen dissociation and

• how this influences what haemoglobin does in different parts of the body (lungs/respiring tissues)

Effects of carbon dioxide concentration – the Bohr effect.

• Haemoglobin has a reduced … for oxygen in the presence of carbon dioxide.

• In the lungs carbon dioxide concentration is…, so haemoglobin has a higher affinity for oxygen (curve to the…). Together with the … concentration of oxygen this means oxygen readily combines with haemoglobin to make oxyhaemoglobin

• In respiring tissues carbon dioxide is … so haemoglobin has a lower affinity for oxygen (curve to …). Together with the … concentration of oxygen this means oxyhaemoglobin will readily dissociate into oxygen and haemoglobin, releasing the oxygen the tissues require.

Missing words - Left right affinity low low high high

Answer -Effects of carbon dioxide concentration – the Bohr effect.

• Haemoglobin has a reduced affinity for oxygen in the presence of carbon dioxide.

• In the lungs P(CO2) is low, so haemoglobin has a higher affinity for oxygen (curve to the left). Together with the high P(O2) this means oxygen readily combines with haemoglobin to make oxyhaemoglobin

• In respiring tissues P(CO2) is high so haemoglobin has a lower affinity for oxygen (curve to right). Together with the low P(O2) this means oxyhaemoglobin will readily dissociate into oxygen and haemoglobin, releasing the oxygen the tissues require.

Why does the Bohr effect occur?Order the cards into 2 sets, check and copy

• In lungs• Carbon dioxide is removed

so is low• pH higher as carbon dioxide

is low• Haemoglobin is a shape that

can load oxygen readily• Oxygen binds haemoglobin • Haemoglobin becomes

‘loaded’ with oxygen

• In respiring tissues• Carbon dioxide is produced

so is high• pH lower as carbon dioxide

dissolves to give a weak acid• Haemoglobin changes

shape into one with a lower affinity for oxygen

• Haemoglobin releases oxygen into the respiring tissues.

• Haemoglobin becomes ‘unloaded’ with oxygen

Loading, transport and unloading of oxygen (fig 3 p.153)

1. Why is it that the more active a tissue is, the more oxygen is unloaded?

2. In humans haemoglobin leaving the lungs is saturated. What does this mean?

3. What is haemoglobin like after passing through an inactive tissue?

4. What is haemoglobin carrying after passing through a very active tissue?

Plenary - Understanding oxygen transport