The oxygen dissociation curve and the Bohr effect

The oxygen dissociation curve is a graph of how saturated haemoglobin is with oxygen at different partial pressures of oxygen (pO₂). It is S-shaped: haemoglobin picks up oxygen readily where pO₂ is high, in the lungs, and releases it where pO₂ is low, in respiring tissues. More carbon dioxide shifts the curve to the right, so haemoglobin gives up even more oxygen. This is the Bohr effect.

What the oxygen dissociation curve shows

Haemoglobin is the protein in red blood cells that carries oxygen. It has a quaternary structure of four polypeptide chains, each with a haem group containing iron, so one haemoglobin molecule can carry up to four oxygen molecules. Haemoglobin with oxygen attached is called oxyhaemoglobin.

• The x-axis is the partial pressure of oxygen, pO₂, in kPa: a measure of how much oxygen is around.
• The y-axis is the percentage saturation of haemoglobin with oxygen: 100% means every haemoglobin is carrying four oxygen molecules.

In the lungs, pO₂ is high and haemoglobin becomes almost fully saturated: it loads oxygen. In respiring tissues, pO₂ is low and the saturation falls sharply: haemoglobin unloads oxygen to the cells that need it. The steep middle of the curve means a small drop in pO₂ releases a lot of oxygen.

Why is the oxygen dissociation curve S-shaped?

The S shape (sigmoid curve) comes from cooperative binding:

1. At low pO₂, it is hard for the first oxygen molecule to bind, because the four chains are tightly packed. The curve starts shallow.
2. Once the first oxygen binds, the shape of the haemoglobin changes, which makes it easier for the second and third oxygens to bind. The curve becomes steep.
3. With three oxygens bound, there are few sites left, so it is harder for the fourth to find a space. The curve levels off near 100%.

AQA A-level Biology (3.4.1) describes this as the change in shape of haemoglobin caused by binding of the first oxygens making the binding of further oxygens easier.

The Bohr effect (Bohr shift)

Respiring tissues produce carbon dioxide. When the concentration of carbon dioxide rises:

1. Carbon dioxide dissolves in the blood and forms an acid, lowering the pH.
2. The lower pH changes the shape of haemoglobin slightly, which lowers its affinity for oxygen.
3. So at the same pO₂, haemoglobin is less saturated: the curve shifts to the right.
4. More oxygen is unloaded in the tissues that are respiring fastest, which are exactly the ones that need it most.

In the lungs the opposite happens: carbon dioxide diffuses out of the blood, haemoglobin's affinity for oxygen rises, and it loads oxygen more easily. Exercise increases this effect, because active muscles produce more carbon dioxide.

Check your understanding.

Curves to the left: fetal haemoglobin and other animals

A fetus gets its oxygen from its mother's blood across the placenta. Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, so its curve lies to the left. At the low pO₂ in the placenta, the mother's haemoglobin unloads oxygen while the fetal haemoglobin loads it.

Many animals have haemoglobin suited to where they live. As a rule, a curve further to the left suits an animal in an environment with little oxygen, because its haemoglobin can still load oxygen at low pO₂. A curve further to the right suits an active animal with a high respiration rate, because its haemoglobin unloads oxygen more readily.

How to read an oxygen dissociation curve

Example 1: how much oxygen is unloaded?

On a dissociation curve, haemoglobin is 96% saturated in the lungs and 30% saturated in a respiring muscle. What percentage of the oxygen carried is unloaded in the muscle?

The saturation drops by 96 − 30 = 66 percentage points. As a percentage of what was carried: 66 ÷ 96 × 100 = 69% of the oxygen is unloaded.

Example 2: a volume of oxygen

Assume 100 cm³ of blood carries 20 cm³ of oxygen when the haemoglobin is fully saturated. Using the values above, how much oxygen does 100 cm³ of blood deliver to the muscle?

At 96% saturation it carries 0.96 × 20 = 19.2 cm³. At 30% it carries 0.30 × 20 = 6.0 cm³. So it delivers 19.2 − 6.0 = 13.2 cm³ of oxygen.

Example 3: the effect of a right shift

If, at the same muscle pO₂, extra carbon dioxide lowered the saturation to 20%, the blood would carry only 4.0 cm³ there and deliver 19.2 − 4.0 = 15.2 cm³: 2.0 cm³ more for every 100 cm³ of blood.

Common mistakes

• Saying the Bohr effect shifts the curve left. More carbon dioxide shifts it right.
• Mixing up affinity and saturation. Affinity is how strongly haemoglobin attracts oxygen; saturation is how much is bound.
• Saying haemoglobin "uses" oxygen. It carries oxygen; the cells use it in respiration.
• Explaining the S shape without cooperative binding. Mention the change in shape after the first oxygen binds.
• Calculating a difference instead of a percentage when a question asks what percentage of the oxygen is released.

Practice questions

1. What does a shift of the curve to the right mean?
2. Explain why the oxygen dissociation curve is S-shaped.
3. Why does fetal haemoglobin have a curve to the left of adult haemoglobin?
4. Haemoglobin is 98% saturated in the lungs and 40% saturated in the tissues. What percentage of the oxygen is unloaded?

Answers: 1) Haemoglobin has a lower affinity for oxygen, so it unloads oxygen more readily. 2) Binding of the first oxygen changes haemoglobin's shape, making it easier for more oxygen to bind (cooperative binding), so the curve is shallow, then steep, then levels off. 3) It has a higher affinity, so it can load oxygen from the mother's blood at the low pO₂ in the placenta. 4) (98 − 40) ÷ 98 × 100 = 59%.

Practise with a short timed paper

Mass transport in animals, including haemoglobin, is in the first year of A-level Biology. The daily A-level Biology: Year 1 paper is a short timed paper of 8 questions, with a new one every day. For how blood is pumped around the body, see the heart.

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