3.1.2 Transport in animals: the role of haemoglobin in transporting oxygen, the oxygen dissociation curve and cooperative binding, the Bohr effect, the higher oxygen affinity of fetal haemoglobin, and the transport of carbon dioxide including the formation of hydrogencarbonate ions and the chloride shift.

What this dot point is asking

OCR wants you to explain how haemoglobin transports oxygen, interpret the sigmoidal oxygen dissociation curve in terms of cooperative binding, explain loading and unloading, describe the Bohr effect and the higher affinity of fetal haemoglobin, and explain how carbon dioxide is carried, including hydrogencarbonate formation and the chloride shift.

The answer

Haemoglobin and cooperative binding

Haemoglobin is a globular protein with quaternary structure: four polypeptide subunits, each containing a haem group with an iron ion that binds one oxygen molecule, so each haemoglobin carries up to four oxygens as oxyhaemoglobin.

Binding is cooperative. The first oxygen binds with difficulty, but it changes the shape of the molecule so that the affinity for the next oxygen increases; subsequent molecules bind more easily. This is why the dissociation curve is S-shaped (sigmoidal) rather than a straight line.

The oxygen dissociation curve

The curve plots percentage saturation of haemoglobin against the partial pressure of oxygen (the concentration of oxygen in the surroundings):

• At the high partial pressure in the lungs, haemoglobin is almost fully saturated: it loads oxygen efficiently.
• At the low partial pressure in respiring tissues, a small fall in partial pressure produces a large fall in saturation (the steep middle of the curve), so haemoglobin unloads (dissociates) a lot of oxygen where it is needed.

This combination of high uptake in the lungs and ready release in the tissues is exactly what an oxygen carrier needs.

The Bohr effect

When carbon dioxide concentration is high (and pH is low), the dissociation curve shifts to the right: at any given partial pressure, saturation is lower, so haemoglobin releases oxygen more readily. This is the Bohr effect. In an actively respiring tissue, more carbon dioxide is produced, so haemoglobin unloads more oxygen precisely where demand is greatest.

Fetal haemoglobin

Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, so its dissociation curve lies to the left. In the placenta, where the partial pressure of oxygen is low, fetal haemoglobin can still load oxygen from the mother's blood (whose haemoglobin is unloading it), allowing oxygen to transfer from mother to fetus.

Transport of carbon dioxide

Carbon dioxide is carried in three ways, but most as hydrogencarbonate ions:

1. As hydrogencarbonate (about 85 percent). Carbon dioxide diffuses into red blood cells, where carbonic anhydrase catalyses its reaction with water to form carbonic acid, which dissociates: $\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$.
2. The hydrogencarbonate ions ($\text{HCO}_3^-$) diffuse out into the plasma. To keep the cell electrically neutral, chloride ions move in as hydrogencarbonate moves out: this is the chloride shift.
3. The hydrogen ions ($\text{H}^+$) are buffered by haemoglobin (forming haemoglobinic acid), which also lowers haemoglobin's affinity for oxygen, contributing to the Bohr effect.

Smaller amounts of carbon dioxide are carried bound directly to haemoglobin (as carbaminohaemoglobin) and dissolved in the plasma.

Examples in context

Example 1. Why people gasp at altitude. At high altitude the low partial pressure of oxygen means haemoglobin loads less in the lungs; over time the body makes more red blood cells, and animals native to altitude have higher-affinity haemoglobin (a left-shifted curve) to load oxygen from thin air.

Example 2. Carbon monoxide poisoning. Carbon monoxide binds haemoglobin far more strongly than oxygen, forming carboxyhaemoglobin and shifting the curve so that oxygen cannot be loaded or unloaded normally, which is why it is so dangerous.

Try this

Q1. Explain why the oxygen dissociation curve is steep in its middle section. [2 marks]

• Cue. Cooperative binding: once the first oxygen has bound and changed the shape, further oxygens bind more easily, so a small rise in partial pressure greatly increases saturation.

Q2. State the direction in which the dissociation curve shifts during the Bohr effect, and the cause. [2 marks]

• Cue. It shifts to the right; the cause is a higher carbon dioxide concentration (lower pH), causing more oxygen to be released.

Q3. Name the enzyme that catalyses the formation of carbonic acid in red blood cells. [1 mark]

• Cue. Carbonic anhydrase.