The effects of carbon dioxide concentration on the dissociation of…

The idea

Respiring tissues release carbon dioxide, which diffuses into red blood cells. The carbon dioxide reacts with water to form carbonic acid, which dissociates to release hydrogen ions, lowering the pH inside the red blood cell. The hydrogen ions bind to haemoglobin and change its tertiary and quaternary structure slightly, so its affinity for oxygen falls. On the dissociation curve this appears as a shift to the right: at any given partial pressure of oxygen the haemoglobin is less saturated, so more oxygen is unloaded. The effect is self-regulating, because the harder a tissue respires the more carbon dioxide it produces and the more oxygen haemoglobin releases to it. In the lungs the reverse happens: carbon dioxide is exhaled, pH rises, affinity is restored and loading is easy.

What earns the marks

• A higher concentration or partial pressure of carbon dioxide in respiring tissues.
• Carbon dioxide leading to the formation of carbonic acid, which releases hydrogen ions and lowers the pH inside the red blood cell.
• The hydrogen ions changing the shape of haemoglobin and so reducing its affinity for oxygen.
• The dissociation curve shifting to the right, so haemoglobin is less saturated at the same partial pressure of oxygen.
• Linking the increased unloading to a greater supply of oxygen for aerobic respiration in an active tissue.

Common mistakes and how to put them right

• Saying carbon dioxide takes the place of oxygen on the haem group.

  Put it right: Carbon dioxide lowers affinity indirectly through hydrogen ions, it does not compete for the haem group.

• Shifting the curve to the left instead of the right, or moving the axes instead of the curve.

  Put it right: A right shift means less saturation at the same partial pressure of oxygen.

• Writing that the Bohr effect changes how much oxygen the blood contains rather than how readily haemoglobin releases it.

  Put it right: It changes the affinity and so the unloading, not the total oxygen carried.

• Leaving pH out entirely and saying carbon dioxide changes affinity directly.

  Put it right: The hydrogen ions and the pH change are the link.

• Claiming the Bohr effect operates in the lungs rather than in respiring tissues.

  Put it right: In the lungs carbon dioxide is lost and affinity rises again.

Exam tips

• Answer in a fixed order: carbon dioxide, hydrogen ions, shape, affinity, curve to the right, more unloading.
• If asked to compare exercising and resting muscle, quote saturations from both curves at the same partial pressure of oxygen.
• The AQA specification does not require the names carbonic anhydrase or carbonic acid, so do not rely on naming the enzyme to gain credit; describe the reaction and the hydrogen ions instead.