Transport in Mammals: Question 4

Worked solution

Part (a): Transport of carbon dioxide

Carbon dioxide, produced as a waste product of aerobic respiration in tissue cells, diffuses out of these cells into the surrounding tissue fluid, into the blood plasma, and then into red blood cells.

Inside the red blood cell, the enzyme carbonic anhydrase catalyses a reaction between carbon dioxide and water, forming carbonic acid. Carbonic acid is unstable and almost immediately dissociates into:

• hydrogen ions (H+), and
• hydrogencarbonate ions (HCO3-).

Most of these hydrogencarbonate ions diffuse out of the red blood cell, down their concentration gradient, into the surrounding blood plasma. This is the form in which the majority of carbon dioxide is actually carried in the blood on its way to the lungs. (A smaller proportion of carbon dioxide combines directly with amino groups on the globin part of haemoglobin to form carbaminohaemoglobin, and a small amount simply remains dissolved in the plasma.)

Because negatively charged hydrogencarbonate ions are constantly leaving the red blood cell, the cell would build up a positive charge unless something replaced them. To maintain electrical neutrality, chloride ions (Cl-) diffuse into the red blood cell from the plasma to take their place. This exchange of hydrogencarbonate ions out and chloride ions in is called the chloride shift.

Part (b): Hydrogen ions, haemoglobin and the Bohr effect

The hydrogen ions released when carbonic acid dissociates (in part (a)) do not simply remain free in the cytoplasm of the red blood cell. Most are taken up, or buffered, by haemoglobin itself: they bind to the haemoglobin molecule and cause a change in its three-dimensional (tertiary/quaternary) shape.

This shape change reduces haemoglobin’s affinity for oxygen. In practice, this means that at any given partial pressure of oxygen, haemoglobin that has bound extra hydrogen ions holds onto its oxygen less tightly, and releases (unloads) a greater proportion of the oxygen it is carrying than it would if fewer hydrogen ions were present. On a graph of percentage saturation against partial pressure of oxygen, this shows up as a shift of the oxygen dissociation curve to the right.

This effect is called the Bohr effect (or Bohr shift), and it must not be confused with the chloride shift described in part (a), which is a separate ionic exchange process. Because the hydrogen ions responsible for the Bohr effect are generated directly from carbon dioxide produced by respiration, the effect is strongest exactly where carbon dioxide production is greatest, at actively respiring tissues. The Bohr effect therefore usefully promotes the unloading of oxygen from haemoglobin precisely at the tissues that need it most.

Final answers

• (a) Most carbon dioxide is transported as hydrogencarbonate ions: carbonic anhydrase catalyses CO2 + water forming carbonic acid, which dissociates into H+ and hydrogencarbonate ions; hydrogencarbonate ions diffuse into the plasma, and chloride ions diffuse into the red blood cell to replace them (the chloride shift).
• (b) Hydrogen ions bind to haemoglobin, changing its shape and reducing its affinity for oxygen (a rightward shift of the oxygen dissociation curve), the Bohr effect, which promotes the release of oxygen from haemoglobin at respiring tissues, where hydrogen ion concentration is highest.