3.1.2(i) - Haemoglobin transport of oxygen and carbon dioxide

Haemoglobin As A Reversible Oxygen Carrier

Haemoglobin is a globular protein found in red blood cells. It has four polypeptide chains, each associated with a haem group containing an Fe²⁺ ion. Each iron ion can bind one oxygen molecule, so one haemoglobin molecule can carry four O₂ molecules. Reversible oxygenation does not oxidise the Fe²⁺ to Fe³⁺. Its oxygen-transport role depends on reversible binding: oxygen can attach to haemoglobin where oxygen is abundant, then detach where cells are using oxygen.

In the lungs, oxygen diffuses from the alveoli into the blood. The oxygen concentration, or partial pressure, is high enough for haemoglobin to bind oxygen and form oxyhaemoglobin. In respiring tissues, oxygen is being used in aerobic respiration, so the local oxygen concentration is lower. Oxygen dissociates from oxyhaemoglobin and diffuses into the tissue cells.

You do not need the oxygen dissociation curve yet. For this lesson, the key idea is simpler: haemoglobin is useful because the same molecule can load oxygen in the lungs and unload oxygen in tissues.

Carbon dioxide has a second connection to haemoglobin. Some carbon dioxide can bind directly to haemoglobin, forming carbaminohaemoglobin, but most carbon dioxide is transported after it has been converted into hydrogencarbonate ions. Haemoglobin also binds hydrogen ions during that conversion, forming haemoglobinic acid. That buffering role is central to carbon dioxide transport.

Carbon Dioxide In Respiring Tissues

Respiring cells produce carbon dioxide. This carbon dioxide diffuses from the tissues into the blood and then into red blood cells. Red blood cells contain a high concentration of the enzyme carbonic anhydrase, which catalyses the reaction between carbon dioxide and water.

The product, carbonic acid, is unstable. It dissociates into hydrogen ions and hydrogencarbonate ions.

Hydrogencarbonate ions are important because they are the main form in which carbon dioxide is transported in the blood. The carbon dioxide molecule itself has changed chemical form, but the carbon from the carbon dioxide is now travelling as part of the hydrogencarbonate ion.

The hydrogen ions must also be dealt with. If hydrogen ions simply accumulated freely in the red blood cell, pH would fall. Haemoglobin acts as a buffer by binding hydrogen ions to form haemoglobinic acid, often written as HHb.

This is a common exam precision point: hydrogen ions bind to haemoglobin. Do not write hydrogen atoms or hydrogen molecules. Do not write that hydrogencarbonate ions bind to haemoglobin; they move mainly into the plasma.

Now check that you can keep the enzyme, the acid and the ions in their proper order.