Haemoglobin-oxygen dissociation curve is a graph that shows the relationship between the partial pressure of oxygen (pO2) and the percentage saturation of haemoglobin with oxygen. The shape of the curve is sigmoid or S-shaped.

Explanation:

The sigmoid shape of the curve is due to the cooperative binding of oxygen to haemoglobin. Haemoglobin consists of four subunits, each containing a heme group that can bind to one molecule of oxygen. When one subunit binds to oxygen, it changes the shape of the protein, making it easier for the other subunits to bind to oxygen. This positive cooperativity leads to a steep increase in oxygen binding at low pO2 levels.

At high pO2 levels, most of the haemoglobin binding sites are already occupied, so additional oxygen binding becomes less likely. This results in a plateau region on the curve where additional oxygen binding does not significantly increase the percentage saturation of haemoglobin.

The shape of the curve is important because it allows haemoglobin to efficiently pick up oxygen in the lungs, where pO2 is high, and release it in the tissues, where pO2 is low. The steep increase in oxygen binding at low pO2 ensures that haemoglobin will be saturated with oxygen in the lungs, while the plateau region at high pO2 ensures that oxygen will be unloaded in the tissues.

The sigmoid shape of the curve can also be affected by various factors, such as pH, temperature, and the presence of other molecules like carbon dioxide and 2,3-bisphosphoglycerate (2,3-BPG). These factors can shift the curve to the left or right, indicating changes in the affinity of haemoglobin for oxygen.

In summary, the shape of the haemoglobin-oxygen dissociation curve is sigmoid due to cooperative binding of oxygen to haemoglobin, and it is important for efficient oxygen transport and delivery in the body.