IB Biology B3.1 Notes

In the HL syllabus, you are expected to learn more about gas exchange - specifically how oxygen is transported around the body once it has diffused into the bloodstream. This is typically performed by haemoglobin, which is found in the red blood cells of adults. Its structure is two α peptides and two β peptides each with a haem group.

The Iron ion in the haem group is what permits red blood cells to carry, transport, and release oxygen around the body. However, the amount of oxygen carried depends on the oxygen pressure in the blood. As oxygen pressure increases, the amount of oxygen carried by haemoglobin increases.

This can be shown by an oxygen dissociation curve, which produces an S-shape. You are expected to know why the curve is shaped like this:

1. When O₂ binds to the haem group of a subunit, it results in a structural change to the haemoglobin, called allostery. 
2. It increases the affinity of other subunits for O₂, increasing the amount of oxygen binding, referred to as cooperative binding.

As a result, oxygen can quickly be picked up during gas exchange in the lungs. However, at the cells, the oxygen must be released. Here:

1. High CO₂ levels cause a decrease in blood pH, increasing acidity.
2. When an H⁺ ion or a CO₂ molecule binds to a subunit of haemoglobin, its affinity for O₂ decreases.
3. Thus, oxygen is released from the other subunits to allow it to diffuse out of the blood and into the cells.