A molecule of hemoglobin carries oxygen molecules.

Hemoglobin is the iron containing protein found in red blood cells.

It is responsible for the transportation of oxygen and carbon dioxide.

It consists of four protein chains and each chain has a haem group attached to it.

Each haem group has an iron molecule. The oxygen binds to the iron molecules.

Since there are four iron molecules, each hemoglobin is capable of carrying four oxygen molecules.