How is oxygen carried by haemoglobin in red blood cells?

Oxygen is carried by haemoglobin in red blood cells through a process called oxygen binding and release.

Haemoglobin is a protein molecule found in red blood cells that plays a crucial role in oxygen transport. It is composed of four polypeptide chains, each containing a heme group. The heme group is the active site where oxygen binds, and it contains an iron atom that can bind one molecule of oxygen. Therefore, each haemoglobin molecule can carry up to four molecules of oxygen.

The process of oxygen binding and release is facilitated by the change in the shape of the haemoglobin molecule. When haemoglobin binds to oxygen in the lungs, it changes from a tense state (T state) to a relaxed state (R state). This change in shape increases the molecule's affinity for oxygen, allowing more oxygen molecules to bind. This is known as cooperative binding.

Once the oxygenated blood reaches the tissues, the oxygen is released from the haemoglobin. This is triggered by the lower pH in the tissues, a condition known as the Bohr effect. The lower pH causes the haemoglobin to return to its T state, reducing its affinity for oxygen and allowing the oxygen to be released into the tissues.

The ability of haemoglobin to pick up oxygen in the lungs and release it in the tissues is vital for maintaining the body's oxygen levels. It ensures that oxygen is efficiently transported from the lungs to the tissues where it is needed for cellular respiration. This process is a perfect example of how the structure of a protein can influence its function, a key concept in biology.

In addition to oxygen, haemoglobin also carries carbon dioxide, a waste product of cellular respiration, from the tissues back to the lungs. However, carbon dioxide is carried in a different way. It binds to the amino acids of the haemoglobin molecule, not the heme group, and is released in the lungs when the blood picks up oxygen. This dual function of haemoglobin highlights its importance in the respiratory system.