A single molecule of haemoglobin can carry a maximum of how many molecules of oxygen?

The correct option is Four.

Explanation

Haemoglobin is the iron-containing respiratory pigment found in the red blood cells (erythrocytes) of vertebrates. Its primary function is the transport of oxygen from the respiratory organs to the peripheral tissues.

Structural Analysis:

• Haemoglobin is a quaternary protein composed of four polypeptide subunits: two alpha (α) chains and two beta (β) chains.
• Each subunit contains a prosthetic group known as heme.
• At the center of each heme group is a ferrous iron atom (Fe²⁺).
• Each iron atom can reversibly bind to one molecule of oxygen (O₂).

Since there are four heme groups in a single haemoglobin molecule, it can bind to a maximum of four oxygen molecules simultaneously. This reaction forms oxyhaemoglobin.

Key Takeaway: A single molecule of haemoglobin contains four heme groups, allowing it to carry a maximum of four molecules of oxygen.