The respiratory pigment in human beings is

The correct option is haemoglobin.

To understand why haemoglobin is the respiratory pigment in human beings, let us break down its role and function step-by-step:
1. Definition of a Respiratory Pigment: A respiratory pigment is a molecule (usually a protein containing a metal atom) that increases the oxygen-carrying capacity of the blood. It binds to oxygen where it is abundant (the lungs) and releases it in tissues where it is needed.
2. The Structure and Role of Haemoglobin: Haemoglobin is an iron-rich protein present in red blood cells (RBCs). Each haemoglobin molecule contains four iron-binding sites that can reversibly bind with oxygen molecules to form oxyhaemoglobin. This allows oxygen to be efficiently transported from the lungs to all the cells of the body for cellular respiration.
3. Why the other options are incorrect:
- Insulin is a hormone produced by the pancreas that regulates blood glucose levels, not a respiratory pigment.
- Enzymes are biological catalysts that speed up chemical reactions in the body, but they do not act as gas transport pigments.
Therefore, haemoglobin is the correct respiratory pigment in human beings.