Haemoglobin, a protein, is primarily found in red blood cells. It plays a crucial role in the transport of oxygen from the lungs to the different tissues and organs of the body. Here is a detailed explanation of why haemoglobin is present in red blood cells:

1. Red Blood Cells (RBCs):
- Haemoglobin is abundantly present in red blood cells, also known as erythrocytes.
- RBCs are specialized cells that are responsible for carrying oxygen and carbon dioxide throughout the body.
- Each red blood cell contains millions of haemoglobin molecules, accounting for approximately one-third of the cell's weight.
- The main function of haemoglobin is to bind with oxygen in the lungs and release it in the body tissues that require oxygen for various metabolic processes.

2. Structure of Haemoglobin:
- Haemoglobin is a complex protein made up of four subunits: two alpha chains and two beta chains.
- Each subunit contains a heme group, which consists of an iron molecule that can bind to oxygen.
- The structure of haemoglobin allows it to easily bind with oxygen in the lungs and release it in tissues with lower oxygen concentrations.

3. Oxygen Transport:
- When we inhale, oxygen enters the lungs and diffuses into the bloodstream.
- The oxygen molecules then bind with the iron molecules in the heme groups of haemoglobin, forming oxyhemoglobin.
- The oxyhemoglobin is carried by the red blood cells through the arteries to various body tissues.
- In tissues with lower oxygen levels, oxyhemoglobin releases the oxygen, allowing it to be used in cellular respiration.
- The released oxygen diffuses into the cells, while carbon dioxide, a waste product, binds with haemoglobin to form carbaminohemoglobin.
- The carbon dioxide is then transported back to the lungs, where it is released during exhalation.

4. Function of Red Blood Cells:
- RBCs have a unique structure that allows them to efficiently transport oxygen.
- They lack a nucleus, enabling them to carry more haemoglobin and maximize the oxygen-carrying capacity.
- The biconcave shape of RBCs increases their surface area, facilitating gas exchange.
- The flexibility of RBCs allows them to squeeze through narrow capillaries and deliver oxygen to even the smallest blood vessels.

In conclusion, haemoglobin is present in red blood cells because it is responsible for binding and transporting oxygen throughout the body. The structure and function of red blood cells make them ideal carriers of haemoglobin, ensuring efficient oxygen delivery to tissues and organs.

A) Red blood cells