Haemoglobin has greater affinity for

Options

A. `O_2`
B. CO
C. `CO_2`
D. `O_3`

Answer

**Step-by-Step Solution:**

1. **Understanding Hemoglobin**: Hemoglobin is a protein found in red blood cells that is responsible for transporting oxygen. It consists of four subunits, each containing an iron atom that can bind to oxygen or other gases.

2. **Identifying the Options**: The question asks for the molecule that hemoglobin has the greatest affinity for among the following:
- O2 (oxygen)
- CO (carbon monoxide)
- CO2 (carbon dioxide)
- O3 (ozone)

3. **Analyzing Affinity**:
- **Carbon Monoxide (CO)**: Hemoglobin has a very high affinity for carbon monoxide. This is because CO binds to the iron in hemoglobin in a way that is nearly perpendicular to the plane of the porphyrin ring, leading to minimal steric hindrance.
- **Oxygen (O2)**: Hemoglobin also binds to oxygen, but its affinity is lower than that for carbon monoxide. The binding of oxygen is not perfectly perpendicular, which reduces its affinity compared to CO.
- **Carbon Dioxide (CO2)**: Hemoglobin has a lower affinity for carbon dioxide. CO2 is bulkier and experiences steric hindrance, making it less favorable for binding.
- **Ozone (O3)**: Similar to CO2, ozone is also a bulky molecule, leading to reduced affinity due to steric hindrance.

4. **Conclusion**: After analyzing the binding affinities, it is clear that hemoglobin has the greatest affinity for carbon monoxide (CO).

5. **Final Answer**: The correct answer is **CO (carbon monoxide)**.

---