A molecule of oxygen diffuses from the alveolar air space into an erythrocyte to from oxyhaemoglobin. How many time dose it need to diffuse through the cell membranes to reach the haemoglobin ?

Options

A. 3
B. 5
C. 6
D. 9

Answer

To solve the question of how many times a molecule of oxygen needs to diffuse through cell membranes to reach hemoglobin in an erythrocyte, we can break down the diffusion process step by step.

### Step-by-Step Solution:

1. **Understanding the Starting Point**: The oxygen molecule begins in the alveolar air space. This is the first location from which it will start its journey.

2. **Crossing the Alveolar Epithelium**: The first barrier the oxygen molecule encounters is the squamous epithelium of the alveoli. This is the first membrane it must diffuse through.

3. **Basement Membrane of the Alveoli**: After crossing the alveolar epithelium, the oxygen molecule must then diffuse through the basement membrane of the alveoli. This is the second layer it crosses.

4. **Interstitial Fluid**: Next, the oxygen molecule moves through the interstitial fluid, which is the space between the alveolar basement membrane and the endothelium of the blood vessels. This counts as the third layer.

5. **Endothelium of the Blood Vessel**: The oxygen molecule then crosses the basement membrane of the endothelium of the blood vessel. This is the fourth layer it must diffuse through.

6. **Endothelium of the Blood Vessel**: Following this, the oxygen molecule crosses the actual endothelial cells of the blood vessel. This is the fifth layer.

7. **Erythrocyte Membrane**: Finally, the oxygen molecule diffuses through the membrane of the erythrocyte (red blood cell). This is the sixth layer it crosses.

8. **Binding to Hemoglobin**: Once inside the erythrocyte, the oxygen molecule binds to hemoglobin to form oxyhemoglobin (HbO2).

### Conclusion:
In total, the oxygen molecule needs to diffuse through **six layers** (5 membranes + 1 erythrocyte membrane) to reach hemoglobin.

### Final Answer:
**6**

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