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 determine 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. **Identify the Starting Point**: The oxygen molecule starts in the alveolar air space. This is where gas exchange occurs in the lungs.

2. **First Diffusion Barrier**: The first barrier that the oxygen molecule encounters is the **endothelium of the alveoli**. This is the thin layer of cells lining the alveoli.

3. **Second Diffusion Barrier**: After crossing the alveolar endothelium, the oxygen molecule must diffuse through the **basement membrane of the alveoli**. This membrane provides structural support to the alveolar cells.

4. **Third Diffusion Barrier**: Next, the oxygen molecule encounters the **basement membrane of the capillary**. This is the membrane that separates the alveolar space from the capillary blood.

5. **Fourth Diffusion Barrier**: The oxygen then diffuses through the **endothelium of the capillary**. This layer lines the capillaries and allows for the exchange of gases.

6. **Fifth Diffusion Barrier**: Finally, the oxygen molecule must cross the **membrane of the red blood cell (RBC)**. Each RBC has its own membrane that the oxygen must pass through to enter the cell.

7. **Final Destination**: Once inside the RBC, the oxygen molecule binds to hemoglobin to form oxyhemoglobin.

### Conclusion:
In total, the oxygen molecule has to diffuse through **5 barriers** (1. alveolar endothelium, 2. alveolar basement membrane, 3. capillary basement membrane, 4. capillary endothelium, and 5. RBC membrane) to reach hemoglobin.

Thus, the answer is **5**.