Oxygen Diffusion Across Five Membranes in Gas Exchange Steps: - Oxygen diffuses from alveolar air across the apical plasma membrane into the alveolar epithelial cell (1st crossing). - It exits the alveolar epithelial cell via the basolateral plasma membrane into the interstitial fluid (2nd crossing). - Oxygen diffuses across the interstitial fluid and enters the capillary endothelial cell through its apical plasma membrane (3rd crossing). - It exits the endothelial cell via the basolateral plasma membrane into the blood plasma (4th crossing). - Finally, oxygen crosses the red blood cell's plasma membrane to reach hemoglobin inside (5th crossing). Why D is correct: - The respiratory barrier requires crossing two plasma membranes per cell layer (alveolar epithelium and capillary endothelium, per diffusion through cytoplasm) plus the RBC membrane, totaling five, as defined in cellular diffusion paths without pores. Why the others are wrong: - A. 2: Counts only the two cell layers as single barriers, ignoring dual membranes per layer and RBC. - B. 3: Treats each layer (epithelium, endothelium, RBC) as one crossing, overlooking two membranes per epithelial/endothelial cell. - …