Final answer:
The plasma membrane of type II alveolar cells is not part of the respiratory membrane; this membrane consists of the capillary endothelium, plasma membrane of type I alveolar cells, and the fused basement membrane.
Step-by-step explanation:
The respiratory membrane consists of several components that facilitate gas exchange between the air in the alveoli and the blood in the capillaries. These components are the capillary endothelium, the plasma membrane of type I alveolar cells, and the fused basement membrane that both these cells share. However, the plasma membrane of type II alveolar cells is not considered a part of the respiratory membrane, even though these cells are present in the alveolar wall and are responsible for secreting pulmonary surfactant to reduce surface tension.