[The Cell Membrane: a Frontier Between 2 Worlds]

The structure and function of the plasma membrane is reviewed. Three of its main functional roles are discussed, i.e. of 1. selective, physico-chemical barrier; 2. key regulator of cell tolerance and cell recognition; 3. effector organelle for long range and short range cell communication. From the structural viewpoint, the fluid mosaic model is presented as a particularly dynamic picture of the membrane machine and its components (pathways for active and passive transport, enzymes, hormonal receptors and other active sites). Some recent aspects fo the modulation of membrane permeability by hormonal or pharmacological agents are discussed by considering two examples of the author's own research: the hydrosmotic action of vasopressin and the vasopressin-like effects of a hallucinogenic drug, harmaline. Finally, the growing list of anomalies of membrane transport and membrane receptors found both in rare and in common clinical entities emphasizes the importance of membrane research in the understanding of human disease.