The rapid development of new concepts and technical possibilities in the investigation
of membrane phenomena, sustained by the growing interest of investigators
from numerous disciplines, has provided in the past years a conspicuous
body of information on the structure and function of cell membranes. Interest
is focused either on the analysis of membrane composition and organization,
or on the description of membrane functions. Among the latter, transport properties,
responsible for the capability of membranes to serve as barriers or vectorial
catalysts for downhill and uphill movements of matter, are a topic of
major physiological and biomedical relevance.