However, membranes are not only passive barriers. They include a set of proteins specialized for promoting or catalyzing various cellular processes (Nelson & Cox 2004, p.369). Molecular transportation is the most important function of membranes facilitated by carrier and channel proteins (McKee 2004, p.62).
General questions of transportation and channeling across biologic membranes are well considered in the encompassing textbooks of Lodish (2003, p.245-300), Nelson & Cox (2004, p.369-420), Kuchel & Ralston (1997, p.171-184), McKee (2004, p.353-366), Garrett & Grisham (1999, p.259-326), etc., in special monographs, e.g. Keizer (2000) and also in a variety of articles.
All cells acquire from its environs the raw materials for biosynthesis and for energy production, and also release to its environment the byproducts of metabolism. Only some nonpolar compounds can cross the membrane unassisted. However, for polar or charged compounds or ions, a membrane protein is essential for transmembrane movement.
Membrane transport mechanisms are vital to living organisms. Ions and molecules constantly move across cell plasma membranes and across the membranes of organelles. This flux must be regulated to meet each cell's metabolic needs. ...