Cell permeability

The permitting or activating of the passage of substances into, out of, or through cells, or from one cell to another. These materials traverse either the cell surface that demarcates the living cytoplasm from the extracellular space or the boundaries between adjacent cells. In many cases the materials also traverse the cell wall. See also Cell walls (plant).

The cell can control many properties of its membranes, including those related to permeability. Control can be exerted in the following ways: (1) by varying the number and variety of membranes; (2) by varying the specific nature of the lipid components in the membrane; (3) by varying the glycocalyx proteins or lipid-associated sugar molecules on the outside of the cell, or the membrane-associated proteins on the inside; (4) by causing large areas of membrane to flow from one place to another, or to fold, indent, evert, or pinch off, carrying with these movements substances bound to one or the other surface of the membrane, or embedded in it; (5) by selectively moving integral membrane proteins in the plane of the membrane, allowing these proteins to carry with them substances, particles, molecules, or other materials bound to them; (6) by varying the properties of a single integral membrane protein or of a closely associated group of them so as to allow or prevent the passage across the membrane of substances such as ions or proteins of a specific character.