Associated membrane protein types are lipopolysaccharide selective barriers. Biological membranes include cell membranes, outer coverings of cells or organelles that allow passage of certain proteins and nuclear membranes, which cover a cell nucleus; and tissue membranes, such as mucosae and serosae. ,The receptors are ligand binding factors of type 1, 2 or 3 and protein-molecules that receive chemical-signals from outside a cell. When such chemical-signals couple or bind to a receptor, they cause some form of cellular/tissue-response, e.g. a change in the electrical-activity of a cell. In this sense, am olfactory receptor is a protein-molecule that recognizes and responds to endogenous-chemical signals, chemokinesor cytokines e.g. an acetylcholine-receptor recognizes and responds to its endogenous-ligand, acetylcholine. However, sometimes in pharmacology, the term is also used to include other proteins that are drug-targets, such as enzymes, transporters and ion-channels.