A biological membrane or biomembrane is an enclosing or separating membrane that acts as a selectively permeable barrier within living things. Biological membranes, in the form of cell membranes, often consist of a phospholipid bilayer with embedded, integraland peripheral proteins used in communication and transportation of chemicals and ions. Bulk lipid in membrane provides a fluid matrix for proteins to rotate and laterally diffuse for physiological functioning. Proteins are adapted to high membrane fluidity environment of lipid bilayer with the presence of an annular lipid shell, consisting of lipid molecules bound tightly to surface of integral membrane proteins. The cellular membranes should not be confused with isolating tissues formed by layers of cells, such as mucous membranes and basement membranes.
Membranous organelles help to compartmentalize the cell so that each reaction in the organelle is separate from those of other organelles and have a variety of organelles that carry out different functions simultaneously, thereby increasing efficiency. Membranes also offer protection. In addition, the organelles can also be in logical proximity to others that have related functions. For example, organelles producing proteins within the cell can be near other organelles involved in transporting the proteins out of the cell. Nuclear membranes around DNA allow for the regulation of different materials into and out of the nucleus and protection of DNA material.