<P> The entire membrane is held together via non-covalent interaction of hydrophobic tails, however the structure is quite fluid and not fixed rigidly in place . Under physiological conditions phospholipid molecules in the cell membrane are in the liquid crystalline state . It means the lipid molecules are free to diffuse and exhibit rapid lateral diffusion along the layer in which they are present . However, the exchange of phospholipid molecules between intracellular and extracellular leaflets of the bilayer is a very slow process . Lipid rafts and caveolae are examples of cholesterol - enriched microdomains in the cell membrane . Also, a fraction of the lipid in direct contact with integral membrane proteins, which is tightly bound to the protein surface is called annular lipid shell; it behaves as a part of protein complex . </P> <P> In animal cells cholesterol is normally found dispersed in varying degrees throughout cell membranes, in the irregular spaces between the hydrophobic tails of the membrane lipids, where it confers a stiffening and strengthening effect on the membrane . Additionally, the amount of cholesterol in biological membranes varies between organisms, cell types, and even in individual cells . Cholesterol, a major component of animal plasma membranes, regulates the fluidity of the overall membrane, meaning that cholesterol controls the amount of movement of the various cell membrane components based on its concentrations . In high temperatures, cholesterol inhibits the movement of phospholipid fatty acid chains, causing a reduced permeability to small molecules and reduced membrane fluidity . The opposite is true for the role of cholesterol in cooler temperatures . Cholesterol production, and thus concentration, is up - regulated (increased) in response to cold temperature . At cold temperatures, cholesterol interferes with fatty acid chain interactions . Acting as antifreeze, cholesterol maintains the fluidity of the membrane . Cholesterol is more abundant in cold - weather animals than warm - weather animals . In plants, which lack cholesterol, related compounds called sterols perform the same function as cholesterol . </P> <P> Lipid vesicles or liposomes are circular pockets that are enclosed by a lipid bilayer . These structures are used in laboratories to study the effects of chemicals in cells by delivering these chemicals directly to the cell, as well as getting more insight into cell membrane permeability . Lipid vesicles and liposomes are formed by first suspending a lipid in an aqueous solution then agitating the mixture through sonication, resulting in a vesicle . By measuring the rate of efflux from that of the inside of the vesicle to the ambient solution, allows researcher to better understand membrane permeability . Vesicles can be formed with molecules and ions inside the vesicle by forming the vesicle with the desired molecule or ion present in the solution . Proteins can also be embedded into the membrane through solubilizing the desired proteins in the presence of detergents and attaching them to the phospholipids in which the liposome is formed . These provide researchers with a tool to examine various membrane protein functions . </P> <P> Plasma membranes also contain carbohydrates, predominantly glycoproteins, but with some glycolipids (cerebrosides and gangliosides). Carbohydrates are important in the role of cell - cell recognition in eukaryotes; they are located on the surface of the cell where they recognize host cells and share information, viruses that bind to cells using these receptors cause an infection For the most part, no glycosylation occurs on membranes within the cell; rather generally glycosylation occurs on the extracellular surface of the plasma membrane . The glycocalyx is an important feature in all cells, especially epithelia with microvilli . Recent data suggest the glycocalyx participates in cell adhesion, lymphocyte homing, and many others . The penultimate sugar is galactose and the terminal sugar is sialic acid, as the sugar backbone is modified in the Golgi apparatus . Sialic acid carries a negative charge, providing an external barrier to charged particles . </P>

A permeable cell membrane acts as a cell