cell membrane

membranes are complex
structures composed of lipids,
proteins & carbohydrates
the various cellular membranes have different compositions, as reflected in the ratio of protein to lipid. this is not surprising, given their divergent functions. membranes are asymmetric sheet-like enclosed
structures with distinct inner and outer surfaces. these sheet-like structures are noncovalent assemblies that are thermodynamically stable and metabolically active. numerous proteins are located in membranes, where they carry out specific functions of the organelle,the cell, or the organism.
the major lipids in mammalian
membranes are phospholipids,
glycosphingolipids, & cholesterol
a. phospholipids
of the two major phospholipid classes present in membranes, phosphoglycerides are the more common and consist of a glycerol backbone to which are attached two fatty acids in ester linkage and a phosphorylated alcohol. the fatty acid constituents are usually even-numbered carbon molecules, most commonly containing 16 or 18 carbons. they are unbranched and can be saturated or unsaturated. the simplest phosphoglyceride is phosphatidic acid, which is1,2 diacylglycerol 3-phosphate, a key intermediate in the formation of all other phosphoglyceride. in other phosphoglycerides, the 3-phosphate is esterified to an alcohol such as ethanolamine, choline, serine, glycerol, or inositol.the second major class of phospholipids is composed of sphingomyelin, which contains a sphingosine backbone rather than glycerol. a fatty acid is attached by an amide linkage to the amino group of sphingosine, forming ceramide. the primary hydroxyl group of sphingosine is esterified to phosphorylcholine. sphingomyelin, as the name implies, is prominent in myelin sheaths.
the amounts and fatty acid compositions of the various phospholipids vary among the different cellular membranes.
b. glycosphingolipids
the glycosphingolipids (gsls) are sugar-containing lipids built on a backbone of ceramide they include galactosyl- and glucosylceramide (cerebrosides) and the gangliosides. they are mainly located in the plasma membranes of cells.
c. sterols
the most common sterol in membranes is cholesterol, which resides mainly in the plasma membranes of mammalian cells but can also be found in lesser quantities in mitochondria, golgi complexes, and
nuclear membranes. cholesterol intercalates among the phospholipids of the membrane, with its hydroxyl group at the aqueous interface and the remainder of the molecule within the leaflet.all of the above lipids can be separated from one anotherby techniques such as column, thin layer, and
gas-liquid chromatography and their structures established by mass spectrometry. each eukaryotic cell membrane has a somewhat different
lipid composition, though phospholipids are the major class in all.
membrane lipids are amphipathic
all major lipids in membranes contain both hydropinghobic and hydropinghilic regions and are therefore termed “amphipathic.” membranes themselves are thus amphipathic. if the hydropinghobic regions were separated from the rest of the molecule, it would be insoluble in water but soluble in oil. conversely, if the hydropinghilic region were separated from the rest of the molecule, it would be insoluble in oil but soluble in water. thus, the polar head groups of the phospholipids and the hydroxyl group of cholesterol interface with the aqueous environment a similar situation applies to the sugar moieties of the gsls
saturated fatty acids have straight tails, whereas unsaturated fatty acids, which generally exist in the cis form in membranes, make kinked tails. as more kinks are inserted in the tails, the membrane becomes less tightly packed and therefore more fluid.
detergents are amphipathic molecules that are important in biochemistry as well as in the household. the molecular structure of a detergent is not unlike that of a phospholipid. certain detergents are widely used to solubilize membrane proteins as a first step in their purification.
the hydropinghobic end of the detergent binds to hydropinghobic regions of the proteins, displacing most of their bound lipids. the polar end of the detergent is free, bringing the proteins into solution as detergent protein
complexes, usually also containing some residual lipids.
membrane lipids form bilayers
the amphipathic character of phospholipids suggests that the two regions of the molecule have incompatible solubilities however, in a solvent such as water, phospholipids organize themselves into a form that thermodynamically serves the solubility requirements of both regions. the closed bilayer provides one of the most essential properties of membranes. it is impermeable to most water soluble molecules, since they would be insoluble in the hydropinghobic core of the bilayer.
lipid bilayers are formed by self-assembly, driven by the hydropinghobic effect. when lipid molecules come together in a bilayer, the entropy of the surrounding solvent molecules increases. two questions arise from consideration of the above. first, how many biologic materials are lipid soluble and can therefore readily enter the cell? gases such as oxygen, co2, and nitrogen—small molecules with little interaction with solvents—readily diffuse through the hydropinghobic regions of the membrane.in general, the permeability coefficients of small molecules in a lipid bilayer correlate with their solubilities in nonpolar solvents. for instance, steroids more readily traverse the lipid bilayer compared with electrolytes. the high permeability coefficient of water itself is surprising but is partly explained by its small size and relative lack of charge.