The lipid bilayer consists of phospholipids. Phospholipids consist of a nonpolar alkyl attached to a phosphate. The nonpolar alkyl resembles a tail. These nonpolar alkyls are hydrophobic, meaning they repel water. As a result, when phospholipids are mixed into water, the nonpolar alkyls will join together and the attached phosphates will have contact with water. The result looks like:
The head of each molecule associates with water, while nonpolar alkyl tails of fatty acyl groups aggregate hydrophobically.
|Glycerophospholipids||A glycerol is attached to 2 fatty acyl groups and one phosphate group. The phosphate may in turn be linked to further groups: ethanolamine (phosphatidyl etholamine), choline (phosphatidyl choline), serine (phosphatidyl serine) or inositol (phosphatidyl inositol).|
|Sphingolipids||A sphingosine linked to 1 fatty acyl group. Sphingolipids are usually linked to a phosphate plus a choline (sphingomyelin), sugar (ceramides) or a complex oligosaccharide (gangliosides). Sphingolipids are absent in most prokaryotes, but are oftentimes present in the outer face of the eukaryotic plasma membrane.|
|Lysophospholipids||Are phospholipids with 1 fatty acyl group removed. This promotes conversion of phospholipid bilayers into micelles, and may destabilize membranes. Substituting cis-unsaturated fatty acids decreases closeness of packing in the nonpolar layer. This increases fluidity and thereby increases movement of small polar molecules across the layer. Cholesterol reduces fluidity and penetration of small polar molecules.|
Fluid Mosaic Model
The fluid mosaic model refers to the freedom of movement of lipids and some proteins in the membrane. This movement was proven by photobleaching, whereby the cell membrane was dyed with fluorescent probes that bound the lipids; and then a high-intensity laser beam bleached a small area, into which the dyed lipids diffused back. The fluidity of the membrane is determined by lipid composition and temperature. At colder temperatures, the membrane is in the gel state (paracrystalline) with almost no movement of lipids; as it gets warmer, it transitions into the liquid ordered state, where it is fluid and lipids in the membrane rotate in place but the lipid bilayer's fatty acid tails remain rather rigid (this is the state at optimal temperature); and at higher temperatures, the membrane is in the liquid disordered state where the fatty acid tails are moving all over the place within the bilayer like flailing arms. Some microorganisms such as E. coli can adjust the lipid composition of their bilayer to maintain ideal fluidity.
In the liquid ordered state, lipids can move around laterally within a leaflet quickly and readily. However, it is very slow for lipids to flip-flop un-catalyzed into the other leaflet; this involves exposing hydrophilic and hydrophobic components to unfavorable environments. Lipids can move between leaflets with the help of certain protein catalysts. Flippase moves lipids from the outer to the cytosolic leaflet; floppase moves lipids from the cytosolic to the outer leaflet; and scramblase moves lipids in either direction, towards equilibrium. Some membrane proteins are able to move within a leaflet of the membrane (one layer of the lipid bilayer), while other membrane proteins (particularly receptors) are restricted or can be clustered in response to stimuli. The membrane skeleton is a peptide grid that gives the cell its shape, and restricts and clusters movement in the membrane. This localization was observed by dyeing membrane proteins with a fluorescent probe and noting that they did not move beyond a certain area of the membrane for long periods of time.
Structures in the lipid bilayer
|Integral Proteins||Cannot be released from a membrane without breaking covalent bonds or disrupting the lipid bilayer. In the laboratory, these can be removed by treatment with detergent; the detergent coats the hydrophobic region of the protein that resides within the bilayer, allowing it to slip out.|
|Peripheral Proteins||Peripheral membrane proteins attach to the surface of the membrane (lipids or proteins) by noncovalent bonds. Peripheral proteins may be removed from a membrane by milder treatments than those needed to remove integral membrane proteins, such as pH adjustments, chelating agents, urea or carbonate.|
|Amphitropic Proteins||Attach and un-attach the lipid bilayer via biological regulation.|
|GPI-Linked Proteins||These are bound extracellularly to glycosyl-phosphatidylinositol (GPI) molecules.|
|Transmembrane Proteins||Transmembrane proteins contain at least one protein domain extended across the lipid bilayer. Some transmembrane integral membrane proteins form ion channels across the membrane; others are active transport or facilitated diffusion carriers; still others are receptors for growth factors or hormones.|
The region of the protein spanning (crossing) the lipid bilayer usually adopts an α-helical structure (ie, bacteriorhodopsin) or a continuous β-sheet (called a β-barrel) (ie, porin) and consists primarily of amino acids with hydrophobic side chains. Thus, the transmembrane region (or regions, if there is a cluster of helices) is recognized by stretches of ~20 hydrophobic amino acids (essentially, N, Q, D and E are the amino acids never within a transmembrane stretch).
Tyrosine and tryptophan are sometimes in the transmembrane region, but close to the membrane's fatty acid heads so that thier polar groups can protrude into the aqueous environment. Also, for some reason intracellular protein regions tend to have positively charged residues. For the transmembrane pore of pore-forming proteins, the outer face is hydrophobic and interacts with the membrane, while the inner face of the pore is hydrophilic and interact with the aqueous environment.
|Lipoproteins||Lipid-linked proteins (lipoproteins) are covalently linked to lipids. Some lipid-linked proteins are fatty-acylated. Myristic acid (14:0) is attached to N-terminal α-amino group of Gly (via an amide linkage): this is a permanent modification and myristoylated proteins are found in many subcellular compartments. Palmitic acid (16:0) is attached to a specific Cys (via a thioester linkage): this is a reversible modification (reversal is catalyzed by palmitoyl thioesterase) and palmitoylated proteins are found on the cytoplasmic face of the plasma membrane. Other proteins are prenylated, whereby isoprene units in the membrane are linked to a C-terminal Cys; and still others have a glycosylphosphatidylinositol (GPI) anchor, located always in the extracellular surface, linked to a C-terminus.|
|Lipid rafts||Lipoproteins congregate into lipid rafts based on the type of lipoprotein. Lipid rafts are mobile membrane microdomains (small parts of the membrane) which are rich in sphingomyelin, glycosphingolipids, cholesterol and membrane proteins (ie, fatty-acylated, prenylated and GPI-anchored lipoproteins).|