(Text: p. 77-84 & 467-476)
STRUCTURAL PROPERTIES OF CELL MEMBRANES
Formation of cell membranes is based upon the properties of lipids.
All are bi-layers of phospholipids with associated proteins. As previously mentioned, phospholipids are amphipathic meaning that they possess both a hydrophobic and hydrophilic end.
Polar head groups are in contact with water while fatty acid tails aggregate in the interior of the membrane.
The four major phospholipids found in cell membranes:
phosphatidyl choline
phosphatidyl ethanolamine
phosphatidyl serine
sphingomyelin - a non-glycerol phospholipid
See figures of these structures in text.
Various glycolipids are also found in the outer leaflet of the cell membrane.
Cholesterol is another important constituent of the animal cell membrane.
Lipid composition differs in the different types of cells and in different types of organisms. The average eukaryotic plasma membrane - ~50% of mass is lipid and 50% protein.
The lipid bilayer behaves as a fluid.
FLUID MOSAIC MODEL proposed by Singer and Nicolson
Lipids and proteins can readily move laterally and can also undergo rotation. The degree of membrane fluidity is determined by temperature and lipid composition.
Lipids with shorter fatty acid chains are less rigid and remain fluid at lower temperatures. This is because interactions between shorter chains is weaker than for longer chains.
Lipids containing unsaturated fatty acids increase membrane fluidity The = bonds introduce kinks, preventing tight packing of the fatty acids.
Cholesterol with its hydrocarbon ring structure plays a distinct role in determining membrane fluidity.
Polar hydroxyl group positions close to the phosphate head group. Rigid rings interact with regions of fatty acid chain adjacent to phospholipid head groups. This interaction decreases the mobility of the outer portions of the fatty acid chains, making this region of the membrane more rigid, even at higher temperatures.
On the other hand...
Insertion of cholesterol interferes with interactions between fatty acids, thereby maintaining fluidity at lower temperatures.
Cholesterol is not present in bacteria or plant cells.
Plant cell membranes do contain sterols which function in a manner similar to cholesterol.
In the fluid mosaic model of the membrane ----there are membrane proteins inserted into the lipid bilayer. The lipids provide the basic structure, but proteins carry out the specific functions of the different types of membranes.
Integral membrane proteins
Amphipathic Properties
Also referred to as transmembrane proteins.
Commonly, integral membrane proteins have membrane spanning domains which are alpha helical. May be 1 , 2, 7 or more membrane spanning alpha helical domains. The alpha helix neutralizes the polar character of the peptide bonds. The hydrophobic side chains assoc. with these amino acids interact with the fatty acid chains of membrane lipids.
Most transmembrane proteins are also glycosylated [have carbohydrate groups attached]
Peripheral membrane proteins
Not embedded in the bilayer but indirectly associated with the membrane through interactions with integral membrane proteins or by weak electrostatic interactions with the hydrophilic head groups of membrane lipids. Located extracellular or associated with the cytoplasmic surface of the bilayer.
3rd Category not described by Singer and Nicolson
Lipid-anchored proteins
Located outside the lipid bilayer, but covalently linked to a lipid molecule that is situated within the bilayer. An increasingly large # of proteins have been found to be linked by a short oligosaccharide to a molecule of glycophosphatidylinositol (GPI) that is embedded in the outer leaflet of the lipid bilayer. These proteins are released when membrane is treated with enzymes that (Phospholipases) that specifically recognized and cleaved inositol-containing phospholipids.
Another group of proteins are actually present on the cytoplasmic side of the membrane and are anchored by long hydrocarbon chains embedded in the inner leaflet of the lipid bilayer.
see appropriate figures in Text
THE GLYCOCALYX
The extracellular portion of the plasma membrane proteins are generally glycosylated. LIkewise, the carbohydrate portions of glycolipids are exposed on the outer face of the plasma membrane. Consequently, the glycocalyx, is formed by the oligosaccharides of glycolipids and transmembrane glycoproteins.
Role:
Protection of cell surface
Markers for cell-cell interactions