Definition –Protective and selective barrier
II. Plasma membranes
1. Separates the intracellular and extracellular compartments.
2. Extraordinarily thin: 4-20 nm.
3. Selective barrier.
4. Detects chemical signals.
5. Attachment site of filaments (e.g. cytoskeleton filaments).
B. Membrane Structure
1. Double layer of lipids molecules in which proteins are embedded;Phospholipid bilayer.
Fatty acid “tails” are hydrophobic but the phospholipid (has phosphate group) “head” is hydrophilic.
2. Lipids –Include Phosphoglycerides (glycerol based – a 3 carbon alcohol), Sphingolipids ( sphingosine based) and Cholesterol.
a. Phosphoglycerides and sphingolipids are amphipathic molecules. One end is polar (water soluble). The other is nonpolar (lipid soluble).
b. Cholesterol– a steroid ring-shaped with C=C bond within the ring; by interacting with phospholipids, cholesterol can reduce permeability by filling gaps between phospholipids; cholesterol can increase membrane fluidity if there is enough cholesterol to block phospholipids from packing closely.
3. Fluidity of the plasma membrane – Fluid Mosaic Model
a. Membrane fluidity depends on properties of membrane lipids which are influenced by the physical environment
b. Longer length of fatty acid chains and saturated chains lead to less fluid membranes & then proteins cannot move
c. Shorter length fatty acid chains and unsaturated chains lead to higher fluidity but cannot be to the point that the membrane liquefies completely and loses its integrity.
d. Note how animals at cold temperatures have more unsaturated tails to keep their membranes fluid at colder temps.
4. Proteins are fewer in number than lipids but larger in size, so can make up to 50% of membrane
a. Integral proteins– embedded in bilayer or spanning across entire membrane. Cannot be extracted without disrupting bilayer
b. Peripheral proteins– located at the surface, usually attached to integral protein
c. Membrane protein functions
-Second messenger systems