"fluid" = soluble, constantly changing movement.
"mosaic" = composed of a plethora of different macromolecules (ie proteins, phospholipids, and fats).

Important Properties

1. selectively permeable
   If it were totally permeable, products made in the cell would not be able stored in the cell, because it wouldn't be able to maintain a specific internal environment.
   Unsaturated fats allow more fluidity because the fatty acids remain separate and not entangled with those of other phospholipids (which brings phospholipids closer to each other).
   1. criteria for selection
      1. solubility
      2. size
      3. charge (compatibity or attraction between the membrane and the molecule or ion)
2. charged
   1. membranes always have a charge and the charge always changes

This basic model was developed to replace the Davson-Danielli sandwich model.  Problems with the sandwich model included

1. the generalization that all membranes of a cell were the same.  It was challenged (membranes are different in structure and function:  mitochondria membranes are 1-2 nanometers--nm--smaller than the plasma membrane)
2. membrane proteins ARE amphipathic.  If proteins were layered on the surface of the membrane (hence the name "sandwich model"), the hydrophobic parts would be exposed to HOH and the hydrophilic parts of the phospholipids would not be in contact with HOH.

DD=Davson-Danielli

What affects the membrane's "fluidity"? (More Properties)

1. The membrane is held together by hydrophobic interaction (fatty acids and protein parts) which is much weaker than covalent bonding.
2. Lipids drift laterally and flip-flopping (although it occurs) is rare because the 'phobic fatty acid chains would touch HOH (and they resist this).  It moves laterally  at 2 micrometers per second.  Some membrane proteins drift as well, but most are anchored in the membrane.
3. The membrane remains fluid as temperature lowers until (@ a critical temp) it solidifies.  If does not solidify at this temperature, then it is rich in unsaturated phospholipids (reason:  unsat do not pack as close together as do sat or straight hydrocarbon chains).
4. The Steroid Cholestoral
   1. helps stabilize fluidity.
   2. at body temperature, it restrains the movements of phospholipids because it hinders close packing together by its presence;
   3. therefore, it raises the membranes tolerance of colder temperatures.
5. If it solidifies, the proteins become inactive.
6. A cell CAN alter lipid composition (sat to unsat and vice versa).
   example:  winter wheat
   1. is a liquid film consisting of proteins dissolved in a lipid bilayer.
   2. is about as fluid as salad oil.

What constitutes "mozaic"-ness?

1. many different proteins in the bilayer
   proteins determine specific functions
2. Types of proteins
   1. integral
      • can be trans-membrane or just partway
   2. peripheral
      • not embedded in the membrane
      • attached to the surface of the membrane; sometimes to integral proteins

The "Inside and Outside Faces" of Membranes

1. different parts of membranes have differing lipid composition
2. Also depending on this composition and the protein itself, the protein has a directional orientation.
3. carbos are restricted to the exterior of the membrane
4. this asymmetrical distribution is determined as the membrane is synthesize by the ER