Membrane Interface


Cell Membrane (Wiki) 
http://en.wikipedia.org/wiki/Cell_membrane 
"The cell membrane ... is the biological membrane separating the interior of a cell from the outside environment.

http://commons.wikimedia.org/wiki/File:Cell_membrane_detailed_diagram_3.svg   

File:Cell membrane detailed diagram 3.svg



"
The arrangement of hydrophilic heads and hydrophobic tails of the lipid bilayer prevent polar solutes (e.g. amino acids, nucleic acids, carbohydrates, proteins, and ions) from diffusing across the membrane, but generally allows for the passive diffusion of hydrophobic molecules. This affords the cell the ability to control the movement of these substances via transmembrane protein complexes such as pores and gates.


Notice that the "
the ability to control the movement of these substances" applies only to hydrophylic (water soluble) substances.  As far as I can tell, the cell has no control at all over the passage of hydrophobic (oil soluble) substances across its membrane.  

"
The cell membrane contains many integral membrane proteins, which pepper the entire surface. These structures, which can be visualized by electron microscopy or fluorescence microscopy, can be found on the inside of the membrane, the outside, or membrane spanning. These may include integrins, cadherins, desmosomes, clathrin-coated pits, caveolaes, and different structures involved in cell adhesion.


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The permeability of a membrane is the ease of molecules to pass through it. Permeability depends mainly on the electric charge of the molecule and to a lesser extent the molar mass of the molecule. Electrically neutral and small molecules pass the membrane easier than charged, large ones.


Note that "
Electrically neutral ... molecules " tend to be hydrophobic (lipid, fatty, nonpolar) while "charged ... ones" are always hydrophylic (water soluble, polar). 


Hydrophilic (Wiki) 
http://en.wikipedia.org/wiki/Hydrophile   
"
...  from the Greek (hydros) "water" and φιλια (philia)  "bonding," refers to a physical property of a molecule  that can transiently bond with water (H2O) through hydrogen bonding. This is thermodynamically favorable, and makes these molecules soluble not only in water, but also in other polar solvents. There are hydrophilic and
hydrophobic parts of the cell membrane. A hydrophilic molecule or portion of a molecule is one that is typically charge-polarized and capable of hydrogen bonding, enabling it to dissolve more readily in water than in oil or other hydrophobic solvents. Hydrophilic and hydrophobic molecules are also known as polar molecules and nonpolar molecules, respectively.


Hydrophobic (Wiki) 
http://en.wikipedia.org/wiki/Hydrophobe   
"
In chemistry, hydrophobicity (from the combining form of water in Attic Greek hydro- and for fear phobos) is the physical property of a molecule (known as a hydrophobe) that is repelled from a mass of water.[1]

Hydrophobic molecules tend to be non-polar and thus prefer other neutral molecules and nonpolar solvents. Hydrophobic molecules in water often cluster together forming micelles. Water on hydrophobic surfaces will exhibit a high contact angle.

Examples of hydrophobic molecules include the alkanes, oils, fats, and greasy substances in general."








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