D.  1     Natural cell barriers -- The ANSWERS
                Barriers such as the  C H A N N E L       P R O T E I N S   within the
                lipid bilayer (2 layers of lipids) in the  CELL   MEMBRANE  (P L A S M A)
                MEMBRANE control the flow of molecules in and out of cells

            
       a.        (Italics = Bio H only)  A barrier which has the ability to choose (select or differentiate)
             which molecules can cross it is known as a semi ( S E L E C T I V E L Y   or  
             D I F F E R E N T I A L L Y)   permeable membrane.

2            Membrane permeability (in decreasing order of priority) is regulated by environmental. factors such as:

a     S I Z E   of the molecule
               
b.    Whether or not the molecule is  P O L A R   (charged) (See water above) 
        Is water polar or non-polar?  _POLAR____ Explain!    
        _  It is charged (hydrogen ends +; oxygen end -) __
c     Molecular  S H A P E  & O R I E N T A T I O N  as it approaches
       P R O T E I N    channels  (See turquoise channel above)

 

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