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SELECTIVE PERMEABILITY OF CELL MEMBRANES

Permeability refers to whether or not a particular molecule can cross the cell membrane. The cell membrane is selective in that it is able to regulate the movement of a number of substances. However, some substances can move freely across the membrane, without the cell's control

Permeability of the cell membrane to a particular molecule depends upon a number of factors:

a) the size of the molecule
b) whether the molecule is hydrophilic (water-loving) or hydrophobic
( water-hating, lipid soluble)
c) whether the molecule is electrically charged or not

These factors also influence the method of transport that the cell uses to move the molecules across the membrane:

1. Passive Transport (PT) - a) Diffusion or b) Osmosis
2. Facilitated Diffusion(FD) - Protein carriers
3. Active Transport(AT) - a) Endocytosis- Pinocytosis or Phagocytosis
b) Exocytosis
c) Protein carriers that use energy

PERMEABILITY
TYPE OF MOLECULE METHOD OF TRANSPORT
1. HIGHEST Small, hydrophilic or Diffusion/Osmosis
hydrophobic uncharged molecules Freely pass between
eg. O2, H20, CO2 phospholipids of bilayer


2. HIGH Hydrophobic, large, uncharged Diffusion
eg. benzene, hydrophobic a.a. dissolve in lipid bilayer


3. MODERATE Hydrophilic, large, uncharged Facilitated diffusion or
TO LOW eg. glucose, sugars, Active transport protein
hydrophilic amino acids carriers



4. LOW Charged ions Facilitated diffusion
eg. H+, Na +, K +, Ca2+, Cl- or Active transport
charged amino acids


5. VERY LOW Very large Macromolecules Active transport
(cannot cross) eg. proteins, starch (endocytosis)

PERMEABILITY OF THE CELL MEMBRANE CONTINUED:

1. the size of the molecule

• the cell membrane is very permeable to small molecules such as
water, carbon dioxide and oxygen
• these molecules freely DIFFUSE across the membrane because they are small
enough to pass between the phospholipid molecules of the lipid bilayer
• the cell CANNOT control the diffusion of these molecules, and their movement
is determined by the concentration gradient
• such as glucose and amino acids, cannot enter by simple diffusion and must
pass through PROTEIN CHANNELS or pores which are large enough to accommodate
them
• such as proteins, or starch, cannot pass through the membrane at all, and
must used ENDOCYTOSIS (membrane shape changes) to enter into a food vacuole.


2. whether the molecule is hydrophilic (water loving) or hydrophobic (water hating or lipid soluble)

HYDROPHOBIC molecules
• such as ether, can pass through the membrane rapidly by
diffusion , regardless of size, because they are able to dissolve in the
lipid bilayer (lipid soluble)

HYDROPHILIC molecules
• such as glucose and amino acids, or ions, must avoid the lipid bilayer, since
they are not lipid soluble.
• therefore, they must enter through a hydrophilic protein channel


3. very small molecules

• molecules which carry a charge are called IONS eg. sodium ion Na+, potassium
ion K+
• the charge makes the molecule hydrophilic and unable to pass through the
lipid bilayer, even though they might be small
• therefore, these ions must enter the cell through a protein channel