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)
b)
c)
the size of the molecule
whether the molecule is hydrophilic (water-loving) or hydrophobic (water-hating, lipid soluble)
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)
2. Facilitated Diffusion(FD)
3. Active Transport(AT)
PERMEABILITY
- a) Diffusion or b) Osmosis
- Protein carriers
- a) Endocytosis- Pinocytosis or Phagocytosis
b)
Exocytosis
c)
Protein carriers that use energy
TYPE OF MOLECULE
METHOD OF TRANSPORT
1. HIGHEST
(able to get across
easily)
Small, hydrophilic or hydrophobic
uncharged molecules
eg. O2, H20, CO2
Diffusion/Osmosis
Freely pass between
phospholipids of bilayer
2. HIGH
Hydrophobic, large, uncharged
eg. benzene, hydrophobic amino
acids
Diffusion
dissolve in lipid bilayer(lipid
soluble)
3. MODERATE
TO LOW
Hydrophilic, large, uncharged
eg. glucose, sugars,
hydrophilic amino acids
Facilitated diffusion or
Active transport
(protein carriers)
4. LOW
Charged ions
eg. H+, Na +, K +, Ca2+, Clcharged amino acids
Facilitated diffusion
or Active transport
(protein carriers)
5. VERY LOW
(cannot cross)
Very large Macromolecules
eg. proteins, starch
Active transport
(endocytosis)
PERMEABILITY OF THE CELL MEMBRANE CONTINUED:
1. the size of the molecule
very small
molecules



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
moderate sized
molecules

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
very large
molecules

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
SEE FIGURE 21-3 p.641
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)
b)
c)
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
These factors also influence the method of transport that the cell uses to move the molecules across
the membrane:
1. Passive Transport (PT)
2. Facilitated Diffusion (FD)
3. Active Transport (AT)
PERMEABILITY
HIGHEST
(able to get across easily)
HIGH
MODERATE
TO
LOW
LOW
VERY LOW
(cannot cross)
-
_________________ or _________________
_________________
______________ -- _____________ or _______________
______________
_________________________________________
TYPE OF MOLECULE
Small, hydrophilic or hydrophobic
uncharged molecules
eg. O2, H20, CO2
Hydrophobic, large, uncharged
eg. benzene, hydrophobic amino
acids
Hydrophilic, large, uncharged
eg. glucose, sugars,
hydrophilic amino acids
Charged ions
eg. H+, Na +, K +, Ca2+, Clcharged amino acids
Very large Macromolecules
eg. proteins, starch
METHOD OF TRANSPORT
PERMEABILITY OF THE CELL MEMBRANE CONTINUED:
1. _____________________________________________________________________________

the cell membrane is permeable to very small molecules such as: _________________,
_________________ _________________ and _________________.

these molecules freely ___________ across the membrane because they are ___________
enough to pass between the phospholipid molecules of the lipid bilayer

the cell _____________ control the diffusion of these molecules, and their movement is
determined by the concentration gradient

moderate sized molecules, 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

very large molecules, 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. whether the molecule is charged or not

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
SEE FIGURE 21-3 p.641