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Membrane Structure &
Function
Functions of the Cell Membrane
Isolates the cytoplasm from the external
environment
Regulates the flow of materials into and
out of the cell
Communicates with other cells
The Fluid Mosaic Model
 Currently accepted model
of the cell membrane
 Proposed by Singer and
Nicolson in 1972
 Phospholipid bilayer
 Hydrophilic “head” –
exposed to the outside
 Hydrophobic “tail” – hides
inside
 Membrane proteins are
randomly dispersed in
phospholipid bilayer
Fluid Mosaic Model
Fluidity of the Membrane
 The lipids and
proteins can drift
throughout the
membrane
 Membrane is NOT
stiff/rigid
 Cholesterol makes
the membrane
stronger by limiting
the movement of
phospholipids
Membrane as a Mosaic
 Lipid bilayer has
membrane
proteins “stuck” in
it
 Integral proteins
Go through the
membrane (both
sides)
 Peripheral proteins
 attached to the
surface of the
membrane
Selective Permeability of the Cell
Membrane
The cell membrane can “choose” what
enters and exits a cell
“Gatekeeper of the Cell”
Passive Transport
Def’n:
Diffusion of a substance that does NOT require
the input of energy by the cell
3 types of passive transport:
Diffusion
Osmosis
Facilitated diffusion
Diffusion
Movement of molecules from high to
low concentration until equilibrium
is reached.
Passive Transport= no energy
required
What substances may diffuse across
membrane? Nonpolar (noncharged) molecules; small polar
molecules
Diffusion
 Each substance
diffuses down its
OWN concentration
gradient and is
unaffected by
concentration
gradients of other
substances
Diffusion
 Does all movement
stop once equilibrium
is reached??
NO!!
Equal rates in all
directions
Osmosis
 Def’n:
The passive transport of
water across a selectively
permeable membrane
Hyper-, hypo-, iso- tonic
 RELATIVE TERMS!!
 Always referring to solute
concentration
 Water moves from areas of
lower concentration of
solutes (hypotonic) to areas
of higher solute
concentration (hypertonic)
Osmosis in Plant and Animal Cells
 Animal Cells:
 Plasmolysis
 Occurs when a cell is in
a hypertonic solution
• Water goes from cell into
solution
 Cytolysis
 Occurs when a cell is in
a hypotonic solution
• Water goes from solution
into cell
 Plant Cells:
 Turgid
 Flaccid
Osmosis in Plant and Animal Cells