TRANSPORT THROUGH CELL
MEMBRANES
Diffusion and Osmosis
Learning Outcomes:
B9 - Analyse the structure and function
of the cell membrane:
 describe passive transport processes
including diffusion, osmosis, and facilitated
transport
 explain factors that affect the rate of diffusion
across a cell membrane
 predict the effects of hypertonic, isotonic, and
hypotonic environments on osmosis in
animal cells
 Recall that the cell membrane is selectively
permeable, allowing only certain things to enter and
exit the cell
 Several mechanisms are involved in the movement of
materials in and out of the cell
 Some of these are passive (do not require energy),
while others are active (require energy)
Diffusion & Osmosis
 Passive - don’t require energy - “goes with the flow”
 Movement of substances with the concentration
gradient (from high to low concentration)
Diffusion
 Movement of molecules from an area of high
concentration to an area of low concentration
until equilibrium is reached
 Substances that can pass
freely through the cell
membrane will do so by
diffusion, if a
concentration gradient is
present (higher
concentration on one
side of the membrane)
 Examples - gases (O2,
CO2), lipid-soluble (nonpolar) molecules
Rate of diffusion
Factors that affect the rate of diffusion:
 Temperature
Size of molecules
 Concentration gradient
(how would each of these affect the rate?)
Osmosis
 Is the diffusion of water
across a differentially
permeable membrane
 Water moves in and out of
cells by osmosis through the
cell membrane
Membrane requirements
 Membrane must be permeable to water but
impermeable to the solute
 Must have different solute concentrations on
either side of the membrane
 Water moves toward higher solute
concentration (ex. toward the salt)
 Solute stays where it is
 (“salt sucks!”)
Osmosis in cells
 Osmotic pressure is pressure due to movement
of water across a membrane
 This is important in the functioning of many
living systems (ex. circulatory system)
Tonicity
 Refers to the relative
solute concentrations of
solutions in two places
 Determines the direction
of water movement (ex.
In or out of cells)
 Hypertonic = higher concentration
 Hypotonic = lower concentration
 Isotonic = equal concentrations
These terms are used to compare two
solutions
Cell in a hypertonic solution
 Solute concentration
is greater outside the
cell
 Net movement of
water OUT of the cell
(cell shrinks)
Cell in a hypotonic solution
 Solute concentration
is less outside than
inside the cell
 Net movement of
water INTO the cell
(cell expands and
may burst)
Cell in an isotonic solution
 Solute
concentrations are
equal inside and
outside of the cell
 No net movement of
water (equal
movement in both
directions)
Animal and Plant Cells
Blood Cells