Diffusion and Osmosis
Chapter 3, Section 4
Of your textbook
Passive Transport
 Particles
are constantly in motion, colliding
and scattering.

This motion is random.
 Does
not require the cell to use ATP /
energy
 The membrane is still semi-permeable, so
only certain substances can cross (in or
out) by passive transport.
Passive Transport
 A concentration
gradient is
the difference in the
concentration of a substance
from one location to another
 When there is a
concentration gradient, the
NET movement is DOWN
their concentration gradient from areas of higher
concentration to areas of
lower concentration
ANIMATION
Diffusion
 Movement
of
molecules in a
fluid or gas
from an area of
higher
concentration to
an area of
lower
concentration
ANIMATION
Equilibrium
 Diffusion
continues until
the solution is at
equilibrium (no
concentration
gradient).
 Molecules still
move, but there
is no NET
movement.
Facilitated Diffusion
 Some
small,
polar
molecules
cannot easily
diffuse across
a membrane.
 They need the
help of
transport
proteins.
Facilitated Diffusion
 The
process of diffusion being “helped” by
transport proteins is called facilitated
diffusion.
 Still passive transport because molecules
are still moving down their concentration
gradient
 Requires no ATP / energy
 ANIMATION
Transport Proteins
 In
facilitated diffusion,
transport proteins,
pierce the cell
membrane and allow
openings for
molecules to pass.
 These proteins are
considered integral
proteins because they
are fully embedded in
the membrane.
Transport Proteins

There are many
types of transport
proteins.
 Most only allow
certain ions or
molecules to pass.
 Some are simple
channels or tunnels
and some are more
complex, shapechanging proteins.
Osmosis
 The
diffusion of water across
a semipermeable membrane
is called osmosis.
 In a solution, there are water
molecules and dissolved
particles (the solute).
 The more dissolved particles
there are, the lower the
concentration of water
molecules.
ANIMATION
Solutions
 Water
is considered the solvent.
 The substance(s) dissolved in water is /
are the solute(s).
 Together, solvent + solute  solution.
Comparing Solutions
 A solution
may be desribed as isotonic,
hypertonic or hypotonic relative to
another solution
 These are comparisons; they require a
point of reference (ie, my hair is shorter…
…than it was last year).
 The comparison in biology is usually to the
inside of a cell.
Isotonic

A solution is isotonic to a
cell if it has the same
concentration of dissolved
particles as the cell.
 This means the water
concentration is also the
same.
 Water molecules move into
and out of the cell at an
equal rate in an isotonic
solution.
 The cell size remains the
same.
Hypertonic

A hypertonic solution has
a higher concentration of
dissolved particles than a
cell.
 This means the water
concentration is lower than
that of the cell.
 Thus, water flows out of
the cell – so, the cell will
shrivel and eventually die.
ANIMATION
Hypotonic

A hypotonic solution has a
lower concentration of
dissolved particles than a
cell.
 Therefore the water
concentration is higher
than that of the cell.
 Thus, water diffuses into
the cell – causing the cell
to expand and potentially
burst.
Impact on Cells

In an isotonic solution (center), water enters / exits
red blood cells at equal rates.
 In a hypertonic solution (like salt water – right),
water rushes out and the cell shrivels.
 In a hypotonic solution (like distilled water – left),
water rushes in and the cell swells / bursts (lysis).
Video clips:
RBC in isotonic solution
RBC in hypertonic solution
RBC in hypotonic solution
Adaptations - Plants
 Plant
cells use the cell wall to prevent bursting.
At center, the plant cell is in an isotonic solution. Water
moves in / out at equal rates (no NET movement)
At left, the plant
cell is in a
hypotonic
solution.
At right, the
plant cell is in
a hypertonic
solution.
Water rushes
in, filling the
vacuole.
Water rushes
out of the
cell, draining
the vacuole.
This cell is
turgid / has
high turgor
pressure.
Video: Elodea in isotonic / hyper / hypo
This is called
plasmolysis.
Adaptations - Protists




Paramecia live in
freshwater
This makes paramecia
hypertonic to their
surroundings
Water is constantly
rushing into the
paramecium
So the paramecium uses
a contractile vacuole to
pump the water back out
(and prevent bursting)
Video: The contractile vacuole in action