1. Osmosis
Diffusion

2. Passive Transport
 What was the job of the cell membrane?
 Help organisms maintain homeostasis by controlling
what substances move into or out of the cell
(semi-permeability).
 Some substances can move across the membrane
without the cell having to expended energy
 This is known as Passive Transport (no energy
required).

3. Diffusion
 Molecules of any substance are in constant random movement in
all fluids. This movement is called Brownian movement.
 The NET movement of molecules from an area of higher
concentration to an area of lower concentration is diffusion.
 The difference in concentrations of molecules across a space is
called a concentration gradient.

4. Equilibrium
 Diffusion will eventually cause the concentration of
molecules to be the same throughout the space that
the molecules occupy.
 Equilibrium exists when the concentration of
molecules is the same throughout a space.

5. Osmosis
 The process by which water molecules diffuse across
a cell membrane from an area of higher concentration
to an area of lower concentration is called osmosis.
 Water moves down its concentration gradient.
 It does NOT require energy to move down gradient, so
it is a passive transport.
 Osmosis is the diffusion
of water!!!!

6. What happened with the potato?
 Water moved (osmosis)INTO the potato cells placed
in distilled water.
 Water moved (osmosis) OUT of the potato cells placed
in salt water.
 WHY?????????????
 Concentration of water outside the potato cells in
distilled water was HIGHer so it moved INTO the cells
where the concentration of water was LOWer.
 Concentration of water inside the potato cells in salt
water was HIGHer so it moved OUT of the cells where
the concentration of water was LOWer.

7. Direction of Osmosis
 The direction of osmosis depends on the concentration of
solutes on the two sides of the cell membrane.
 We have to compare the solution inside the cell to the
solution outside the cell to determine where the
molecules will go.
 Remember: The cell membrane is selectively permeable
(semi-permeable) and water can always cross from areas of
high concentration to areas of low concentration but some
other molecules cannot. If the other molecules can’t cross
then water will be the only molecule to move.

8. Types of Solutions: Compare
solutes inside and outside cells.
 Hypotonic – Concentration of solutes outside the cell are
lower than inside the cell.
 Isotonic – concentration of solutes outside the cell are
equal to inside the cell.
 Hypertonic – concentration of solutes outside the cell are
higher than inside the cell.
 How would the concentration of water molecules compare
to the concentration of solute molecules?
 TIP: concentration of water is opposite that of solutes. If
solutes are high then water is low, if solutes are low then
water is high. Water diffuses from hypotonic solutions to
hypertonic solutions.

9. Direction of osmosis

10. How cells deal with osmosis
 Which environment do you think is easiest for cells to
deal with? Is it the same for plant and animal?
 Isotonic: this is the case with the cells of most
vertebrate animals on land and of most other
organisms living in the sea.
 Many cells function in a hypotonic environment like
freshwater unicellular organisms.
 Water constantly diffuses INTO these organisms.
 The organisms must rid themselves of the extra water
that enters through osmosis. How?

11. Contractile Vacuoles
 http://www.youtube.com/watch?v=iG6Dd3COug4
 Paramecium collects excess water into a vacuole that contracts
expelling the water (sends it back out of the cell).
 Are contractile vacuoles a form of passive transport?
 NO: It requires energy from the cell to contract.
 Can you think of another way organisms living in a hypotonic
environment could deal with this problem?
 Pump solutes out to keep the solutions inside the cell isotonic to
those outside the cell making is less likely water will diffuse into
the cell. This also requires energy. What term describes
transport that requires energy?
 Active transport
 What happens to cells that can’t compensate for osmosis?
 Cytolysis (bursting of cells) or shrivel (shrink)

12. What type of environment is best
for plant cells?
 Hypotonic: water moves into the plant cells by
osmosis. (What do plant cells need water for?)
 Plant cells swell as they fill with water (stored where?)
 The swelling stops when the cell membrane is pressed
up against the cell wall.
 The cell wall is strong enough to resist the pressure
exerted by the water inside the expanding cell. (What
would happen to an animal cell? Why?)
 The pressure that water molecules exert against the
cell wall is called Turgor Pressure (TP). High TP is
good!

13. What happens to plant cells in a
hypertonic environment?
 Water leaves the cells through osmosis.
 The cell membrane shrinks away from the cell wall.
 Turgor Pressure is lost resulting in Plasmolysis
 Plasmolysis is the reason plants wilt if they don’t get
enough water.

14. Facilitated Diffusion
 Process used for molecules that cannot diffuse rapidly
through cell membrane even when there is a
concentration gradient.
 Molecules could be insoluble in lipids or too large to pass
through pores in the membrane.
 Any solutions? Ideas? Phospholipids and what?
 Movement of these molecules is assisted by carrier
proteins.
 Acts as a door specific to certain molecules and only allows
passage from HIGH concentration to LOW con.
 Glucose gets in and out this way. Why would that be
important?

15. Diffusion through Ion Channels
 How does this work? Does it require energy? Why?
 Ions are not soluble in lipids.
 Proteins act as ion channels and are specific to each.
 Some are always open, others have “gates” that open
and close to allow or stop ions
(Calcium, Sodium, Potassium, Chloride).
 Gates will open or close in response to 3 kids of
stimuli: stretching of membrane, electrical signals, or
chemicals. What controls the gates?
 Stimuli!