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Transcript
Do Now
1. What maintains homeostasis for cells?
THE CELL MEMBRANE
2. What is the term that means when the
organelle from #1 only lets certain things in
and out?
•
SELECTIVELY PERMEABLE
Getting through the barrier of the
cell membrane
• Some substances pass through the membrane
without any energy needed from the cell 
called passive transport
• Some substances need energy from the cell to
pass through the membrane  called Active
Transport
Passive Transport
Passive Transport: movement of molecules
across a membrane WITHOUT any energy
– Three types: 1. Diffusion
2. Facilitated Diffusion
3. Osmosis
The Goal: maintain homeostasis using the cell
membrane as a barrier.
Diffusion
Diffusion = movement of molecules of a substance
from an area of HIGH concentration to an area of
LOW concentration
 Ex: sugar cube in water
food coloring
home cooking
air freshener
Diffusion
• There is a difference in
concentration of the 2 substances
(food coloring & water, water &
sugar, air freshener & air)
• The difference in concentration
across space/area 
concentration gradient
• When the concentration of a
substance is the same all over 
Equilibrium!
(molecules still moving… at random)
Diffusion
• Molecules tend to move DOWN the
concentration gradient…
– more (hi) concentration  to less (low)
concentration
Diffusion Across Membranes
• Some molecules can diffuse across a
membrane (with no aid/help) if they are
soluble in lipids
– NOT all molecules can do this
– Also depends on 1. size & type of molecule
2. structure of molecule
Facilitated Diffusion
• The movement of molecules with the gradient (to an area of
lower concentration) using a carrier protein for passage
• Carrier proteins are molecule specific, so only one type of
molecule can move through them
• Occurs when regular diffusion cannot occur fast enough as
needed (large molecules)
• Can go either into or out of cell depending on gradient
OSMOSIS
Osmosis
Diffusion of water molecules through cell
membrane
Direction of osmosis is determined by the SOLUTE
concentration
DIRECTION OF OSMOSIS
Depends on the concentrations of solutes & water
.
THE WATER MOVES WHERE THERE IS MORE
SOLUTE!
Hypertonic Solution
• HIGH concentration of SOLUTE
OUTSIDE the cell,
• LOW concentration of SOLUTE INSIDE
the cell
• RESULT: WATER MOVES OUT OF THE
CELL.
HYPOTONIC
• LOW concentration of SOLUTE
OUTSIDE the cell
• HIGH concentration of SOLUTE
INSIDE the cell,
• RESULT:
– WATER MOVES INTO THE CELL.
ISOTONIC
• inside and outside concentrations
are equal.
• equal concentrations of solute &
water outside and inside the cell
RESULT: water diffuses in and out at
constant rate, no net movement
Osmosis in Cells
OSMOSIS
How does osmosis affect cells?
• Many cells live in isotonic solutions so
equilibrium is easily attained
• But some do not:
– Some unicellular freshwater organisms
– Plant cells
Unicellular freshwater organisms
Organisms live in hypotonic environments (lots of salt
inside their cells) so they constantly take in water.
• If too much water enters cell dies, called cytolysis
• They have an organelle called a contractile vacuole
= that collects & pumps out the excess water. This
requires energy to be expended
Contractile Vacuole of Amoebae
Osmosis in Plants
• Plant cells also live in hypotonic solutions, so
water enters the cell.. They like it 
• Why?... Osmosis continues until vacuole is full
& the cell membrane is forced against the cell
wall, called  turgor pressure
• When a plant cell loses turgor pressure, the
membrane collapses away from the cell wall,
causing the plant to wilt, called  plasmolysis
Plasmolysis