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Cell Membranes
Diffusion, Osmosis
Tonicity
Function of
Plasma Membrane-
Controls what enters and
exits the cell.
Also acts like a boarder,
preventing cells internal
environment from coming in
contact with external
environment.
Fluid Mosaic Model
The model, which was devised by SJ Singer and GL Nicolson in
1972, describes the cell membrane as a two-dimensional liquid
in which phospholipid and protein molecules diffuse easily.
Phospholipid
Bilayer
Hydrophilic
Hydrophobic
Phospholipid 
OUTSIDE
Heads
Tails
Heads
INSIDE
5
6
7
1
2
3
4
Cholesterol = adds to the fluidity of the phospholipids
5
6
7
3
4
Protein (Integral and Peripheral) Functions
5
6
7
Surface Carbohydrates (Glycoproteins and Glycolipids)
= Function in cell recognition, cell signaling, and cell adhesion
Diffusion- when molecules pass through a
membrane.
Osmosis – when water only passes through a
membrane.
Permeable- molecules pass through membrane.
Impermeable – molecules can’t pass through membrane.
Selective or Semi-permeable – membrane will allow
certain size molecules to pass through.
Cell Membrane’s Function?
Controls what enters and exits the cell
Selective Permeability =
allows certain items to pass in/out
What items pass easily?
Small
Uncharged (neutral)
Transport Processes:
Passive Transport
-Diffusion (Simple and Facilitated)
-Osmosis
Active Transport
Passive Transport =
No energy required
Concentration
Gradient
High  Low = Reach
+Equilibrium
Diffusion

molecules
other
than H2O
------------------------------------------------------------------------------------------

What else operates by Diffusion?
Facilitated Diffusion = Diffusion through a protein (integral)
What type of molecule? Large and/or charged
Simple Diffusion
Facilitated Diffusion

Diffusion
molecules
other
than H2O
------------------------------------------------------------------------------------------

Osmosis
water
Passive Transport
Active Transport
Fig. 8.16 Both diffusion and facilitated diffusion are forms of passive transport of molecules down
their concentration gradient, while active transport requires an investment of energy to move
molecules against their concentration gradient.
Active Transport
Energy?
Requires energy
Concentration Gradient (with or against)?
Why?
HIGH 
 LOW
Would this be an example of (passive or active) transport?
Sodium Potassium Pump (Na+ K+ pump)
Passive
Active
Passive
Why do you think they call the proteins in the
plasma membrane the gatekeepers?
Name some of the molecules that must enter and exit the cell
on a daily, hourly, minute by minute bases.
What is the cellular environment like?
Surrounded by water
?
?
3 Types of “Solution”:
• Isotonic Solution = solutions with equal solute
concentrations
• Hypotonic Solution = solution with the higher
concentration of solutes
• Hypertonic Solution = solution with the lower
concentration of solutes
“passive transport”
Tonicity
Animal
Cells
#1
#2
#3
Plant
Cells
Q. What kept the plant cell from bursting in picture #1?
Isotonic
Hypotonic
Hypertonic
Define Plasmolysis: occurs in a plant cell during a hypertonic
situation, plasma membrane pulls away from
cell wall due to shrinking of water vacuole.
Isotonic
Hypertonic
(Plasmolysis)
1.
2.
1.
2.
3.
4.
5.
What does the blue represent? (water or molecules of solute-salt)
What do the purple dots represent? (water or molecules of solute)
*count molecules on each side of membrane.
Identify membrane, notice it’s location.
What does selectively permeable membrane mean?
Notice water level in start U-tube. Describe.
Notice water levels in finished U-tube. Describe.
Remember you are looking at concentration levels of WATER. On start U-tube
which side has higher concentration of water. Go HL, what happens?
6. Which molecule was allowed to pass through the
selectively permeable membrane?
7. Remember all membranes have pores they just
vary in size. So which molecule is smaller
(water or solute-purple dots).
8. Would the U-tube example represent (osmosis
or diffusion)? Explain your answer.
9. Would the U-tube example represent (passive
or active) transport of molecules? Explain your answer.