1. Diffusion
• Particles are always
moving
• Particles move from areas
of high concentration to
low concentration
• Particles move until
equilibrium is reached
• Particles move passively
– No energy (ATP) required
– PASSIVE TRANSPORT
http://highered.mcgraw-
hill.com/sites/0072495855/stud
ent_view0/chapter2/animation_
_how_diffusion_works.html

2. Proteins
Recall the
structure of the
cell membrane…
• Selectively permeable = semipermeable
– Regulates what enters and exits the cell
• Lipid bilayer (two layers of lipids = fats)
• “Mosaic Model”
– Contains many types of proteins, pumps, channels, and receptors
Inside of cell (cytoplasm)
Outside of cell
Protein
channel Lipid bilayer
Carbohydrate
chains
Diffusion Across the Cell Membrane

3. Diffusion Across the Cell Membrane
• Cell membranes are selectively-permeable - they allow
some materials to pass across, but not others
(also called semipermeable)
• If the membrane is permeable to a substance, movement of
that substance (solute) will occur as shown in the diagram
below:

4. Facilitated Diffusion
• Facilitated Diffusion
– No energy required – PASSIVE TRANSPORT
– Uses a protein channel in the membrane
• Molecules move from high to low concentration
High
Concentration
Low
Concentration
Cell
Membrane
Glucose
molecules
Protein
channel

5. Osmosis
• Diffusion of water through a selectively permeable
membrane (Osmosis is a form of facilitated diffusion.)
• PASSIVE TRANSPORT

6. The Effects of Osmosis on Cells
As a result of osmosis, cells respond differently when the
water concentration inside the cell is equal to, greater than,
or less than the water concentration in the environment.
Isotonic Hypertonic Hypotonic

7. Molecule to
be carried
Molecule
being carried
Energy
Active Transport
• Requires energy
• Moves materials from
area of LOW to HIGH
concentration (against a
concentration difference)

8. Active Transport

9. Diffusion, Osmosis, Facilitated Diffusion,
or Active Transport?
9

10. Diffusion, Osmosis, Facilitated Diffusion,
or Active Transport?
10