 The cell membrane’s purpose is to control what goes in
and what goes out of a cell
 The cell membrane also provides protection and
support
The Lipid Bilayer
 The cell membrane is made up primarily of two sheets
of lipids called the lipid bilayer
 In addition to lipids, some carbohydrates and proteins
are embedded in the bilayer. Because of these
embedded molecules we refer to the lipid bilayer as a
fluid mosaic.
Concentration
 How does a cell membrane control what enters and
exits a cell?
 The concentration of the liquids on either side of the
membrane is the key.
 Concentration is the measure of the amount of “stuff”
(solutes) in any given amount of a solvent.
 Concentration = mass of solute / volume of solvent.
 8 grams of an salt in 2 liter of water gives us a
concentration of 4 g/L.
Which of these is more
concentrated for sugar?
 2 gallon of iced tea with one spoonful of sugar
 3 gallons of iced tea with two spoonfuls of sugar
Which of these is more
concentrated for sugar?
 3 gallons of iced tea with two spoonfuls of sugar
Diffusion
 http://www.uh.edu/phar2362/animations/simple_diff
usion.html
Diffusion
 Diffusion is the random spreading out of molecules
 In diffusion molecules move from areas of high to
areas of low concentration
 The cell membrane is only permeable to some
substances. It is selectively permeable.
 Diffusion can only occur across a cell membrane if that
membrane is permeable to the solute in question.
Osmosis
 Cell membranes usually permeable to water, so a
special kind of diffusion called osmosis can occur.
 Osmosis is diffusion of water.
 Osmosis moves water from high concentration of
water to low concentrations of water.
Concentration of Water
 If a water solution is highly concentrated for a solute,
it has a low concentration of water.
Which of these is more
concentrated for iced tea?
 2 gallon of iced tea with one spoonful of sugar
 3 gallons of iced tea with two spoonfuls of sugar
Which of these is more
concentrated for iced tea?
 2 gallon of iced tea with one spoonful of sugar
Where will salt move?
Where will water move?
Transport
 When substances move across a cell membrane we call
it transport
 There are 2 types of transport
 Passive
 Active
Passive Transport
No energy
Moves from high to low
concentration
Facilitated Diffusion
 Special protein channels allow
substances to pass through a
membrane
 High to low concentration
Active Transport
 Uses energy
 Requires
transport protein
 Low
concentration
to high
concentration
Tonicity
Hypotonic
Solution –
concentration is
lower
Tonicity
Isotonic Solution
– concentration
is equal
Tonicity
Hypertonic
Solution –
concentration is
higher
Why can’t we drink sea
water?
 Salt water is
hypertonic to our
cells
Water intoxication
 Drinking extremely large
amounts of water in a
short time period can lead
to death
 Our cells are hypotonic to
pure water