Movement across the
Cell Membrane
Section 7-3
Cell Membrane
Passive Transport
Movement across the
cell membrane that
takes no energy to be
done.
Diffusion
Movement of
substances from an
area of high
concentration to low
concentration.
Concentration
 Solute vs. Solvent
Water: 93%: Solvent
Sugar: 7%: Solute
We have a 7% sugar solution.
Example of Sugar in Water

Inside the Cell

7% Glucose
 ________________________

Outside the Cell

3% Glucose
Osmosis
Movement of water
across the cell
membrane.
Example with Water
Inside the Cell

98% water
 ______________________

Outside the Cell

96% water

Different Solutions
Bozeman Cell Membrane Video Clip
Hypertonic:
 The solution has a higher concentration of solute
(= lower solvent-liquid) when compared to another solution.
Hypotonic:
 The solution has a lower concentration of solute
(= higher solvent-liquid ) when compared to another
solution.
Isotonic:
 The solutions have the same
concentration of solute.
Isotonic Solution

Inside the Cell

93% Water-7% Salt
 ______________________

Outside the Cell
 93% Water-7% Salt
Hypertonic vs. Hypotonic

Inside the Cell
 92% Water-8% Salt
 ______________________

Outside the Cell
 95% Water-5% Salt
Hypertonic vs. Hypotonic

Inside the Cell
 98% Water-2% Salt
 _____________________

Outside the Cell
 94% Water-6% Salt
Diagram Of Osmosis Through A Cell Membrane
Osmosis: Movement of water molecules (blue circles) through a cell
membrane (red) from a region of high (water) concentration (inside cell) to
a region of lower (water) concentration (outside cell). Inside the cell the
solution is hypotonic with a low solute (salt) concentration. Outside
hypertonic
the membrane the solution is
with a high solute (salt
ion) concentration shown by the orange circles. The membrane is not
permeable to the salt ions. Since the concentration of water molecules
per unit area is higher inside the cell than outside, water moves out
of the cell.
The internal body fluids of a marine fish
are hypotonic compared with the
hypertonic sea water; therefore, water
molecules diffuse out of the fish
through the gill region. To cope with this
steady loss of water, the marine fish
has greatly reduced urine with little or
no water loss, continuously drinks
water, and excretes excess salt through
the gills by active transport.
Conversely, a freshwater fish is
hypertonic compared with the water of
a lake or pond; therefore, it continually
absorbs water through the gill region.
To cope with this steady influx of water
molecules, the freshwater fish has
copious urine, drinks very little water,
and absorbs salts through the gills by
active transport..