Getting In & Out of a Cell
Osmosis & Diffusion
Cell Membrane
• Remember: The Cell
Membrane is SEMIPERMEABLE
– It doesn’t let
EVERYTHING inside the
cell
• Cells do this to
maintain homeostasis
– Stable internal
environment
PASSIVE TRANSPORT
• Substances that
cross the cell
membrane
without taking
energy from the
cell
Diffusion
Diffusion
DIFFUSION
• The movement of
molecules from an
area of high
concentration to low
concentration
– Concentration
gradient = difference in
concentration!
• Eventually this leads to
equilibrium
– Equal concentration
throughout space
Equilibrium
• When the concentration of the
molecules of a substance is the same
throughout the space.
H
H
H
O
H
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H
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H
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H
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H
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O
Sugar Molecules
H
O
H
H
H
H
O
O
H
H
H
H
O
O
H
O
H
H
H
O
H
H
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O
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O
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O
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Sugar Molecules
H
O
H
O
H
O
H
H
H
H
H
O
H
H
O
H
H
H
H
O
H
H
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O
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Sugar Molecules
O
H
H
H
H
H
O
O
H
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O
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H
Sugar Molecules
H
H
O
H
O
H
H
H
O
H
O
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H
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O
Osmosis
The process by
which water
molecules
diffuse through
a membrane
from an area of
greater
concentration
to an area of
lesser
concentration.
OSMOSIS
Directions of Osmosis
• The net direction of
osmosis depends on
the concentration of
solutes outside &
inside the cell
– Hypotonic
– Hypertonic
– Isotonic
HYPOTONIC
• Concentration of
molecules are
lower on OUTSIDE
of cell
• Water diffuses
INTO the cell
Hypotonic Solutions
• Animal cells
swell & burst if
they take in too
much water
• Plant cells do
best in
hypotonic
solutions
HYPERTONIC
• Concentration of
molecules are
lower on the
INSIDE of the cell
• WATER diffuses
OUT of the cell
Hypertonic Solutions
• Cells will shrink
& die if too
much water is
lost
• Plant cells
become
flaccid (wilt);
called
plasmolysis
ISOTONIC
• Concentration of
molecules is equal
inside & outside
the cell
– At equilibrium
• Water diffuses
equally into and
out of the cell
– No NET movement
of water
Isotonic Solutions
• Animal cells do
best in isotonic
solutions.
• Plant cells
become flaccid.
This is when the
plasma
membrane is not
pressed tightly
against
the cell wall
Examples
Direction of Osmosis
Condition
If solute
concentration in env.
is less than the cell
If solute
concentration in env.
is more than in cell
If solute
concentration in env.
is equal to the cell
Environment
solution is:
Cell Solution
is:
hypotonic hypertonic
Water will
move:
Into the
cell
Out of the
hypertonic hypotonic
cell
isotonic
isotonic
Water will
not move