Diffusion and
Osmosis
The Cell Membrane

The cell membrane is:
 Selectively
permeable
Permeable = Pass through (Latin)
 Cell membrane = Gate-Keeper that
determines what can and can’t enter the
cell.

The Gate Keeper: The Cell
Membrane

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Who comes in for the
party?
Small molecules like
water, carbon dioxide,
or oxygen can easily
pass through the
pores of the
membrane.

Who gets bounced?

Large molecules like
glucose
Charged molecules
like salts.

Molecule Movement

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Recall cells are small because they need to
obtain sufficient nutrients and dispose of their
wastes.
This is done by the cell membrane by three
means:
1. Diffusion
2. Osmosis
3. Active Transport (Senior Science)
Definitions.
Solution = A homogenous, liquid mixture
of two or more substances.
 Solvent = The dissolving agent of a
solution. Water is the most versatile
solvent known.
 Solute = Substance that is dissolved in a
solution.

Diffusion

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The process by which molecules tend to move
from an area of higher concentration to an area
of lower concentration.
Concentration= amount of substance in a given
volume.
Think of the perfume example (high low)
Think of the food-dye example (high low)
Why Diffusion?
Kinetic molecular theory.
 Molecules are always moving
 Collisions occur between molecules, which
causes the molecules to move away from
one another.
 If there is space these molecules want to
spread out evenly.

What direction will the molecules move?
Diffusion and the Cell

Movement of molecules by diffusion is how the
cell fulfills its needs.

Nutrients: Low concentration in the cells, high
concentration outside of the cells.

What direction will the nutrients diffuse?
From outside the cell to inside the cell.
High to Low Concentration!!


Diffusion and the Cell

Wastes: High concentration in the cell, low
outside of the cell.
What direction will the wastes move?
 From inside the cell to outside the cell.
 High to Low concentration

A special case of diffusion:
Osmosis



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Water moves through the pores of the cell
membrane like other molecules.
The movement of water from a high
concentration to a low concentration (thru a
semi-permeable membrane) is called: Osmosis
Osmosis = movement of water (the solvent)
Diffusion = movement of dissolved materials
(the solutes)
Osmosis is the movement of water (red dots) through a
semipermeable membrane to a higher concentration of
solutes (blue dots).
Activity — Predicting
Movement of Water

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Selectively Permeable Membrane
This “U” tube has a
selectively permeable
membrane separating
side A and B.
Side A has a 40%
solution of sugar and
side B has a 60%
solution of sugar.
Which side has more
water?
Which way will the
water move?
Osmosis – a cure for the flu?
Who here has taken penicillin to combat
sickness?
 Penicillin weakens bacterial cell walls
 Water is at higher conc. outside the
cell…which way will water move?
 With a weak cell wall, the bacterial cell
swells and…POP!

**Summary**
Both osmosis and diffusion involve the
movement of molecules from an area of
high concentrations to a area of low
concentrations.
 Osmosis = movement of water (the
solvent)
 Diffusion = movement of dissolved
materials (the solutes)

Water Balance

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The cell membrane doesn’t actually control the
movement of water.
The environment around the cell determines the
direction of water movement.
For example, a cell is placed in a salt water tide pool.
The sun evaporates some water.
The concentration of water inside the cell is higher than
outside.
What happens?
Water leaves the cell.
Possible shrinking of the cell.
Water Balance
On the contrary imagine it rains.
 A higher concentration of water outside the
cell is the result.
 What happens?
 Water moves into the cell
 Possible swelling of the cell.

Water Regulation
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Without other mechanisms the cell would burst
or shrivel.
Plant vacuoles store water even when the
environment is dry.
The cell wall is rigid to prevent bursting or
shriveling.
Animals can increase or decrease their amount
of water intake and outtake (Urine
concentration/amount of water that is drank)
Hypertonic Solution – More
water inside cell than outside
Plasmolysis
Plasmolysis
Cells shrink
and die
Iso-osmotic Solution – same
concentration of water inside and outside
cell
Animal cell
Plant cell
No net gain or
loss of water
Hypotonic Solution – more water
outside cell than inside
Cytolysis –
cell swells and
bursts
Builds up turgor
pressure – cell
becomes stiff,
keeping plant
upright
Cell’s are about 90% water.

What would happen
to a cell placed in
pure water?
Human Red Blood Cell Before:
90% water
10% solute
After
100% water
Cell’s are about 90% water.

What would
happen to a cell
placed in a 20%
salt solution?
Human Red Blood Cell Before:
90% water
80% water
After
10% solute