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Date: ________________________
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Diffusion and Osmosis
Diffusion is the random movement of particles from an area of high concentration to an area of
low concentration until they are evenly spread out. When the molecules are evenly distributed
throughout the space, they have reached “equilibrium”.
1
A concentration gradient is a difference between concentrations in a space. In the boxes
above, the particles in the first box show a concentration gradient, with a high concentration in
the upper left corner and a low concentration in the lower right corner. Molecules will always
move down the concentration gradient from an area of high concentration to an area of low
concentration. Think of food coloring spreading out in water, or perfume moving through the air
in a room.
After a time, the particles will reach equilibrium so that the particles are equally distributed and
moving equally in all directions.
2
Unequal distribution
1
2
Equilibrium
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Osmosis
Osmosis is the diffusion of water across a semi-permeable membrane. A semi-permeable
membrane acts like a filter that allows some substances to pass through but not others. Water
can pass through the membrance, and always moves from an area of high water concentration
(low solute concentration) to an area of low water concentration (high solute concentration).
Types of Solutions
Solutions separated from one another by a semi-permeable membrane are named based on the
relative amounts of solute in each solution. We label solutions as either isotonic, hypertonic or
hypotonic. These names help us understand the direction that water will move, which is
especially helpful when discussing cells and their surrounding environments.
1.
Isotonic Solution
The prefix “iso-” means “the same”. In an isotonic
solution, the concentration of solute is equal on both
sides of the semi-permeable membrane. Water
molecules will move back and forth across the
membrane but the overall amount of water on both
sides of the membrane remains the same. For every
molecule that moves into the cell, another water
molecule moves out.
3
2. Hypotonic Solutions
The prefix “hypo-” means “less”. In this case there are
fewer solute molecules outside the cell, and more solute
inside the cell. Since water moves toward an area of
high solute concentration, the water moves into the cell.
The cell will swell.
In plants, the cell wall prevents the cell from bursting
when water enters. In animal cells, the cell may be in
danger of bursting because it does not have a cell wall to
limit its expansion.
3
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3. Hypertonic Solutions
The prefix “hyper” means “more”. In hypertonic
solutions, there are more solute molecules outside the
cell, which causes the water to move out of the cell. In
plants, this loss of water causes wilting. In animals, the
cells also shrink. In both plants and animals, the cells
may die.4
It is because of this movement of water out of the cells
that drinking sea water increases the rate of dehydration,
because adding the salt to your body causes water to move out of the cells instead of
into them.
4
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