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Diffusion/Osmosis Notes
In Preparation for the Plankton Lab
I. Definitions:
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solute: a substance that gets dissolved (ex: salt)
solvent: a substance in which a solute is dissolved
(ex: water)
solution: a mixture in which a solute is dissolved evenly
throughout a solvent (ex: ocean water)
concentration: the percent represented by either the solute
or solvent
In ocean water, salt is the solute, water is the solvent, and the
result is a solution. Salt and water can be in a variety of
concentrations which make the water more or less salty.
diffusion: the movement of molecules in a solution from an area of
higher concentration to an area of lower concentration
osmosis: the diffusion of water molecules from an area of high to low
concentration
semi-permeable membrane: a membrane which allows the passage of some
molecules (ex. water), while preventing the passage of other substances (some
molecules are prevented from passing through due to their size and/or
charge)
Single-celled water organisms have a semi-permeable membrane that lets water
through, but salt is too large to pass through.
water
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II. Osmosis and Cells - 3 Examples
A. Hypotonic - a situation where there is a solution with a lesser
concentration of solute outside the cell compared to the
concentration of solute inside the cell
Ex: a single-cell salt water
organism is placed in a
fresh or brackish water
solution
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brackish water
Since salt is too big to
move through the
membrane, water moves
from an area of high
concentration in the
brackish water through the
cell’s membrane, causing
the cell to swell. This will
continue to happen until
the concentration of water
inside the cell equals the
concentration outside the
cell - or - until the cell
explodes
B. Hypertonic – A situation where there is a solution with
a greater concentration of solute outside the cell
compared to the concentration of solute inside the cell
Ex: a single-cell fresh water
organism is placed in a sea water
solution
sea water
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Since salt is too big to
move, water moves from
an area of high
concentration in the cell
through the cell’s
membrane causing the cell
to shrink. This will
continue to happen until
the concentration of water
inside the cell equals the
concentration outside the
cell - or - until the cell
collapses
C. Isotonic – A situation where there is a solution with an
equal concentration of solute outside the cell compared
to the concentration of solute inside the cell
Ex: a single human red blood cell is
placed in a 0.9% saline (salt)
solution.
0.9% saline
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Since the concentration of
salt and water is the same
inside and outside the cell,
neither the salt nor the
water has to move to
balance. Water can still
pass through the cell’s
membrane but there is no
net gain or loss of water
inside the cell
Osmosis Overview
Osmosis
Animation
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Some organisms have developed adaptations to prevent themselves
from swelling or shrinking:
III. Facilitated Diffusion
A. Moves molecules from a high to low concentration
B. NO energy required (In the direction nature intends)
C. molecules called carrier proteins imbedded in the cell
membrane facilitate the movement
D. Organisms now can move salt in or out of their cells to
balance themselves instead of being flooded/emptied of water
E. Ex: Red blood cells carry and deliver glucose into the
bloodstream
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IV. Active Transport
A. Moves molecules from an area of lower concentration to an
area of higher concentration (the opposite direction than
nature intends)
B. an adaptation requiring both carrier proteins and energy
C. carrier proteins and high energy molecules pump molecules
from low to high concentration
D. Ex: the gills of marine fish have cells that remove salt
from the body and pump it back into the water
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