Lab Manual - eScience Labs

Lab 6: Diffusion
cule to cross the membrane is also determined by its
size. Small, uncharged molecules pass easily, while
most large or charged molecules are prevented from
passing. Molecules which cannot diffuse across the
membrane may be able to cross through other regulated
gateways.
Hemodialysis is a method of removing toxic
substances from the blood when the kidneys
are unable to do so. It is frequently used for
paents with kidney failure, but may also be
used to quickly remove drugs or poisons in
dangerous situaons.
Dialysis is the separaon of molecules through diffusion.
A differenally permeable membrane is used to
separate the components of a mixed soluon containing
more than one type of molecule. This membrane allows the free passage of water, but limits
the movement of molecules by their size. In one of the following exercises you will dialyze a soluon of
glucose and starch to observe membrane permeability.
We will test the rate of diffusion for two dyes. Their different molecular weights (M.W.) will enable you
to observe the effect of molecular size on the rate of diffusion.
Experiment 1: Diffusion Through a liquid
Materials
Blue dye (M.W.= 793 g/mol)
Red dye (M.W.= 496 g/mol)
4 Micropipees
Corn syrup
1 Petri dish (top and boom)
Ruler
Concepts to explore:
Watch*
Viscous liquid from your cupboard*
* You must provide
IMPORTANT NOTES:
Perform the experiment on only one side of the plate at a me.
Once the syrup has reached the edges of the dish, place the drop of dye on the plate right away.
If you wait too long, the syrup will develop a “skin” and the dyes will not be able to penetrate into
the soluon.
Procedure
1. Separate the boom and top of the petri dish. Place the two dishes over a ruler, making sure
that you can read the markings on the ruler through the dish.
2. Fill the two halves of the petri dish with corn syrup unl the enre boom surface is covered.
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