Prac : Diffusion across a Selectively Permeable Membrane
Name:
Introduction
Atoms & molecules are in a state of rapid random motion. If a substance is heated up the particles
gain kinetic energy and move at a faster rate. Conversely, as substances are cooled down the
particle movement slows down.
In solids the particles do not have sufficient energy to move freely, in liquids the particles can move
to a much greater extent and in gases the particles are free to move in space.
Moving particles bump into one another, objects in their path and the walls of containers.
Whenever a collision occurs the direction of the particles movement is modified.
A solvent is a substance which can dissolve another substance. A solute is a substance that can
dissolve in a particular solvent. Water is a solvent. Salt (eg sodium chloride) is a solute and salt
water is a solution. A solute (or many solutes ) are able to dissolve in a solvent. The resulting
mixture is a solution.
In gas mixtures and liquid solutions diffusion is a dynamic and important process.
Diffusion is:
In living organisms, cells separate the internal environment from their external environment by a
plasma membrane. These membranes are selectively permeable which means they are able to
regulate what gets into and out of the cell. A selectively permeable membrane allows some
types of molecules to diffuse across the membrane and prevents other types of molecules from
crossing the membrane.
What problems do you think a cell would have if it could not control what got into or out of it?
In this practical we investigated the diffusion of molecules across a synthetic selectively
permeable membrane that models the cell membrane. Smaller molecules can cross this
synthetic selectively permeable membrane, but larger molecules cannot. The substances utilized
were water, glucose, starch and iodine.
Which of the following four particles do you think will be able to cross the membrane?
Molecule
Molecular Formula
Glucose
Starch
Iodine
Water
Hypotheses/ Predictions
Solute or
Solvent?
Will it cross
the
selectively permeable
membrane?
Materials/ Reagents
 Glucose solution
 Starch solution
 Iodine solution
 Dialysis tubing
 Thistle funnels
 Gas jars or tall beakers
 Retort stands
 Glucose test strips
Method/Procedure
To test our predictions, we attached synthetic
membrane bags to thistle funnels and suspended
these in gas jars or tall beakers in a similar manner to
the visking osmometer illustrated on the right .
Solutions of starch and glucose were poured carefully
via the thistle funnel into the bags until full and the
liquid extending up the funnel to within 2-3 cm from
the top. The bag was gently squeezed to remove any
air bubbles
The thistle funnel and attached bag was rinsed, then
suspended in a gas jar or beaker.
The beaker in.which the starch bag was suspended
had distilled water added to within 1 cm of the top of
the bag and iodine was added to the water until there
was a definite yellow tinge
The beaker into which the glucose bag was
suspendedhad distilled water added to within 1 cm of the top of the bag
The immediate level of the liquid in the funnel of both conditions was marked on the funnel.
The water in the beaker containing the glucose bag was tested for the presence of glucose using
test strips
The apparatus was then observed over a period of 30 -60 minutes and further results recorded.
Observations & Results
Initial Condition Starch model
Final Condition Starch model
Initial Condition Glucose model
Final Condition Glucose model
We tested for the movement of starch and iodine using the starch Iodine reaction which results in
a starch iodine complex appearing as a blue-black colour.
We tested for the movement of glucose using glucose test strips which change colour in the
presence of glucose.
Water is the main component of both the solution inside the tube and the solution in the beaker
outside the tube. If water can cross the selectively permeable membrane, water could diffuse into
the tube and out of the tube.
To test for net movement of water into out of the tube we..
Did starch move out of the tube? How do you know?
Did iodine move into the tube? How do you know?
Did glucose move out of the tube? How do you know?
Did water move into or out of the tube? How do you know?
Conclusions:
.
Molecule
Molecular Formula
Did it cross the
selectively permeable membrane?
Glucose
Starch
Iodine
Water
The characteristic of the molecules in this experiment that appears to determine the ability to
cross the selectively permeable membrane is most likely….
These observations support the conclusion that the dialysis tubing is a selectively permeable
membrane because.
There was water on both sides of the membrane, in the tube and in the beaker. Water moved in
the direction that it did because..
Each living cell is surrounded by a selectively permeable cell membrane which allows water to
move into or out of the cell by diffusion. The diffusion of water across a selectively permeable
membrane plays such an important role in biology that this process has been given a special
name, osmosis.
What will determine whether osmosis moves water into a cell or out of a cell?
Living cells can use energy provided by ATP to pump an ion or molecule across the cell
membrane in a process called active transport. Active transport can move a substance from a
region of lower concentration to a region of higher concentration; this is useful for a cell to maintain
the correct concentration of ions and substances like glucose. What type of molecular transport
was demonstrated in your investigation—diffusion or active transport? How do you know?