Lab 5a
Diffusion and Osmosis
Remember the Membrane?
Terms
• Solutions: molecules (solutes) dissolved in a
liquid (solvent)
– All molecules are constantly in motion
– Random motion causes mixing
• Brownian Motion: random tendency of ALL
molecules to move due to their inherent kinetic
energy.
• Concentration is the amount of solute in a
solvent
– Concentration gradient: more solute in one region of a
solvent than in another
Passive transport: Diffusion
• Due to: random motion and collision of
molecules
• Movement “down” a concentration
gradient (from area of high concentration
to area of low concentration)
• Occurs until a dynamic equilibrium is
reached
Diffusion – solid in water
What factors Affect Diffusion
Rates?
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Distance
Molecule size
Temperature
Gradient
Electrical force
What can/can’t diffuse through
the cell membrane?
Osmosis = Water Movement
• Water molecules diffuse across membrane
toward solution with more solutes
• Volume increases on the side with more
solutes
• Can think of it like diffusion of water: water
moves from an area in which it is more
concentrated (less solute) to area where it
is less concentrated (more solute)
Osmosis
How Osmosis Works
• More solute molecules = lower
concentration of water molecules
• Key to osmosis: membrane must be freely
permeable to water, selectively
permeable to solutes.
(i.e. some solutes must be impermeable.
Otherwise, diffusion would occur)
Osmosis
• Osmosis is the net “diffusion” of water
across a membrane.
Osmosis only occurs when solutes cannot
cross a selectively permeable membrane
(no diffusion) so the solvent, water,
crosses instead
Lab Exercise 5a - Demonstrations
• Activity 0: Brownian motion demo – view in
scope up front
• Activity 1: Diffusion in a solid - you measure
• Activity 3: Diffusion through non-living
membranes - I’ll do tests, you observe
• Activity 5: Diffusion through living
membranes using eggs and in blood cells
• Activity 6: Filtration
Activity 0 – Brownian Movement
• Observe the demonstration setup of india
ink molecules under the microscope
• Note how the “very large” ink molecules
appear to vibrate quickly
– The fluid may also appear to be flowing in one
direction or another as well. This is NOT
Brownian movement, ONLY the vibration is
Brownian motion
Activity 1 – Diffusion in a solid
• Each group will get:
– Agar (a water based solid) in a Petri dish
– Two bottles
• one of methylene blue (same dye we used for the cheek
cells) MW = 320
• one of potassium permanganate, MW = 158
• Place a drop of each off-center in the dish
• After approximately one hour, measure the size
of each dot to determine the diffusion rate of
each in mm/min
• Which will have a faster rate?
Activity 2 – Diffusion in a liquid
• Temperature effects demo
• How fast will dye diffuse in hot vs cold water?
Activity 3 – Diffusion through
membranes
• Demonstration
• I will set up a beaker full of iodine
• I will place a “cassette” of starch solution in
the iodine
• One of two things will happen: iodine will
enter the sac, or starch will leave
Activity 3.
Starch +
iodine =
purple
Purple will
be inside or
outside bag
Activity 5: Osmosis in eggs
• Eggs are very large single cells, allow us to
measure osmosis easily
• When shell is decalcified, water easily passes
through
• Eggs have a relative concentration of 14%
• We will place eggs into solutions of different
solute concentrations (hypertonic, isotonic,
hypotonic)
• What will happen to the egg if the solution is
hypertonic and why? What about hypotonic?
Activity 5: Osmosis in eggs
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Get 2 eggs and weigh them in the weigh boats
Get 2 plastic trays and label them 1 and 2
Fill 1 halfway with dH2O
Fill 2 halfway with 30% sucrose
Put one egg in each container (noting which one you put
in which)
After 20 minutes, gently remove each egg and weigh
them
Replace and repeat at 40 min and 60 minutes
Fill in the table “Data from Expt 1” on page 58
Graph each
Activity 5 Osmosis in blood
• Take a slide :
– put one drop of physiological saline (0.9%) on it
– Add one drop of blood to it
– Coverslip and look at under high power
• On a second slide:
– place a drop of 5% saline
– add one drop of blood,
– coverslip and observe under high power
• While slide is still on the scope
– Add a drop of dH2O to this slide right next to the
coverslip.
– Fold up a Kimwipe and blot near the liquid on the
other side of the coverslip.
– Watch what happens to the cells
Osmosis in cells
Osmosis
• Isotonic  cell ok
• Hypotonic  Swelling or hemolysis
(burst). Like in the bathtub
• Hypertonic  crenation (shrinkage)
Activity 6 – Filtration
• Follow instructions in Lab book page 60
• You will pour a solution containing starch,
charcoal and copper sulfate into a funnel lined
with filter paper (held over a beaker)
• Count the number of drops that fall through in a
10 sec period (after the steady stream stops)
• When funnel is half empty, again count the
number of drops in a 10 second period
• At the end, note what passed and what was
retained
– If filtrate is blue, copper sulfate passed through
– Sample 2 ml and add Lugol’s solution (purple = starch)
– Look for charcoal
Clean Up
• Clean off balance, wipe off table (egg
mess)
• Put everything back where you got it from
• Return everything up to front including
putting eggs back in the vinegar
Now What?
• To do now:
– Start egg experiment (5)
– Stars solid diffusion demo(1)
• Then:
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Make and observe blood slides (5)
Observe Brownian motion demo (0)
Observe liquid diffusion demo (2)
Don’t forget about the egg!
• After about an hour:
– Measure solid dots (1)
– Observe and record dialysis; (3)
Due Next Thursday
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Rates of diffusion for two dyes in a solid
Table on Page 58 (egg weights)
Review Sheet pages 63 – 66
Cut some questions (3, 5, 8, 11)
Add:
– for the starch iodine: which one moved and
how do you know?
– For the dyes in water: which diffused faster
hot or cold. Why?