Diffusion and Osmosis
Lab 1: Diffusion & Osmosis
Lab 1: Diffusion & Osmosis
Description
dialysis tubing filled with starch-glucose solution in
beaker filled with
KI solution
potato cores in
sucrose solutions
Lab 1: Diffusion & Osmosis
Concepts
Selectively permeable membrane
diffusion
osmosis
solutions
hypotonic
hypertonic
isotonic
water potential
Procedure A: Diffusion
Glucose/starch solution in bag made of dialysis tubing
Place bag in a beaker with
dH2O and KI solution
Use glucose test strips to
test both bag and beaker
for the presence of glucose
Glucose test strip turns green/brown in the presence of
glucose
KI turns dark blue/purple in the
presence of starch
Analysis of Results
1.
Which substance(s) are entering the bag and which are leaving
the bag? Which experimental evidence supports your answer?
2.
Explain the results you obtained. Include the concentration
differences and membrane pore size in your discussion.
3.
Quantitative data uses numbers to measure observed changes.
How could this experiment be modified so that quantitative
data could be collected to show water diffused into the dialysis
bag?
Analysis of Results
4. Based on your observations, rank the following by relative size,
beginning with the smallest: glucose molecules, water molecules, IKI
molecules, membrane pores, starch molecules.
5. What results would you expect if the experiment started with a glucose
and IKI solution inside the bag and only starch and water outside?
Why?
Procedure B: Osmosis
6 dialysis tubing bags with the following:
Distilled water
Purple – o.2M sucrose
Green – 0.4M sucrose
Blue – 0.6M sucrose
Yellow – 0.8M sucrose
Red – 1.0M sucrose
Find the mass of each bag and record
Place each bag in a beaker of dH2O and let sit for 30 minutes
At the end find and record mass
Graph the results for your data and class average
Analysis of Results
1.
Explain the relationship between the change in mass and the
molarity of sucrose within the dialysis bags.
2.
Predict what would happen to the mass of each bag in this
experiment if all the bags were placed in a 0.4M sucrose solution
instead of distilled water. Explain your response.
3.
Why did you calculate the percent change in mass rather than simply
using the change in mass?
Analysis of Results
4. A dialysis bag is filled with dH2O and the placed in a sucrose solution.
The bag’s initial mass is 20 g and its final mass is 18 g. Calculate the
percent change of mass, showing your calculations here.
5. The sucrose solution in the beaker would have been ___________ to the
dH2O in the bag.
Isotonic
Hypertonic Hypotonic
Procedure C: Water Potential
Potato cores are massed and then put in an assigned solution
(dH2O, 0.2M, 0.4M, 0.6M, 0.8M, 1.0M sucrose solution)
overnight
The next day, potatoes are removed and massed
Calculate percent change in mass (individual and class average)
Graph individual and class average for percent change in mass
Determine the molar concentration of the potato core.
Molar Concentration of the Potato core
Find sucrose molarity in which the mass of potato core
does not change
The point at which this line crosses the x-axis represents
the molar concentration of sucrose with a water potential
that is equal to the potato tissue water potential.
At this concentration there is no net gain or loss of water
Molar concentration of sucrose = ________M
Procedure D: Calculation of Water Potential from
Experimental Data
Calculating Solute Potential
Ψs = -iCRT
i = Ionization constant ( sucrose = 1.0)
C = Molar concentration (Procedure C)
R = Pressure constant (R = 0.0831 liter bars/mole oK)
T = Temperature oK (273 + oC of solution)
Procedure D: Calculation of Water Potential from
Experimental Data
Ψp = 0
Knowing solute potential and pressure potential allows
you to calculate water potential (Ψ)
Ψ = Ψp + Ψs
Procedure D: Calculation of Water Potential from
Experimental Data
Water potential at equilibrium will be equal to the
water potential of potato cells
What is the water potential of the potato cells?
Analysis of Results
1. If a potato core is allowed to dehydrate by sitting in
open air, would the water potential of the potato cells
decrease or increase? Why?
2. If a plant cell has a lower water potential than its
surrounding environment and if pressure is equal to
zero, is the cell hypertonic or hypotonic? Will the cell
gain water or lose water? Explain.
Procedure E: Onion Cell Plasmolysis
Wet mount slide of an onion
Observe under 100x magnification, sketch and describe
Add 15% NaCl solution, sketch and describe
Flood with fresh H2O, sketch and describe
Analysis of Results
1.
What is Plasmolysis?
2.
Why did the onion cells plasmolyze?
3.
In the water, grass often dies near roads that have been
salted to remove ice. What causes this to happen?
Lab 1: Diffusion & Osmosis
Conclusions
water moves from high concentration of water to low
concentration of water
hypotonic=low solute
hypertonic=high solute
solute concentration &
size of molecule
affect movement
through
semi-permeable
membrane
Lab 1: Diffusion & Osmosis
ESSAY 1992
A laboratory assistant prepared solutions of 0.8 M, 0.6 M, 0.4 M, and
0.2 M sucrose, but forgot to label them. After realizing the error, the assistant
randomly labeled the flasks containing these four unknown solutions as flask A,
flask B, flask C, and flask D.
Design an experiment, based on the principles of diffusion and osmosis, that the
assistant could use to determine which of the flasks contains each of the four
unknown solutions.
Include in your answer:
a. a description of how you would set up and perform the experiment;
b. the results you would expect from your experiment; and
c. an explanation of those results based on the principles involved.
Be sure to clearly state the principles addressed in your discussion.