Diffusion and Osmosis
Lab 4: Diffusion & Osmosis
Lab 4: Diffusion & Osmosis
Description
Surface area and cell size (celery stick and agar cubes)
Modeling osmosis
(simulate cell with dialysis
tubing)
Osmosis in living plant
cells ( Elodea and potato
cores)
Lab 1: Diffusion & Osmosis
Concepts
Selectively permeable membrane
diffusion
osmosis
solutions
hypotonic
hypertonic
isotonic
water potential
Part 1A – Osmosis and Diffusion
Observe celery stick soaked in water.
Record your observations.
Break the celery stick. Record your
observations.
Observe a celery stick soaked in salt
water. Record your observations.
Break the celery stick. Record your
observations.
Part 1B – Osmosis and Diffusion
Measure the dimension of a set of agar
cubes
Place the three cubes in a cup and cover
with vinegar. Keep cubes submerged for 10
minutes.
Calculate the Surface area, volume and
Surface Area to Volume ratio.
After 10 minutes, remove agar cubes and
cut
open.
Measure distance vinegar diffused into
each
cube.
Calculate the rate of diffusion for each cube
(cm/min).
Calculate the volume of the portion of each
cube which has not changed color
Calculate the extent of diffusion into each
cube as a percent of the total volume.
Graph the rate of diffusion (cm/min, x-axis)
relative to surface area to cell volume ratio
(1/cm, y-axis)
Graph the extent of diffusion (x-axis) relative
to cell volume and surface area (y-axis).
Analysis of Results
1.
Why are celery sticks soaked in salt water more flexible than those soaked in
plain water?
2.
Examine your data from Part A. What dimension supported the fastest diffusion
rate? Why?
3.
What dimensions supported the greatest diffusion percent total volume? Why?
4.
Construct a graph of the relationship between cell dimension to the extent of
diffusion.
5.
Why can’t humans drink seawater for hydration?
6.
The size of some human cells is 0.01 mm. Using the formulas in this activity,
calculate the surface area to volume ratio of such a cell (assume 0.01 mm cube).
Describe the extent of diffusion into this living cell as compared to the smallest
agar cube. Explain.
Part 2A – Procedure: Modeling Osmosis
5 dialysis tubing bags with the following:
Salt
Sucrose
Glucose
Sucrose
Protein
Find the mass of each bag and record
Place each bag in a beaker of dH2O or Salt and let sit for 30 minutes
At the end find and record mass
Calculate the percent change in weight
Analysis of Results
1.
Examine the initial and final weights of the model cells. What causes
the mass of the dialysis bags to change? Was there more or less water
in the dialysis bags at the conclusion of the experiment? Explain.
2.
From your results, which solutes, if any, diffused across the
membrane, and which, if any, were restricted? Why do you think this
occurred?
Part 3A – Procedure: Osmosis in Living Plant Cells
Wet mount slide of an Elodea
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 Elodea cells plasmolyze?
3.
In the water, grass often dies near roads that have been
salted to remove ice. What causes this to happen?
Part 3B – Procedure: Osmosis in Living Plant Cells
Potato cores are massed and then put in an mystery solutions
(dH2O, 0.2M, 0.4M, 0.6M, 0.8M, 1.0M sucrose solution) for 30
minutes.
After 30 minutes, 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 3B: 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 3B: 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 3B: Calculation of Water Potential
from Experimental Data
Water potential at equilibrium will be equal to the
water potential of potato cells
Analysis of Results
1. Which color mystery solution had the highest
concentration of sucrose? How do you know this?
2. Knowing that the mystery solutions were composed of
sucrose at various concentrations (0.2 M, 0.4 M, 0.6M,
0.8 M, and 1.0M), calculate the water potential of the
potato core. Show your calculations and explain your
reasoning.
3. What is the concentration o each of the solutions?
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.