Diffusion and Osmosis lab
APBiology
Dr. Trexler
Cells have kinetic energy. This causes the molecules of the cell to move around and bump
into each other. Diffusion is one result of this molecular movement. Diffusion is the random
movement of molecules from an area of higher concentration to areas of lower
concentration. Osmosis is a special kind of diffusion where water moves through a selectively
permeable membrane (a membrane that only allows certain molecules to diffuse
though). Diffusion or osmosis occurs until dynamic equilibrium has been reached. This is the
point where the concentrations in both areas are equal and no net movement will occur from
one area to another.
There are three classifications of solutions based on their relative solute concentrations. If
two solutions have the same solute concentration, the solutions are said to be isotonic. If the
solutions differ in concentration, the area with the higher solute concentration is hypertonic and
the area with the lower solute concentration is hypotonic. Since a hypertonic solution contains a
higher level of solute, it has a high solute potential and low water potential. This is because
water potential and solute potential are inversely proportional. A hypotonic solution would have
a high water potential and a low solute potential. An isotonic solution would have equal solute
and water potentials. Water potential () is composed of two main things, a physical pressure
component, pressure potential (p), and the effects of solutes, solute potential (s). A formula
to show this relationship is  = p + s. Water will always move from areas of high water
potential to areas of low water potential.
The force of water in a cell against its plasma membrane causes the cell to have turgor
pressure, which helps maintain the shape of the cell. When water moves out of a cell, the cell
will lose turgor pressure along with water potential. Turgor pressure of a plant cell is usually
attained while in a hypotonic solution. The loss of water and turgor pressure while a cell is in a
hypertonic solution is called plasmolysis.
In this lab we will be examining movement of water and solutes into onion cells, potato
pieces, and dialysis tubing.
You will need to be able to:
1)
2)
3)
4)
5)
accurately weigh samples,
construct wet mount slides,
draw samples viewed under a microscope,
assemble dialysis tubing,
test for the presence of starch and glucose using chemical colorimetric tests.
Diffusion and Osmosis lab
APBiology
Dr. Trexler
You will need to answer the following before beginning activities: (for pre-lab quiz)
1)
2)
3)
4)
5)
What is dialysis tubing and how does it work?
What does Lugol’s solution test for?
What does Benedict’s solution test for?
What is the mass of 1 ml of H2O?
If given a set of sucrose solutions ranging from 0 to 1 M in concentration and a supply of
membrane that was only permeable to water, how would you find the concentration of
a test solution?
6) The water potential equation is  = p + s, where the solute potential s = i MRT.
The variables and constants in the equation are
, M = molarity and T
= absolute temperature in Kelvin. i is the van 't Hoff factor, which depends on the manner
in which a solute dissolves. i = 1 for nonelectrolytes, but it is approximately equal to the
number of ions generated during dissolution for electrolytes. For example i for NaCl is
2, but for sucrose, i = 1. Calculate the water potential for the following solutions
a. 1 M sucrose
b. 0.1 M MgCl2
c. 0.3 M ethanol
d. 5 M NaCl
7) Based on the water potential calculations above, determine the direction of water
movement between b and c & a and d.
8) Using solution a as a reference, classify b, c and d as hypertonic or hypotonic relative to
a.
Diffusion and Osmosis lab
APBiology
Dr. Trexler
Materials:
Lab 1A - 1 segment presoaked dialysis tubing, 2 binder clips, Iodine Potassium Iodide (IKI or
Lugol’s) solution, 15% glucose/ 1% starch solution, Benedict’s solution, distilled water, and a 250mL beaker, test tube, boiling water bath.
Lab 1B - 6 segments presoaked dialysis tubing, distilled water; 0.2M, 0.4M, 0.6M, 0.8M, and a
1.0M sucrose solution; Benedict’s solution, 1 M HCl, six 250-mL beakers or cups, and a 3 beam
balance.
Lab 1C -a potato, knife, potato core borers, distilled water; 0.2M, 0.4M, 0.6M, 0.8M, and a 1.0M
sucrose solutions, and a 3 beam balance.
Lab 1D - During Lab 1D, only paper, pencil, and a calculator will be needed to make the
calculations.
Lab 1E - n Lab 1E these items are needed: a microscope slide, cover slip, onion epidermis, scissors,
light microscope, and a 15% NaCl solution.
Procedures:
Testing for glucose with Benedict’s solution: Take 2 to 5 ml of solution to be tested in test tube,
add 3 to 8 drops of Benedict’s solution and mix. Place test tube in boiling water bath for 2 to 5
minutes. A color change within 5 minutes indicates glucose was in the solution.
Testing for sucrose with Benedict’s solution: Take 2 to 5 ml of solution to be tested in test tube,
add 2 to 3 drops of 1 M HCl and mix. Place test tube in boiling water bath for 5 minutes. Add 5
drops Benedict’s solution. A color change will occur rapidly if sucrose was in the solution.
Testing for starch with Lugol’s solution: Take 2 to 5 ml of solution to be tested in test tube, add 2
to 3 drops of Lugol’s solution. A color change will occur rapidly if starch was in the solution.
Lab 1A -1) Test the glucose/starch solution for presence of glucose by adding 5 drops to 3 ml
water and following procedure above. 2) Test the glucose/starch solution for presence of starch
by adding 5 drops to 3 ml water and following procedure above. 3) Test distilled water for
presence of glucose and starch. 4) Pour 10 ml glucose/starch solution into dialysis tubing and
close the bag with binder clips. 5) Put the dialysis bag into a beaker filled with distilled water and
let stand for 30 minutes. 6) When time is up test both the bag and the beaker for presence of
glucose. 7) Also test the bag and the beaker for presence of starch. Record all data in a table.
Diffusion and Osmosis lab
APBiology
Dr. Trexler
Lab 1B - Obtain the six strips of dialysis tubing and fill each with 10 ml sucrose solution of a
different molarity. Mass each bag. Put each bag into a beaker of distilled water and let stand for
half an hour. After 30 minutes is up, remove each bag and determine its mass. Record all data in
an appropriate table.
Lab 1C - Using the potato core borer, obtain 24 cylindrical slices of potato, four for each cup.
Determine the mass of the four cylinders. Immerse four cylinders into each of the six beakers or
cups with the assortment of sucrose solutions. Let stand overnight. After time is up, remove the
cores from the sucrose solutions, blot gently and mass them. Record all data in its appropriate
table.
Lab 1D - Using the paper, pencil, and calculator collected, determine solute potentials of the 6
sucrose solutions.
Lab 1E - Prepare a wet mount slide of the epidermis of an onion (See figure below for method to
obtain epidermis). Draw what you see of the onion tissue under the microscope. Add several
drops of the 15% NaCl solution to the slide and wick across to replace the solution surrounding
the mounted onion tissue. Now draw the appearance of the cells. Add several drops of distilled
water and wick across the cover slip as before. Draw the appearance of the cells.