AP Biology
Cerveny
2015-2016
Diffusion Lab: Part 2
Osmosis Through a Membrane
Learning Objectives:
1. To design an experiment to measure water potential in plant cells.
2. To analyze the data collected in experiments and make predictions about
molecular movement through cellular membranes.
3. To work collaboratively to design experiments and analyze results.
4. To connect the concepts of diffusion and osmosis to the cell structure and
function.
All life takes place in water – either external water or internal water – so we need to
address the special case of water movement across cell membranes while
studying diffusion. The diffusion of water across a semi-permeable membrane is
called osmosis. As with the diffusion of solutes, water moves from a region of
higher concentration to a region of lower water concentration. The free energy
associated with the water in this system is called water potential (). Distilled
water has the highest concentration of water and at standard conditions has  = 0
bars (a bar is a measurement of pressure). The concentration of water decreases
as solutes – like sugars and salts – dissolve in the water. Accordingly, the water
potential of the solution decreases.
Using the principles you learned in the first part of this lab, we will investigate the
movement of water in and out using a model cell, followed by testing in real plant
cells. From the data you acquire you will generate a curve related to concentration
of a sucrose solution then use that curve to determine the water potential in a
sample of plant tissue (aka baby carrots).
Key Vocabulary:
Define these words in your lab notebook:
Water Potential
Solute Potential
Turgor Pressure
Key Equations:
w =  solute +  pressure
 solute = -iCRT
i = ionization constant
C = molar concentration of solute
R = pressure constant, 0.0831
(L*bars)/(moles*K)
T = temperature (273 + oC)
Percent Change
= ((original – final)/original) x 100
For this lab your numbers will be the appropriate
masses.
Pre-lab questions:
1. What is kinetic energy, and how does it differ from potential energy?
2. What environmental factors affect kinetic energy and diffusion?
3. Why are gradients important in diffusion and osmosis?
4. What would happen if you applied saltwater to a freshwater plant?
5. How does a plant cell control its internal (turgor) pressure?
AP Biology
Cerveny
2015-2016
Materials:
Graduated cylinder
Ruler
Carrot samples
Plastic wrap
plastic cups
dialysis tubing
scissors
Mystery solutions
balances
Distilled water
Safety:
The materials in this laboratory are safe. Don’t run with the scissors or poke your
eye out with the carrot. When finished the solutions can be poured down the drain
and the carrots discarded in the collection bucket (I’ll put them in my yard waste)
Determining sucrose concentrations in mystery solutions:
1. Day 1
a. Cut a 6 pieces of dialysis tubing from the roll. Each piece should be
~15 cm long.
b. Soak the tubing pieces in water until they are soft enough to work
with. Tie one end of each bag off like you are tying a balloon.
c. To each piece of tubing add enough of a single mystery solution to fill
it a little more than halfway. Repeat with the other 5 solutions.
d. Tie off the end of the tubing (leave some space for expansion) and
blot it dry with paper towels.
e. Mass the samples. Record in a table in your notebook.
f. Place the samples into different plastic cups that are arranged on the
white paper towels. Write each sample’s weight on the appropriate
cup.
g. Add 0.5 M sucrose until the samples are clearly floating in the cup.
h. Cover the cups with plastic wrap and let sit overnight.
2. Day 2
a. Carefully remove samples from the beakers and blot dry. Mass each
sample and record it. This is your final mass.
b. Determine the percent change in mass from your original mass. Plot
this data using the sucrose concentrations of:
0, 0.2, 0.4, 0.6, 0.8, and 1.0 M.
You will need to think about which percent change would go with
which concentration. Show ALL of your calculations
AP Biology
Cerveny
2015-2016
Determining the sucrose concentration of a carrot sample:
1. Day 1
a. Pour ~50 mL of each mystery solution into 6 separate plastic cups.
Make them deep enough that the carrot is fully covered.
b. Obtain 6 baby carrots, blot them dry and determine their mass.
c. Place one carrot into each of the mystery solution containers. Keep
track of which carrot is in which solution so you can measure the
changes in mass.
d. Cover the cups with plastic wrap and let sit overnight.
2. Day 2
a. Make any observations you would like to of the carrots in the cups.
b. Carefully remove the carrots, blot them dry and determine their mass.
c. Determine the % change in mass of the carrots from their original.
Plot this data against the sucrose concentrations from above and
determine the concentration of sucrose in the carrot. Show ALL your
calculations.
Analysis: answer these questions in your lab notebook
1. What is the solute potential of the 0.5 M sucrose solution.
2. If you looked at the carrot cells under the microscope, describe what you
think you would see.