Lab 4
Diffusion and Osmosis
Lab Objectives
Upon completion of this lab you should be able to do the following.

Define and/or use properly the following terms:
solute, solvent, concentration gradient, diffusion, differentially permeable membrane, equilibrium,
osmosis, osmotic pressure, hypotonic, hypertonic, isotonic, dialysis

Describe the processes of diffusion and osmosis

State what factors affect the rate of diffusion and explain how the rate is affected.

Predict the direction of net flow of water and/or solutes for two different solutions which are
separated by a differentially permeable membrane.

Propose and test a hypothesis for an osmosis experiment, and predict the outcome.

Interpret the results of an osmosis experiment.

Identify controlled and uncontrolled variables.

Write a scientific lab report.
Introduction
All living cells must maintain an internal constancy in their ever-changing environment. They do this by
regulating the flow of materials into and out of the cell through the plasma (cell) membrane.
This
complex membrane differentiates between different substances, keeping some outside the cell while
allowing some to pass through, and is therefore called a semi-permeable or differentially permeable
membrane. Although most dissolved solutes are too large to pass through the plasma membrane, water
is not, and easily travels back and forth across it.
All substances that do pass through the plasma membrane must be dissolved in water. Water in this case
is called a solvent. The substances that dissolve in the water are called solutes. Solutes include salts,
proteins, sugars and other biological molecules.
In this lab, you will study the processes governing
movement of materials across membranes by using either living cells or artificial membranes that model
the behavior of a cell’s plasma membrane.
DIFFUSION and RELATED PROCESSES
The process of diffusion is evidence that molecules, atoms and ions of any substance are constantly in
motion. Diffusion refers to the natural movement of molecules down a concentration gradient, that is,
from an area of higher concentration to an area of lower concentration. Eventually the molecules will
become evenly distributed within a given space, so a concentration gradient will cease to exist.
Equilibrium is the term used to describe this final condition. Even when equilibrium is reached, the
molecules will continue to move randomly and retain an even distribution. In addition, currents in a
solution will increase the rate of mixing, bringing the solution to equilibrium much quicker than diffusion
alone. Most liquids and gases experience some type of random currents due to uneven heating of the
solution.
Temperature is determined by the average amount of motion of individual molecules or atoms, either by
their vibration, rotation and/or translation (movement through space). For gases and liquids, all three of
these motions increase as temperature goes up. Solids are limited to only changes in the rate of their
vibration: their atoms or molecules are fixed in place and therefore unable to rotate, or move to a new
location. Diffusion is actually a result of the third type of molecular motion: translation. Therefore, we do
not typically observe diffusion in solids. In a mixture of substances, the molecules of each substance
diffuse at different rates according to their concentration gradient. A number of other factors, in addition
to temperature, also affect the rate of diffusion: the molecular weight of the substance diffusing, the
steepness of its concentration gradient, the nature of the solvent, atmospheric pressure, and the presence
of other dissolved substances.
OSMOSIS
The term osmosis refers to the movement of water molecules through a membrane from an area of high
water concentration to an area of low water concentration. Osmosis is an extremely important process for
the survival of living cells. The process of osmosis can be demonstrated by using potato pieces and
several glucose solutions of known concentration. Each potato piece is weighed, immersed in a glucose
solution of known concentration for a specified period of time then removed from the solution and
weighed again. Since the potato pieces are composed of living cells which are permeable to water, the
comparative weights of the pieces before and after they were immersed in the glucose solution will
indicate the net direction of water movement. Next week, you will design and perform an osmosis
experiment using slices of potato.
There are three possibilities for the net flow of water: from the solution into the cells, from the cells into
the solution, or into and out of the cells at an equal rate (equilibrium). If the potato cells are immersed in
a glucose solution that leads to a net loss of water from the cells (they shrink in size and weight), the
glucose solution is said to be hypertonic. If the potato cells have a net gain of water (they increase in
size and weight), the glucose solution is hypotonic. If the cells neither gain nor lose water, the glucose
solution is said to be isotonic to the cells.
The process of dialysis is similar to osmosis, but dialysis is the movement of solute particles through a
membrane. You are probably most familiar with this process in terms of kidney dialysis which is used to
remove waste materials from the blood when the kidneys are not functioning properly. During dialysis,
solutes (such as potassium, calcium, and urea) pass from a higher concentration in the blood, through the
dialysis tubing, into the dialysis fluid which removes the waste materials.
Procedures/Questions
I. Egg Osmosis
Prior to lab, eggs were soaked in a bowl full of vinegar (enough to cover eggs) for 24-72 hours.
Afterwards, each egg was rubbed carefully by hand to remove any remaining shell residue. The treated
eggs were rinsed in water and then place in a bowl of corn syrup. The eggs remained in the syrup for at
least 12 hours prior to the start of lab.
Each Group of Students:
A. Remove one de-shelled egg (raw) from the bowl of corn syrup, rinse it off in the sink, and pat it dry
with a paper towel.
B. Measure the mass of your egg in grams. Use one of the plastic cups to hold the egg while measuring
its mass. “Tare out” the mass of the cup using the following method: place the cup on the scale, press
the tare button to re-zero the scale, then carefully place the egg in the cup, and record the egg’s mass
on the Lab Data Sheet.
C. Place the egg in the bowl of distilled water at your table, for 30 minutes. Re-measure the mass of the
egg. Record the mass on the Lab Data Sheet.
D. Calculate the percent change in mass of the egg using the formula below.
Use a positive sign to
indicate an increase in mass, or a negative sign to indicate a decrease in mass. Record the percent
change on the Lab Data Sheet.
E. Now place the egg in a clean beaker and cover it with corn syrup. After at least 30 minutes, remove
and wash your egg, and record its mass on the lab data sheet.
F. Again, calculate the percent change in mass of the egg. Use the mass of the egg after it came out of
the distilled water as the initial mass, and be careful to indicate with a positive or negative sign
whether there has been a loss or gain of mass. Record the percent change on the Lab Data Sheet.
G. Return your egg to the corn syrup bath where you originally found it. Discard the used corn syrup
from your beaker in the waste container provided and wash out your beaker.
H. Record all data on the class data sheet at the front of the class and record the other lab groups’ data
on your Lab Data Sheet. Calculate the Standard Error for the percentage change in mass rows.
Questions:
1.
What happened to the egg after it was moved to the bowl of distilled water? Use the terms hyper,
hypo and/or isotonic in your answer, and describe what is moving and why.
2.
What happened to the egg after it was then moved to the beaker of corn syrup? Use the terms hyper,
hypo and/or isotonic in your answer, and describe what is moving and why.
3.
If the egg were placed into a sugar solution for one hour, but did not show any change in mass, what
could you conclude about the concentration of the sugar solution relative to solution inside of the
egg?
II. Potato Osmosis Experiment
This experiment will involve the use of potato pieces and glucose solutions, to determine what
concentration of glucose is isotonic to potatoes. This will also involve identifying which glucose solution
(or solutions) is (are) hypertonic and hypotonic to the potatoes. The potato pieces will be obtained by
using a cork borer to remove potato sections from a whole potato. You will prepare 10 glucose solutions
(ranging from 2 to 20% glucose) by mixing specified amounts of glucose and distilled water (as indicated
in Table 1 below). The potato pieces will be weighed, and then two pieces will be placed in each of the
solutions. After approximately one hour, the potato pieces will be weighed again. This experiment will be
replicated (conducted twice) to reduce variability.
A. Proposing a Hypothesis
Before beginning your experiment it is necessary to state a hypothesis about what concentration of
glucose is likely to be isotonic to the cytoplasm of potato cells and how it will differ between the types of
potatoes in the lab. In other words, what percentage of glucose is water likely to move into and out of
the potato cells at an equal rate? In the space below, state what concentration of glucose you
hypothesize will be isotonic to the cytoplasm of the potato cells. Also include a statement about which
concentrations of glucose will be hypotonic and which will be hypertonic to the potato cells. The results
of your experiment will either support or refute your hypothesis. Useful information is obtained even if
the hypothesis is refuted.
Hypothesis Statement
B. Predicting Your Results
Plot point on Graph 1 that represents the glucose concentration that you predict is isotonic to the potato
cells. Consider what change, if any, will occur over time in the mass (weight) of the potato pieces that are
immersed in an isotonic glucose solution. Plot a predicted point for each of the glucose solutions, and
connect the plotted points on your graph.
Percent Increase or Decrease in Mass
Graph 1 - Percent Change in Mass of Potato Pieces
25
20
15
10
5
0
-5
-10
-15
-20
-25
2
4
6
8
10
12
14
16
18
20
Concentration of Glucose (% Glucose)
C.
Materials and Methods
Preparing the Glucose Solutions
1.
2.
The glucose solutions should be prepared as in Table 1. Dispense approximately 110 ml each of
distilled water and 20 % glucose into separate beakers.
Use separate graduated cylinders to add the required amount of distilled water first to each test
tube and then add the required amount of 20% glucose.
Table 1
Test Tube No.
1
2
3
4
5
6
7
8
9
10
Distilled water (ml)
9
8
7
6
5
4
3
2
1
0
20% Glucose (ml)
1
2
3
4
5
6
7
8
9
10
2%
4%
6%
8%
10%
12%
14%
16%
18%
20%
Concentration
Glucose
of
Preparing and Weighing the Potato Pieces
1.
2.
3.
4.
5.
6.
7.
Each group will be given a number of potatoes with which to experiment. Do not place the potato
section or potato pieces on paper towel.
Use your cork borer to cut out 20 potato pieces from the potatoes. The cork borer will have two
parts (cylinders). Cut the potato pieces with the cylinder that has the greater diameter. The
potato sections you cut out should be complete cylinders and not have any potato skin.
Remove the potato pieces from your cork borer by pushing the cylinder with the smaller diameter
into the one containing the potato pieces.
Weigh two potato pieces to the nearest 0.01 g and record the weight in Table 3 (Lab Data
Sheet) in the appropriate box of the initial mass column.
Place the two potato pieces in the black film canister with the same number as the test tube.
Repeat steps 4 and 5 for the remaining potato pieces by recording their weights and placing
them in the appropriate film canisters.
After all 20 potato pieces have been weighed, transfer each pair to the test tube with the glucose
solution that has the same number as the black film canister.
Replicating the Experiment
To gain the most accurate data, a good experiment should be repeated to reduce the effect of
variables. Once trial 1 has been set up, a second trial will be set up using a second piece of
potato.
C.
Collecting and Graphing Your Results
1.
After one hour, pour off the glucose, remove each pair of potato pieces from the test tube,
briefly blot with paper towel to remove excess water and reweigh.
2.
Record the final weight (mass) in the appropriate box of Table 3 (Lab Data Sheet).
3.
Calculate the percent change in weight (mass) for each pair of potato pieces and record the
results in Table 3 (Lab Data Sheet). The formula for the percent change in mass is:
(Final Mass  Initial Mass)
 100
Inital Mass
4.
5.
Calculate the average percent change from trials 1 and 2. Plot the average on your graph. Due to
experimental error and uncontrolled variables your points may not form a line. To show the
relationship between % glucose and change in mass of the potatoes create a straight line that
best fits your data points. Use this trend line to determine the concentration of glucose that
would be isotonic to the potato cells.
Record your data on the class Data Sheets and record the other groups’ data on the Lab Data
Sheet.
Formal Lab Requirements
The following must be included in the formal lab report.




A copy of your notebook pages with any notes or calculations.
The Lab Data Sheets
The typed responses to the question in Part I
A formal report, typed, with the following sections (pertaining to Part II of the experiment
only)
o Title
o Name
o Introduction discussing the nature of osmosis
o Your Hypothesis statement
o A summary of the procedures used
o Tabulated Data for your group and from the class
o Graph of Group Data
o A discussion of the data in relation to your hypothesis
o A discussion of uncontrolled variables that may have influenced the data
o References (if needed)
Lab Data Sheet
Table 2: Egg Osmosis
Data Collected
Initial Mass (g)
1
2
Groups
3
4
5
6
Std.
Error
Final Mass after distilled water (g)
% Change in Mass
Final Mass after corn syrup (g)
% Change in Mass
Table 3: Potato Osmosis
Trial 1
Glucose
Concentration
2%
4%
6%
8%
10 %
12 %
14 %
16 %
18 %
20 %
Initial
Mass
Final
Mass
Trial 2
Percent
Change
in Mass
Initial
Mass
Final
Mass
Percent
Change
in Mass
Average
Percent
Change
in Mass
Table 4: Group Data Sheet
Glucose
Concen.
2%
4%
6%
8%
10%
12%
14%
16%
18%
20%
1
Avg. Percent Change in Mass by Group
2
3
4
5
6
Type of
Potato