Diffusion and Osmosis Lab
Objectives:
 Relate structure to function for the components of plant and animal cells. Explain the role of cell membranes as a highly selective barrier
(passive and active transport).
 Compare and contrast the general structures of plant and animal cells. Compare and contrast the general structures of prokaryotic and
eukaryotic cells. AA
Purpose of the Lab/Activity:
 Address the movement of material through a semi-permeable membrane.
 To observe the process of osmosis and how it occurs in cells.
 Examine and describe the different environmental conditions that can affect a cell and its processes.
Prerequisites:
 The movement of ions and molecules across membranes is discussed.
 Iodine indicator is used to test for the presence of starch. It turns blue-black when starch is present.
 Basic understanding of the structure and function of cells.
Problem Statement(s): What is the movement of material through a semi-permeable membrane? (Activity 1) What is the osmotic effect of
varying sodium chloride (NaCI) solutions on the physical characteristics of a potato core? (Activity 2)
Background: (Source: www.explorelearning.com)
Diffusion is the process in which there is a net movement of molecules from a high area of concentration to a low area of concentration.
Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane (Semi-permeable membranes
are very thin layers of material which allow some things to pass through them but prevent other things from passing through.) to a region of
low water concentration. This is seen in cell membranes. When there is a higher concentration of one type of molecule outside of a cell, water
will move through a membrane out of the cell in order to make the water concentrations equal. This causes the cell to shrink (hypertonic). If the
concentration of certain molecules is higher inside of the cell, then the water will move into the cell causing it to swell (hypotonic). When the
molecule concentrations are equal on both sides of the membrane, water does not move (isotonic).
In the human body, many salts and enzymes help to regulate a cell’s state and the processes necessary for the human body to function such as
potassium and calcium channels in the heart. These functions are carried out by having constant changes in concentration of molecules from
one side of the membrane to another. Cell membranes will allow small molecules like oxygen, water, carbon dioxide, ammonia, glucose, amino
acids, etc., to pass through. Cell membranes will not allow larger molecules like sucrose, starch, protein, etc., to pass through.
Vocabulary: cell, cell membrane, permeable, diffusion, semi-permeable membrane, osmosis, hypertonic, hypotonic, isotonic
Extension: Osmosis & Diffusion Gizmos
Part A: Diffusion (Demonstration):
Drop several drops of food coloring into a glass beaker of water. Ask students to observe what happens over the next couple of minutes and
have them note changes to water and food coloring in their journals. Have students predict what will happen after 5, 10 and 20 minutes.
Introduce the concept of “diffusion”.
Part B: Diffusion & Osmosis
Materials (per group):
 15% Glucose / 1% Starch Solution (5 of Starch in 500ml of
hot / stirred water. Then add 75g of dextrose)
 Beaker (600ml) x 2
 Benedict’s Solution x 10ml
 Distilled water x 500ml
 Forceps




Lugol’s solution (3g KI and 1.5g of I crystals on 500ml of
distilled water) x 10ml
Marker
Ziploc bag (small) x 2
Masking Tape
Procedure
Note: If students never performed qualitative analysis of organic molecules lab, teacher should demonstrate Lugol and Benedict’s solution
tests, explain what each of them tests for, and show what a positive results look like.
1. Using masking tape and marker, label two 600ml Beakers as A & B
2. Add 250ml of distilled water to both beakers.
3. Using a 10ml syringe place between 5-15ml of Lugol solution inside it Beaker A. Agitate solution to mix as you add iodine to it. Stop
when the solution turns brown (teacher will demonstrate to class). Record observations of the color of the solution in the data table.
4. Repeat step 3, but instead of Lugol Solution, add Benedicts solution instead (turns water blue).
5. Fill 2/3 of the first zip lock bag with 15% Glucose / 1% Starch Solution. Record color of solutions in the data table. Note that it appears
that no water or iodine is leaking or can leak out of the bag.
6. Place one bag in each beaker.
7. Carefully observe and record the changes over time to both the beakers and the bags. You will need to carefully lift the bags part way out
of the water with a forceps to observe. Then lower the bag back into the water and return the container as directed by the teacher.
Results:
Initial Observations
Final Observations
Beaker A
Beaker A Bag
Beaker B
Beaker B Bag
Analysis/Discussion Questions
1. What does Benedict’s solution test for? What color indicates a positive result?
2. What does Lugol solution test for? What color indicate positive result?
3. Based on your answers for questions 1-2 and on the results of your experiment, complete the following chart by marking an X in a location
where the indicated chemical is present.
Before
After
Benedict’s
Lugol’s
Benedict’s
Glucose
Starch
Iodine
Glucose
Starch
Solution
Solution
Solution
Beaker A
Beaker A Bag
Beaker B
Beaker B Bag
4.
5.
Provide an explanation for why the color did or did not change:
a. In Beaker A:
b.
Inside Bag in Beaker A
c.
In Beaker B
d.
Inside Bag in Beaker B
Based on your answer for question 3. Sketch the experiment, use arrows to show the way the diffusion and osmosis was occurring. For
each Beaker, include the movement of: Water (Osmosis); Indicator Solutions; Glucose; & Starch
6.
7.
8.
Observing the diffusion process in real cells is difficult because they are too small to be seen easily. In this activity you created a giant
model of a cell so that you can observe the effects of diffusion through a membrane. In your cell model describe the role of each of the
following parts:
a. Ziploc bag:
b. Contents of the bag:
c. Area outside the bag:
What kind of membrane was the zip lock bag? Permeable, impermeable, or semipermeable? Explain your answer
What has this experiment taught you about the function of the cell membrane and the reason why it is called semi-permeable?
Part C: Inquiry Extension: Diffusion & Temperature
 Students are design an experiment following all steps of the scientific method to examine the effect of temperature on diffusion rates
 May use the available lab resources, but must try to bring their own
 At the conclusion, groups will have to submit full lab reports in accordance with lab report template / rubric
 Before doing the lab, submit the following to the teacher: Problem Statement / Background / Material List /Procedure
Note: Before you write a full background you may want to check you problem statement and a preliminary idea of how to test it with the
teacher. Students often decide to switch experiments after they realize their procedures are not practical. At that point, you may have
wasted your time already writing a full background)