Mr. Tripp
7th Grade
Lab #4
Diffusion & Osmosis Lab
Lab Station # _______
Lab Members:
Seat A:_____________________________
Seat B:_____________________________
Seat C: _____________________________
Seat D: _____________________________
Standard(s):
S7CS1 Students will explore the importance of curiosity, honesty, openness, and skepticism in science and will
exhibit these traits in their own efforts to understand how the world works.
S7CS2 Students will use standard safety practices for all classroom laboratory and field investigations.
S7CS4 Students will use tools and instruments for observing, measuring, and manipulating equipment and
materials in scientific activities.
S7CS6 Students will communicate scientific ideas and activities clearly.
S7CS8 Students will investigate the characteristics of scientific knowledge and how that knowledge is achieved.
S7CS9 Students will investigate the features of the process of scientific inquiry.
S7L2. Students will describe the structure and function of cells, tissues, organs, and organ systems.
a. Explain that cells take in nutrients in order to grow and divide and to make needed materials.
b. Relate cell structures (cell membrane, nucleus, cytoplasm, chloroplasts, mitochondria) to basic cell
functions.
Essential Question:
How do the higher levels of organization serve the needs of the cell?
Answer:
Part 1Introduction: In this lab you will observe the diffusion of a substance across a
semi permeable membrane. Iodine is a known indicator for starch. An indicator is
a substance that changes color in the presence of the substance it indicates.
Watch as your teacher demonstrates how iodine changes in the presence of
starch.
Prelab Observations: Describe what happened when iodine came into contact
with starch.
Procedure:
1. Fill a plastic baggie with a teaspoon of corn starch and a half a cup of
water tie bag. (This may already have been done for you)
2. Fill a beaker halfway with water and add ten drops of iodine.
3. Place the baggie in the cup so that the cornstarch mixture is submerged in
the iodine water mixture.
4. Wait fifteen minutes and record your observations in the data table
5. While you are waiting, answer the questions.
Questions:
1. Define diffusion.
2. Define osmosis
3. What is the main difference between osmosis and diffusion
4. Why is iodine called an indicator?
5. Molecules tend to move from areas of _______ concentration to areas of
______ concentration.
What's in the Bag?
We're going to think about concentrations now, which substances are more or
less concentrated depends on which one has the most stuff in it.
1. Is the baggie or beaker more concentrated in starch?
2. Is the baggie or beaker more concentrated in iodine?
3. Iodine solution: is the baggie or the beaker hypertonic?
4. Starch solution: is the baggie or the beaker hypertonic?
5. Which one is hypotonic in relation to starch, baggie or beaker?
Make Some Predictions
1. If the baggie was permeable to starch, which way would the starch move, into
the bag or out of the bag? ________
2. If the baggie was permeable to iodine, which way would the iodine move, into
or out of the bag? _______
3. If the baggie was permeable to iodine, what color would you expect the
solution in the baggie to turn? _______ What about the solution in the beaker?
___________
4. If the baggie was permeable to starch, what color would you expect the
solution in the baggie to turn? ________ What about the solution in the beaker?
_________
5. Make a prediction about what you think will happen:
Data Table
Starting Color
Color after 15 minutes
Solution in Beaker
Solution in Bag
Post Lab Analysis
1. Based on your observations, which substance moved, the iodine or the
starch?
2. How did you determine this?
3. The plastic baggie was permeable to which substance?
4. Is the plastic baggie selectively permeable?
5. Sketch the cup and baggie in the space below. Use arrows to illustrate how
diffusion occurred in this lab.
6. What would happen if you did an experiment in which the iodine solution was
placed in the baggie, and the starch solution was in the beaker?
Be detailed in your description.
7. Why is it not a good idea to store iodine in a plastic bag?
Part 2Lab: Osmosis in Red Onion Cells
INTRODUCTION
The passage of water molecules through a selectively permeable membrane is a special case of diffusion known as
osmosis. Osmosis results in the
movement of water from an area of lower concentration of dissolved material to an area of higher concentration of
dissolved material. For a
demonstration of this principle, a purple onion cell is going to be bathed in different concentrations of salt (NaCl) water.
BACKGROUND
In comparing two solutions of unequal solute concentration, the solution with a greater concentration of solutes is said to
be hypertonic. The solution
with the lesser solute concentration is hypotonic. These are relative terms that are only meaningful in a comparative
sense. For example, tap water is
hypertonic to distilled water, but hypotonic to seawater. In other words, tap water has a higher concentration of solutes
than distilled water, but a lower
concentration than seawater. Solutions of equal solute concentration are said to be isotonic.
The diffusion of water across a selectively permeable membrane is a special case of passive transport called osmosis. In
a sense, the water is
diffusing down its concentration gradient. The solution with the greater concentration of solute has the lesser
concentration of solvent. The direction of
osmosis is determined only by a difference in total solute concentration, not by the nature of the solutes. Water will move
from a hypotonic to a
hypertonic solution even if the hypotonic solution has more kinds of solutes. Seawater, which has a great variety of
solutes, will lose water to a solution
of sugar that is very concentrated, because the total solute concentration of the seawater is less. It should be noted that
there is no net movement of
water between isotonic solution separated by a membrane.
In this demonstration, the purple pigment visibly responds to the changes in the salt concentration. Sodium molecules are
too large to pass through the
cell membrane so the water moves in and out of the cell to maintain the proper cell pressure.
Problem: How do solutions of various salt concentrations influence osmosis
in relation to an onion cell?
Hypothesis:
______________________________________________________________________________________
_____________________
______________________________________________________________________________________
_____________________
Materials: (per student student group): red onion epidermis, forceps, dropper,
distilled water, 5% or high percentage sodium chloride (table salt) solution,
paper towels, microscope, slide, and cover slip
PROCEDURE:
1. Make a wet mount of the red onion epidermis.
2. Examine under low power.. When you have a clear view of several cells, switch to high power.
Make a labeled drawing, properly labeled in your data. This will give you a record of the original
appearance of the onion cells.
3. Take a dropper and add several drops of salt solution to one side of your cover slip while placing
a small piece of paper towel along the opposite edge of the cover slip. The paper should draw out
the water and draw in the salt solution.
4. Observe the effects of the saline (salt) solution on the onion cells. Make a properly labeled, careful drawing
of the cells' appearance in a second properly labeled carefully constructed drawing in your data.
5. Replace the sodium chloride solution with distilled water in the same way that the salt solution was added.
Make a properly labeled drawing of the cells' appearance in your data.
6. Answer the questions which follow your data.
Observations:
____Original appearance
Saline solution____
Magnification ______________
Description ________________________________
Magnification ____________________
Description _______________________
_____Saline solution replaced with distilled water