DIFFUSION ACROSS MEMBRANES
Week 2, BIOL 241
INTRODUCTION
Today, you will complete this worksheet (Worksheet 2)
as you learn about diffusion. Remember that this is
group work, and you will upload your completed
worksheet to Blackboard before the start of your next
lab period.
Group members present:
1.Lucy Alcala
2.Carmen Jewkes
3.Shayleigh Smith
4.Kaden Fisher
WEEK 2 LEARNING OUTCOMES
Diffusion is the movement of molecules from an By the end of this lab you will be able to
area of higher concentration to an area of
 Define diffusion and provide examples
lower concentration. How does this happen?
 Predict how physical properties (e.g.,
Molecules possess kinetic energy which allows
temperature) affect diffusion rate
them to be in constant motion. They bump into
 Determine the effect of solute size on
each other and bounce off in random directions,
diffusion across a semipermeable
transferring energy to each other as they
membrane
do. Eventually, the molecules become equally
distributed in the space. Do the molecules stop
moving? No, but their concentration has become evenly spread out.
You are already familiar with this process. Think of a bottle of ammonia that you place in the
middle of the room and then open. Students sitting next to the bottle will smell the ammonia
first
and
more
intensely.
The
ammonia
molecules are bumping into
each
other and also other molecules in the
air.
Eventually, the ammonia molecules will disperse
throughout the room and increasingly more
students will smell it as it spreads. If the room is
large enough, at some point the ammonia may be
in too small a concentration for your nose
to even detect. The same process would happen
if you were to drop a cube of sugar into a glass of
Figure 1. Illustration showing the movement of the sugar
water (Figure 1). The sugar molecules would
molecules in a beaker of water.
https://schoolworkhelper.net/selective-permeability-ofbounce against each other and against the water
dialysis-tubing-lab-explained/
molecules until the sugar was evenly mixed in the
water. The molecules would continue to move but there would no longer be a concentration
gradient, or difference in concentration.
What is another example of diffusion that you can think of from everyday life (1 point)?
air freshners/perfumers
PART 1: HOW DOES TEMPERATURE AFFECT DIFFUSION RATE?
While the presence of a concentration gradient is the principal force driving diffusion, other
factors can affect the rate of diffusion. In today’s experiment, you will investigate the way in
which temperature affects diffusion rate. First, you will add similar amounts of food coloring into
2 beakers. One beaker will contain room temperature water and the other will contain heated
water. Because of the random movement of dye particles, a dense solution forms in the water at
the base of the beaker. The colored solution will then slowly spread into the rest of the water
throughout the beaker creating a less dense but evenly colored dye solution.
1. Make a prediction: In which of the beakers will the dye diffuse more quickly? What is your
reason for this prediction (1 point)?
The dye will diffuse faster in the beaker with heated water because as temperature increase
diffusion will also increase due to increased energy of the therefore molecules move more
rapidly.
Protocol:
1. Working as a group, obtain a 100mL beaker and add 50mL of tap water from the cold tap.
Then obtain a 100 mL beaker filled with 50mL of hot tap water, which your TA will have
prepared for you using a hot plate. Let the water sit in both beakers so that you no longer
see any movement in the water. At this time you can obtain a bottle of food coloring.
2. With the food coloring dropper as close to the surface of the water as possible without
touching the water, gently add one drop of food coloring to the beaker with cold tap
water. Then, immediately add one drop of food coloring to the beaker with hot water.
3. Start a timer, and time how long it takes for the color of the dye to be evenly dispersed
throughout the water in each beaker. Do not touch/move the beakers once you start the
timer!!
a. Note: Observe your beakers initially, but you may want to advance onto Part 2 of
this lab as you wait for the dye to become evenly dispersed throughout the water.
Just don’t forget to check back on your beakers often, so that you know when they
reach this state!
4. Record your times in the table below and also in the table on the whiteboard (class
data). From the class’ raw data, you can calculate the average time. Remember to include
your units of time (2 points).
Temperature in Beaker
Room
Heated
Your Diffusion Time
Class Average Time
2578 sec
1319.3 sec
978 sec
524 sec
By compiling diffusion time from the entire class, we are increasing the replication in this
experiment. Why is it important to have replication in experiments (1 point)?
Replicants are important because it makes experiments more reliable by decreasing the variablity
that one may come across when conducting an experiment.
Construct a box plot in Excel using the entire class’ raw data (not just the class average time) on
the board. If you cannot remember how to make a box plot, refer back to the Week 1 lab or the
“Making a Box Plot” video on Blackboard. Insert your graph here, and then re-size it if necessary
(2.5 points):
Based on the data you collected, what can you conclude about the effect of temperature on
diffusion rate (1 point)?
Temperature decreases diffusion rate
Temperature increases diffusion rate
Temperature does not affect diffusion rate
Do your conclusions match your predictions?
Other properties can also affect the rate of diffusion. Using your textbook, class notes, or an
online resource, identify another factor that affects diffusion rate and explain how (1 point).
Other factors that affect diffusion rate:
-Extent of concertation gradient-the greater the concentration gradient the more rapid diffusion
occurs
-Mass of molecules-heavier molecules move slower therefore affecting speed in which diffusion
can happen
-Solvent density-denser the solvent, the slower the rate of diffusion due to molcules moving slower
passing through the medium
-Surface Area- More surface area, higher rate of diffusion because more particles can pass in more
places
-Solubility-nonpolar molcules pass easier through the membrane
-Distance-the greater the distance, the slower the rate of diffusion
PART 2: DIFFUSION ACROSS A MEMBRANE
Cells are surrounded by selectively permeable membranes that
allow certain substances to enter and leave cells, and can
prevent others from entering or leaving. One way that molecules
can pass through the cell membrane is by diffusion. Molecules that
diffuse through the cell membrane do so by moving through pores
in the walls of the membrane. If Molecule A is larger than the pore,
it cannot pass through. If Molecule B is smaller than the pore then
will it pass through? It might! What would determine
whether the Molecule B will pass from one side of the membrane
to the other? A difference in concentration of that type
of molecule on each side of the membrane. Because the
movement of these molecules does not require energy from the
cell, it is called passive diffusion.
Figure 2. Initial set up for experiment
showing the glucose and starch in the
tube and the water with Iodine in the
beaker. https://biodifferences.com/dif
ference-between-osmosis-anddiffusion.html
In this next experiment, you will investigate the diffusion of
molecules across a synthetic selectively permeable membrane
that will imitate a cell membrane. The membrane is considered
selectively permeable, or semi-permeable, because it has pores embedded in its surface that
allow small molecules to pass through, in either direction.
Fill in the chart below, indicating which molecule(s) you think will be able to cross the membrane
and why (4 points).
Molecule
Molecular Formula
Glucose
C6H12O6
Will it cross the selectively
permeable membrane?
Yes
Why or why not?
Small particle size
Large particle size
Iodine
Polysaccharide made up of No
many glucose molecules
Yes
I3 -
Water
H2O
Yes
Small particle size
Starch
Small particle size
To conduct this experiment, you will put solutions of starch and glucose into an imitation “cell”
that you will construct from dialysis tubing. You will then place this tubing into a beaker of
distilled (DI) water with Lugol’s iodine added to the water (Figure 2). You will run this experiment
at room temperature for 20 minutes. To determine if the starch or iodine moved, you will look
for changes in color in the tubing and in the beaker. Under normal conditions, iodine is a
yellowish- brown color (Figure 2), but it will turn a dark blue/black in the presence of starch. You
can also test for the presence of glucose using glucose strips.
Using your predictions from above, fill in the expected color of each liquid after diffusion has
occurred (2 points).
Initial State
Color
Final State
Color
Liquid in Tube
Clear
Liquid in Tube
blue/black
Liquid in Beaker
Yellowish Brown
Liquid in Beaker
yellow brown
We will test your predictions by following this experimental protocol:
Note: Review the “How to pipet” and the “How to make a tube with dialysis tubing” video
on Blackboard to see how to set up your tubing.
1. Obtain one 20cm piece of dialysis tubing.
2. Soak the tubing in distilled (DI) water for at least one minute.
3. Rub the tubing together with your fingers to separate the tubing and form an open-ended
tube.
4. Tie a knot in one end of the tubing.
5. Before moving on, make sure you’ve watched the “How to pipet” video on Blackboard.
A couple of questions to review pipetting:
You need to add 472 uL (microliters) to a tube. Which pipettor will you use? (0.5 points)
P10
P20
P200
P1000
P5000
When you adjust the volume on the pipettor in the previous question, how will the pipettor
volume display appear? (0.5 points)
You need to add 30uL to a tube. Which pipettor will you use? (0.5 points)
P10
P20
P200
P1000
P5000
When you adjust the volume on the pipettor in the previous question, how will the pipettor
volume display appear? (0.5 points)
You need to add 3mL (milliliters) to a tube. Which pipettor will you use? (0.5 points)
P10
P20
P200
P1000
P5000
When you adjust the volume on the pipettor in the previous question, how will the pipettor
volume display appear? (0.5 points)
Now, let’s return to your protocol…
4. Obtain a p5000 pipettor and adjust the volume to 3mL (3000 uL). Check with your TA to
make sure you have set the pipettor correctly. Using a pipettor (make sure the tip is on
the pipettor!), add 3mL of glucose and 3mL of starch solution to the tube. Change pipet
tips in between adding the glucose and the starch.
5. After the tube is filled, tie another knot in the tubing to close off the liquid. Cut off any
extra dialysis tubing from the end.
6. Rinse off the tube in fresh DI water.
7. Weigh the bag and record in the table below. Be sure to record your units of
measurement.
8. Add 100 ml of DI water to a 250 ml beaker. Make sure you don’t add extra water.
9. Use a glucose test strip to confirm there is no glucose in the water in the beaker.
a. Remove one glucose test strip from the bag.
b. Reseal the bag as soon as possible.
c. Do not touch the test pad with your fingers.
d. Completely immerse the test pad into the specimen for 1-2 seconds.
e. Run the test strip along the edge of the container to remove excess liquid.
f. Compare to the color chart after 3 minutes
10. Add a few drops of iodine to the water in the beaker. Stir. Be careful to avoid staining
your clothes or fingers with the iodine.
11. Put the tube in the water.
12. Wait 10-20 minutes for substances to move across the selectively permeable membrane.
13. Document observed changes in the color of the liquid in the bag and the liquid in the
beaker.
14. Weigh the bag, blotting it dry first. Be sure to include your units of measurement
when recording your results in the table below.
15. Use a glucose test strip to test for the presence of glucose in the liquid in the beaker. Be
sure to include your units of measurement when recording your results in the table below
(2.5 points).
Observed Final State Color
Liquid in Bag
Liquid in Beaker
clear
yellow-brown
Beginning
Weight
6.18g
Ending Weight Glucose concentration
6.89g
1000 mg/dL
Based on your observations, what can you conclude about the movement of glucose, starch,
iodine, and water across your selectively permeable membrane? If any of your conclusions did
not support your original predictions, explain why and what you have learned from your new
observations (4 points).
One can conclude that the iodine, water, and glucose molecules are permeable through the
membrane. This indicates that these molecules are able to pass through the dialysis tube.
Indicated throught the color change within the tube, one can see that the clear glucose, starch
solution turns into a blue/black color indicating that iodine diffused through the membrane, down
the concentration gradient. Water also rushes inside the tube to balance the gradient and that is
noticed with the increase in weight within the tube. Glucose proceeds to go down its
concentration gradient and move across the membrane into the contents of the beaker. Starch is
too large of a molecule to move pass the membrane hence why one observes color change only
within the dialysis tube and not in the contents of the beaker.
PART 3: WRITING A RESULTS SECTION
When scientists conduct research, they share their findings in the form of peer-reviewed
scientific papers. These are reports of their experiments, including the methods they used, the
results from their research (including tables and figures), and a discussion of how they interpret
their results. These papers also contain an introduction that contextualizes the research in the
wider scientific field and justifies the need to conduct the research in the first place. You will be
writing reports in the style of scientific papers this semester.
For homework (part of Prelab Assignment 2), you will practice recognizing what a good Results
section of a scientific paper looks like. You will use a rubric to “grade” two example results
sections. The rubric is the same one that your TA will use to grade your lab reports later on this
semester, so practicing using the rubric will help you prepare for writing your own results sections
later on.