This lab experiment investigated how the rate of diffusion through a semi-permeable membrane is affected by the surface area of the membrane. Starch-filled dialysis tubing of three different lengths were submerged in an iodine solution, and the color change of the starch over five minutes was measured. It was hypothesized that tubing with smaller surface area would diffuse more slowly. The results supported this, as the shortest tubing produced the lightest color after five minutes, indicating slower diffusion. Areas for improving the experiment were also identified.

1. Kristin Coad
IB Biology 2
September 2010
Effect of diffusion through semi-permeable membrane
INTRODUCTION:
Diffusion occurs when molecules of a high concentration move to a low concentration. This is more
formerly known as moving down the concentration gradient. The objective of diffusion is to reach
equilibrium or where everything is equal (refer to figure one). In this research, the relationship between
the rate of diffusion through a semi-permeable membrane and surface area of dialysis tubing which is
filled with starch will be investigated. Because of the nature of the membrane, this enables diffusion to
occur. The results which would be tested is the colour of the starch after being in the water based iodine
solution for five minutes. A colourmeter is the main device that would help to record the colour
reading. The colourmeter is a piece of equipment which tests for the accurate amount of light that is
being absorbed in a solution. The reading of the starch will happen after the meter is calibrated with
water and starch. This has to happen to get a base reading of no absorbance, and the regular
absorbance of starch. Distilled water came up with a reading of 99.9 nanometres, where as the starch
had a reading of 90.5 nanometres (The light colourmeter is read in nanometers, and the light being used
for this experiment is a blue-violet light with a reading of 430 nanometers). The transmittance, or how
much light is being absorbed in a solution, is what was being tested for this experiment. The way that
the colour was tested was by having the same amount of starch, 10 milliliters, in three different size
dialysis tubing bags; the average lengths were 194.75mm, 80mm, and 35mm and the height of all the
bags were 45mm, which was all submerged in the same bath of water/ iodine solution. Because of the
chemical reaction, when starch is diffused with water/iodine, the end colour will turn up to be in the
purple spectrum.
What is expected to happen during
this experiment is that the bags
that have smaller surface area
should diffuse slower, rather than
the longer bags, which take less
time to diffuse. The obvious way
that the colour will be figured out
is by the how dark or light the end
reaction is.
It is predicted that at the end of
the five minutes, the longer bags Figure 1: This is a diagram of basic diffusion.
will have a darker, more prominent purple colour, As labeled, the water molecules between
rather than the small surface area that will be around a the membrane on the right are equal, or
light purple, and nearly see-through. This is because the have reached equilibrium because the
larger bags have less starch per square centimeter,
so it would be almost like diffusing less liquid at one time, instead of more starch like in the little bags.

2. Semi-permeable Question: How does the amount of surface area affect the rate of diffusion through a semi-permeable
membrane membrane?
Independent variable: Surface area of the semi-permeable membrane
Dependent variable- The time in which the cell takes to reach equilibrium by passing through the
membrane
Table 1: The controlled variables of the experiment.
Controlled variables How it was controlled
I2KI For each trial being in the same bucket so the
iodine is the same for each tube.
Time Setting a time, and having a “set time” of five
minutes
Amount of Starch Measuring five milliliters in a graduated cylinder,
measuring at the bottom of the meniscus.
Width of dialysis tubing By using the same roll of dialysis tubing, and by
measuring one to get the size.
Temperature of water Putting the tubing in the same large bucket filled
with water and iodine simultaneously for all the
trials for each length.
Table 1: This table shows all of the different controlled variables that went into the experiment. Along
with that, the table shows how they were being controlled through-out the entire experiment.
MATERIALS:
• Semi-permeable membrane (dialysis
tubing), 3 different lengths x4
• Ruler
• Scissors
• Water
• Bucket
• Iodine
• Starch
• Computer
• Colourmeter
• Curvettes
Figure 2: This figure shows some of the materials
• Clamps that were being used through out the experiment.
Part of the materials includes the ruler, scissors, and
the semi-permeable membrane.

3. • Funnel
• Graduated cylinder
• Stop watch
• Beaker
PROCEDURE:
Before the research began, three different lengths of semi-permeable membrane was measured with a
ruler and cut four times to create 4 trials. All of the tubes were soaked in plain water for a few minutes
so they can be opened. The three lengths averages were 194.75mm, 80mm, and 35mm. After every
membrane was cut and tied at one end only, five millimeters of starch was poured into each bag via
funnel. Then all the air remaining in the bag was squeezed out to help add to the accuracy of the results.
While the bags were being filled and measured, the bath containing the water and iodine mix solution
was being prepared. The bucket was large enough to be able to hold 12 bags. To ensure that the bags
would not stick to each other, slight agitation
was necessary, while a partner on the computer
set up and calibrated the colour meter. Once
everything was calibrated with water and starch
samples, and the five minutes of agitation was
over, each bag is opened one by one, poured
into a curvette via funnel. Each bag was tested
in the colourmeter to get an average reading
for each of the three lengths. Once all of the
curvettes were measured, and the data
recorded, everything was cleaned up, and put
back where it belonged.
Figure 3: This figure shows the set up of the colourmeter
HYPOTHESIS: (on the left) with the computer (on the right) to record
the transmittance.
It was predicted that the bag with the smallest
surface area to amount of starch would take
more time to diffuse through the semi-permeable membrane. It was predicted like this because since
the bag is completely full of starch, it would take longer for everything to completely diffuse. Where as,
the bags with more surface area wont need as much time because the starch would pan out, and then
there would be less starch in one area. Because of the starch to iodine reaction, it is expected that the
starch will be tinted a purple- blue colour after some time, and the end colour of all the three different
size bags won’t differ too much.

4. DATA:
Table 2: Recorded average lengths and transmittance (Uncertainty of ± 0.05)
Trials (length of Average length Transmittance Average
Size of the bag bag in mm) (colourmeter in transmittance
nanometers)
Small 1.214 1.11.2
2. 190 2. 10.9
193.75 11.625
3. 187 3. 14.0
4. 184 4. 10.7
Medium 1.80 1.38.7
2. 81 2. 39.6
80 43.95
3. 80 3. 36.1
4. 79 4. 34.4
Large 1.36 1.63.7
2. 35 2. 65.1
34 66.25
3. 33 3. 69.0
4. 32 4. 67.2
Table 2: This data table represents the relationship between the length of the membranes and colour for
each of the four trials. The table includes four trials of each of the three lengths (small, medium and

5. large), and the transmittance of each of the trials. Also included in the table are the averages of the
length and transmittance. The bags were measured in millimeters, and the transmittance was measured
in nanometers.
Figure 4: The water/iodine bathThis isthe three photo of the starch
Figure 5: with the after
semi-permeable membrane baggies. Diffusion
different sizes. All four trials are submerged with in
the one bucket. This ishas occurred due happened.
before diffusion to the purple-blue stain.
Graph 1: The relationship between the average length and transmittance (Uncertainty ± 0.05)

6. Graph 1: The graph above shows the relationship between the average length of the membrane and
transmittance of light. As the membrane length increases, the transmittance decreases. This means that
for instance, the membrane with 193.75mm of surface area, the transmittance is low. In other words,
the darker the colour after diffusion, the less percent of transmittance there.
CONCLUSION/EVALUATION:
The way that surface area affects the rate of diffusion is by amount of surface area. In the research,
there were different size bags of semi-permeable membrane, though they all contained the same
amount of starch inside. What ended up was that the bag with the most surface area, 200mm, took
shorter than five minutes to fully diffuse, where as the bag that was completely full of starch would have
taken longer than the given five minutes to finish fully diffusing. The research does not fully support the
hypothesis in the sense that what it explained about the time because of the surface area was correct.
What was not supported was the colour factor being that the end product resulted in drastically
different shades of the purple stain in the starch. After the five minutes in the bath, the bag that had the
smallest surface area, average of 34mm, had the weakest colour of 66.25 nanometers. After that, the
medium bag of 80mm had the transmittance of 43.95 nanometers, and lastly, the longest bag of
193.75mm had the littlest transmittance of 11.625 nanometers. The biggest bag that had the lowest
nanometer reading was the darkest all the bags, then the medium, and the smallest bag.

7. Things that could be improved for the next experiment include things such as accurate readings, lengths
and sample sizes. For the evaluation, refer to table three.
Table 3: Observations through-out the experiment
Qualitative observations The immense
difference of colour
between the three
different bags.
Maybe if the
smallest bag was the size of the longest bag, then the colour would be darker,
or close to the same range.
Figure 6: The above figure shows the four trials after
diffusion. The liquid is presented in curvettes which
were all sampled in the colourmeter.
Quantitative observations Things such as the temperature could have affected the end result of the
colour, but since every bag was in the same bucket of room temperature, all
the bags received the same temperature.
Another observation that was taken account for is the amount of starch in the
bags. Since there was the amount in every bag was 10 mililetres, this greatly
took a role in how the results came out.
Another observation was the amount of time the bags were in the solution.
They were in the bath for five minutes giving each set of bags to diffuse in the
amount of time that was given.
Table 3: The table above shows the qualitative observations such as colour, and the quantitative
observations like temperature and amount of starch through out the experiment.
Table 4: Table of weaknesses and limitations of the experiment
Weakness/ limitations What(how it was effect it) Solution

8. Length Because rulers are unreliable, Perhaps to use something that
this made it hard to get a very accurately measures the length
accurate measurement. of the membrane.
Ruler This weakness is by not having Requesting to have better
very accurate equipment. measuring equipment.
Sample size This weakness was made Take longer sample sizes. Instead
because of the uncertainty of of making the first one short, by
how long each different piece making it longer and going from
should have been. there will improve the
experiment.
Table 4: This table shows the weaknesses and limitations that were faced through-out the experiment. It
also shows how it affected the experiment, and the solution towards fixing it.
This is a lab that I did on diffusion in the beginning of biology SL in senior year. This lab was to
see the how the length of a semi-permeable membrane effects the rate of diffusion. The outcome was
that the longer pieces diffused faster. The major WoK in this lab is reason and language.
The way that reason partakes in this lab is because, if the data cannot be read right, then a
reasonable conclusion cannot be processed. With that, then all of the data, including the entire lab will
be off and misunderstood. The limitations that reason has on this lab is because, like said above, that if
one thing is misinterpreted, then the entire thing is off. What I learned from this lab regarding reason is
that coming to a conclusion is harder without the knowledge of finding true reason. Senior Seminar has
helped me come to a fuller understanding of what should really go into analyzing something through
reason, therefore, I have really expanded my knowledge so I can make reasonable and logical
conclusions.
The second way of knowing that is incorporated in this lab is language. I chose language because
I find it hard to comprehend something that is out of my language ability. That is either because it is not
in English, or the level is too hard, and it contains too many difficult words. The limitation of language
creates a boundary of how much you can really understand. For instance, if I didn’t understand what
diffusion was (as the audience) then fully comprehending this lab will be difficult because the whole
concept of the lab is diffusion through a semipermeable membrane. I learned that, even though
language can be difficult to fully understand, that finding a way to get past that boundary is crucial.
Senior Seminar really helped me to understand and broaden my knowledge of language.
This lab is meaningful to me because I feel that I did a good job executing what I was trying to
say, even though I know I can improve on the next lab. Knowing my love for biology, I am happy to make
thoughtful comments on this lab, which in result would actually help me understand what I do best. I
learned a lot from Seminar to help to broaden my knowledge, and put that to good use.