Investigating the Movement of Materials across Selectively Permeable Membranes
Grisha Agamov and Ali Murad Büyüm
Abstract:
The reason of this experiment was to observe diffusion and osmosis, and to
understand the sizes of molecules and the chemical reactions taking place because of the
diffusion. A glucose/starch solution was used inside the dialysis tube for the first experiment,
whereas cellulose solutions of different molarities were used for the second experiment. The
starch was inside the dialysis tube in order to show its reaction with potassium iodide, as the
latter can pass from the beaker into the tube. The glucose/starch solution's change of color
showed that the potassium iodide was able to pass through the tube into the solution. During
the second experiment, solutions of different molarities were used, and a relation between the
molarity of a solution and the percentage change in mass was sighted, both being
proportional to each other after osmosis. The change in mass of the tubes explained that
water had entered the tubes, because of the molar concentration.The concentration of water in
each tube was lower than the concentration of water in each cup.
Keywords:
Membrane, diffusion, osmosis, dialysis tubing, pore, semi-permeable.
Introduction:
A membrane is a structure through which matter transfer may be realized by the help
of different forces (Koros et. al. 1996). The Plasmalemma, also known as cell (plasma)
membranes are biological membranes that separate the inner environment of every cell from
the outer one. They have a complex structure, having two layers that include hydrophobic
(lacking affinity for water) tails and hydrophilic (having a strong affinity for water) heads.
Each head normally has two tails.
1 The bilayer is a natural barrier that prevents ions, proteins and other molecules that are
needed, from diffusing (Taylor, 2005). It is like a building material that forms the biggest part
of the membrane and consists of phospholipids and cholesterol. However it is not the only
thing that forms the membrane. There are integral and transmembrane proteins.
Transmembrane proteins usually play the role of ionic channels or transporters of
elements from the outer environment to the inner and vice versa. Integral proteins often work
as receptors. Also, there are other important structural elements - carbohydrate chains, which
mark cells, and help other substances inside it - cell interaction, guiding the hormones to its
receptor, etc.
Diffusion is the event of the movement of some substances from where they are high
in concentration towards where they are less, according to the concentration gradient.
Membranes allow the diffusion by letting certain molecules pass through it. Small pores in
the semi (selectively) permeable membranes, which flow only towards one way, allow the
process of osmosis to continue. These pores let only tiny molecules to go through them (e.g.
Dihydrogen monoxide or ions of Potassium and Sodium).
Another way of transportation is called facilitated diffusion, which is not a type of
diffusion, but a method of element movement through membranes, with the help of integral
proteins. Facilitated diffusion, osmosis and diffusion are energy-free processes, so they do
not need the cells to spend energy on them. Together, they are called "Passive transports"
(Baggott, 1995).
There are few more ways of transportation of material in and out of cells.
1) "Active Transport", which is the opposite of Passive Transport, because energy is needed
so that the proteins can move against the concentration gradient.
2 2) "Endocytosis", which is divided into "Phagocytosis", the absorption of solid elements by
the vacuoles of the cell, and "Pinocytosis", the absorption of liquid elements by forming
bubbles (vesicles) to pull the substance into the cell.
3) "Exocytosis", which is the opposite of "Endocytosis". The word "Exocytosis" means
getting rid of something. This is done by the cell throwing it out in a bubble.
Osmosis was first observed by Aristotle, when he placed salty sea water inside a wax
can and saw that distilled water came out the wax can's walls. It was discovered by JeanAntoine Nollet in 1748, but the exploration started nearly a century later. Zacharias Jansen
invented the "compound microscope" in the year 1590, which helped scientists observe
microscopic matter. Robert Hooke (1635 –1703) began exploring the microscopic substances
and eventually, he discovered a thin sheet of cork, which looked under his microscope like a
bee comb pattern with tiny holes. Years later, the Royal Society of London discovered an
important manuscript, where plants were seen and inside these plants, an amount of
“bubbles” were discovered. These bubbles were first thought to be filled with air. (Ling,
2007)
The objectives of these experiments were to distinguish the chemical reactions taking
place inside the dialysis bag (like the iodine combining with starch to create a purple colored
substance), to understand the size of the molecules and whether they could pass through the
membrane or not (like glucose and starch, the latter being too big to pass through), to
calculate the difference and the percent change between the initial and the final mass of
solutions that have different molar concentrations, after diffusion.
The knowledge of Osmosis is crucial, because it is everywhere around us. As for
human beings, it is very important for us to stay alive, so it is important to know that drinking
salt water would be bad for us as salt absorbs water from the human body and dehydration
occurs with the organism.
3 For example, during any surgical operation, there is always blood loss which is why a plasma
solution is injected into the patient's body, which consists of 0,9% of NaCl, in order to
balance the concentration of water and salt in our bodies, as the red blood cells would burst if
stored in water, because Osmosis would draw too much water into the cells. Similarly, if the
cells were placed in a solution with a higher solute concentration, or hypertonic solution,
osmosis would draw water out of the cells until they dry up.
Materials and Methods:
Diffusion:
15 ml of glucose/starch solution was prepared and was put into a graduated cylinder.
Meanwhile, a dialysis tube was soaked in water and a firm knot was tied on one end. The
other end of the tube was carefully unrolled. The already-prepared solution was poured inside
the dialysis tube. The color was indicated on the sample table 1. In order to see whether the
glucose that was originally inside the solution was still present, a glucose indicator paper was
put inside. The information was written on the sample table 1. The other end of the tube was
tied with enough space provided for the possible expansion of the tube. A 250 ml beaker was
filled with distilled water, and 1 ml of potassium iodine was set inside. Again, the color was
observed and was noted on the sample table 1. The glucose level was measured one more
time with a glucose indicator strip, and the data was collected. The dialysis bag, now tied on
both ends, was submerged in the solution. Half an hour later, the dialysis bag was taken out
from the beaker, and the color was checked for the last time. Also, the quantity of glucose
inside the beaker and the dialysis bag was found out. To measure the glucose level in the bag,
scissors were used to cut a hole on the bag. The presence/absence was copied on the table.
Osmosis:
Six plastic cups were marked "water, 0.2 M, 0.4 M, 0.6 M, 0.8 M, 1.0 M". Six
dialysis bags were soaked in water. A knot was tied on one of the ends of the tube.
4 One of the ends of the tube was unrolled. 25 ml of the still water was poured inside the tube
and the open end was tied to create a bag. The tube was dried up and was put inside the cup
that contained water. For each cup this technique was repeated by putting a different sucrose
solution to each tube, and by matching them with the corresponding cup.
The bags were dried up, their weight was measured and recorded on the sample table 2.
These bags were placed in six new cups, that contained 3/4 distilled water.
Half an hour later the bags were taken out, dried and had their weight measured. A formula
was utilized in order to calculate the percentage change in mass of each bag.
% Change = (Final mass - Initial mass) / Initial mass x 100
Each result was written down. The results of each person in the class and the average was
recorded.
Results: Table 1 contains the usage of color and glucose content in order to see the diffusion,
during 30 minutes. The glucose/starch solution inside the tube was transparent at the start, but
since starch is too big to pass through the membrane of the dialysis tube, the potassium
iodide, which was inside the beaker, and gave the beaker a light brown (amber) color, passed
the membrane, inside the tube, and combined with starch in 30 minutes that gave a dark
blue/purple colored substance. Not all of the potassium iodide passed through, so the water in
the beaker kept its amber color, after 30 minutes. Also, a diffusion of glucose was observed,
because at the start, glucose was only inside the tube, whereas after 30 minutes, traces of
glucose were found inside the beaker.
Table 1. The measurement of diffusion on the basis of color change and glucose content. 5 Table 2 contains the differences of mass of each dialysis tube, containing water, 0.2
M sucrose solution, 0.4 M sucrose solution, 0.6 M sucrose solution, 0.8 M sucrose solution
and 1.0 M sucrose solution, before and after they're put in water. The percentage change in
mass increases as the molar concentration of the solution gets higher.
Table 2. The measurement of the initial mass, final mass, change in mass, and % change
in mass in the movement of water through various sugar solutions.
Discussion and Conclusion:
The results shown in Table 1 proves that with the use of the iodide potassium solution
in beaker and the glucose/starch solution in the dialysis tubing, the iodide reacted with the
starch inside the tubing and the color of the solution inside the tube changed to dark blue, but
in the beaker the color stayed amber (did not change), because there was still potassium
iodide inside. Analysis of the solution in the beaker showed that it also contained glucose but
in a very small rate (only 100 mg.). Before the experiment, large amounts of glucose were
found inside the dialysis, but in the beaker there was none.
After the experiment, glucose was found in both the tubing and the beaker, but inside the
beaker, only small amounts were found.
Potassium iodide was only in the beaker before the experiment but after half an hour,
diffusion occured and iodide was in both the beaker and the tubing.
6 This means that pores of the membrane are big enough for the potassium iodide to pass
through them.
The results shown in Table 2 proves that osmosis will be in larger amounts when the
molarity of the solution is higher, and thus the change in mass and the % change in mass are
greater. The cellulose solutions each had different molarities (0.2 M, 0.4 M, 0.6 M, 0.8 M,
and 1.0 M) and their change in mass was in ascending order respectively, after they were put
in distilled water for about half an hour.
In conclusion, the bigger the molarity of the solution, the higher the change in mass.
Also, starch molecules are too big to pass through the membrane of the dialysis tube, which
is why only the solution inside the tube turned dark blue, and small amounts of glucose
passed through the dialysis tube, into the beaker. The difference between the Initial and the
Final Mass shows that Osmosis has occured, which is the change of the concentration of
water inside the dialysis tube and the cup. The concentration of water in each tube was lower
than the concentration of water in each cup. So, water went from the cup inside the dialysis
tubes, increasing their mass. As the molarity goes higher, the % change increases.
There could be a few mistakes made during the experiment. First of all, the beakers
were used by other students before the experiment and they probably weren't washed in a
proper way after the previous usage, so some substances could have been stuck on the walls
of the beakers. The same thing could have happened with cups in the second experiment.
Also, some dialysis tubings were dropped. The pipettes were very actively used by students
from different groups, they were all around the place and substances with different molarities
were probably taken with the same pipettes, so that the molarity balance wasn't accurate.
Knots weren't tied properly on the ends of the tubes, probably causing a leakage of the
sucrose solution in the cups. There also some possible improvements. The beakers must be
carefully washed before the next lab. Dialysis tubings shouldn't be dropped.
7 Experiments are usually more accurate with the equipment given, to be sure that someone
else did not use any tool (for example, the pipette) for a different substance.
Just to be sure that the results are realistic, the whole sequence should be repeated one more
time.
References Cited:
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Cambridge University Press, Cambridge.
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Beginning in Mid-19th Century to Its Disproof 45 Years Ago — though Still Taught
Worldwide Today as Established Truth", (2007), Physiol. Chem. Phys. & Med. NMR
(2007) 39: 1–67
Baggott, James; Dennis, Sharon E."Membranes", (1/5/1995), Netbiochem.
Koros, W.J.; Ma, Y.H. and Shimidzu, T. "Terminology for Membranes and Membrane
Processes", (1996), IUPAC.
Reece, Jane B.; Urry, Lisa A.; Cain, Michael L.; Wasserman, Steven A.; Minorsky, Peter V.;
Jackson, Robert B. and Taylor, Martha R. "Study Guide for Campbell Biology",
(2005), Benjamin Cummings (Pearson).
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