Abstract

A close scrutiny over the membranes suggests two crucial forces, diffusion and

osmosis, to reach a state of dynamic equilibrium. Diffusion represents the molecules’

natural proclivity to travel down its concentration gradient. A type of diffusion of water

molecules is called osmosis. The three experiments examined the molecules’ movements

through a selectively permeable membrane, depending on the variation of molarity of the

solution. Using IKI and starch and glucose solution, the first experiment inspected the

plausibility of IKI and glucose molecules to diffuse across a dialysis bag, a selectively

permeable membrane., The following experiment observed the percent change in mass of

the dialysis bags, which contained a solution of unknown sucrose molarity, in respect to

the different molarity of an equilibrium point, where the concentration of molecules

between the two solutions was equal. Similarly, the final experiment utilized potato cores

and different sucrose molarity solutions to find the equivalent point, 0.305M. Each

experiment ascertained the principle of diffusion and osmosis through molecules gradient

movement depending molarity of the solution. The coloration of the dialysis bag and

glucose and test strip proved that relative size of the molecules that could pass through

cell membranes. Due to the molecules’ tendency to move from higher concentration to

lower concentration, the mass of potato cylinders increased before the equivalent point,

0.35M, was reached; however, once the molarity achieved at 0.4M, the mass decreased,

losing its water to the surroundings.

1. Introduction

Diffusion and osmosis are important for all living organisms. Also, people apply

osmosis and diffusion to their lives like making Kim chi in Korea and sometimes use the

properties of diffusion to make better perfumes.

Diffusion is the movement of particles from higher concentration to lower

concentration. Once diffusion occurs, the higher concentration of a certain area will

decrease, and the lower concentration near the higher concentration area will increase1.

Then, as time goes on, the concentration will be in equilibrium. The driving force of

diffusion and osmosis is constantly moving particles of substances. In addition, diffusion

and osmosis is related to the entropy, which is a measure of randomness, or disorder. The

second law of thermodynamics applies to diffusion and osmosis because it states that all

energy transformation increases the entropy of universe6. For example, in the dorm, when

somebody makes popcorn and after few minutes, the smell of popcorn diffuses from

microwave to the whole hallway and even in the rooms.

Osmosis is one kind of diffusion, but it is little bit different because osmosis is the

diffusion of water. Osmosis is movement of water molecules through a semi-permeable

membrane from higher water potential to lower water potential. The water potential

represents the measure of free energy of water6, in other words, it represents how much

water can move. Osmosis works from lower solute concentration to higher solute

concentration. Also, Osmosis is passive, which means no energy input is required. Three

conditions of osmosis can exist: hypotonic solution, isotonic solution, and hypertonic

solution. Hypotonic solution is same as fresh water. Isotonic solution has equal

concentration, and there’s no net flow of water. Hypertonic solution is higher

concentration solution. In Korea, people apply osmosis when they make Kim chi. So,

Koreans put cabbage in salty water, and the water comes out from the cabbage. Osmosis,

then, makes cabbage shrivels and increases the concentration of Kim chi flavor by letting

out the water from cabbage.

Diffusion and osmosis is a crucial factor for all the living organisms. Diffusion

and osmosis are ways of eating for unicellular organisms such as amoeba. The substances

needed for amoeba diffuse through its membrane. For another example, in the human

body, lipids are diffusing through phospholipid bilayer, which is selectively permeable

and allows certain substances to go through the membrane. It enables organisms to let

waste out of cell and permits nutrition in to the cell needed by using concentration

gradient. However, since plasma membrane forms a selective barrier between the inside

of the cell and outside of the cell to prevent necessary nutrition going out or harmful toxic

going into the cell, not all substances can go through the plasma membrane. Osmosis in

plants is a vital action because it makes plants alive, but sometimes it also can kills

plants. Plants absorb water by using osmosis. However, if soil has higher concentration of

fertilizer than that of plants, the plants will give water out of the cell and shrivels. Thus,

Plants in hypertonic soil is very harmful.

2. Materials

Balance

250mL beaker

Cork borer

Dialysis tubing

Distilled water

15% glucose and 1% starch solution

Graduated cylinder

Lugol’s (Iodine Potassium) solution

Solution of unknown sucrose molarity

Pipet

Potato

Six plastic cups

Stirring rod

Sucrose

Test strip

3. Methods

Exercise A)

An approximately 30cm long and 2.5cm wide piece of dialysis tubing that had

been submerged in water was obtained. One end of the tubing was tied to form a

bag. The other end was rubbed together in order to separate the edges.

A 15mL of the 15% glucose and 1% starch solution was placed in the bag. The

other end of the bag tied off, leaving sufficient space for the expansion of the

contents in the bag. The color of the solution was recorded.

A 250mL beaker was filled with distilled water. Approximately 4mL of Lugol’s

(Iodine Potassium) solution was added to the distilled water. The color of the

solution was recorded.

The bag was immersed into the beaker of solution.

The setup was left for 30minutes or until a distinct color change in the bag or in

the beaker was displayed. The final colors of the solution in the bag and of the

solution in the beaker were recorded.

The liquid in the beaker was tested for the presence of glucose and the result was

recorded.

Exercise B)

Six 30cm strips of presoaked dialysis tubing were obtained.

A solution of unknown sucrose molarity, which was prepared by the instructor,

was poured into the six dialysis bags. Most of the air was removed from each bag

by drawing the dialysis bag between two fingers. Sufficient space, about one-third

to one-half of the piece of tubing, was left for the expansion of the contents in the

bag.

After each of the six dialysis tubing bags were dried, their initial mass was

measured and recorded.

150mL of each of the following solutions: distilled water, 0.2M sucrose, 0.4M

sucrose, 0.6M sucrose, 0.8M sucrose and 1.0M sucrose were prepared in the

separate 250mL beakers. Each beaker was labeled accordingly to indicate the

molarity of the solution.

Each bag was completely submerged into the beakers, which were filled with

different molarity of solution.

The setup was left for 30 minutes.

At the end of 30minutes, the bags were removed from the beaker.

Six bags were carefully blotted and their mass was determined. The final mass of

the bags were recorded.

Exercise C)

150mL of each of the following solutions: distilled water, 0.2M sucrose, 0.4M

sucrose, 0.6M sucrose, 0.8M sucrose and 1.0M sucrose were prepared in the

separate 250mL beakers. Each beaker was labeled accordingly to indicate the

molarity of the solution.

A cork borer was used to cut six pairs of four potato cylinders. Any potato skin

was peeled. Four potato cylinders were needed for each beaker.

Each set of four potato cylinders was separately weighed on the balance and their

mass was recorded.

Each set of potato cylinders was put into their respective beaker, which contained

different molarity of solution.

The beaker was covered with plastic wrap to prevent evaporation.

The setup was left for overnight.

The potatoes were taken out from the beaker and gently blotted on a paper towel.

Each pair of four potatoes’ mass was measured on the balance. Their mass was

recorded.

4. DataPresence of Glucose in Solution Through Dialysis Tubing

InitialContents

Solution Color Presence of GlucoseInitial Final Initial Final

Bag 15% glucose & 1% starch

Milky; foggy

Blue black Yes Yes

Beaker H2O & IKI Amber Amber No Yes

The data above, which recorded the color transformation of the solution in

dialysis tubing and presence of glucose in the beaker, justified the movement of both

glucose molecules and IKI molecules.

Percent Change in Mass of Dialysis Bags

Contents in Dialysis Bag

Initial Mass Final Mass Mass Difference

Percent Change in

Massa) 0.0M Distilled Water

7.29g 7.91g 0.62g 8.50%

b) 0.2M Sucrose

17.47g 17.56g 0.09g 0.52%

c) 0.4M Sucrose

10.22g 9.99g -0.23g -2.25%

d) 0.6M Sucrose

13.33g 12.44g -0.89g -6.68%

e) 0.8M Sucrose

13.48g 12.46g -1.02g -7.57%

f) 1.0M Sucrose

14.55g 12.85g -1.7g -11.68%

By comparing the initial and final mass of the six dialysis tubing, which were

submerged into the different solutions, the following data explicitly demonstrated the

variation in mass of the dialysis tubing.

Percent Change in Mass of Potato Cylinders

Contents in Beaker

Initial Mass Final Mass

Mass Difference

Percent Change in Mass

a) 0.0M Distilled Water

13.29g 16.19g 2.90g 21.82%

b) 0.2M Sucrose

13.50g 14.15g 0.65g 4.96%

c) 0.4M Sucrose

13.10g 11.60g -1.50g -11.45%

d) 0.6M Sucrose

13.28g 10.85g -2.43g -18.30%

e) 0.8M Sucrose

13.04g 9.41g -3.63g -27.84%

f) 1.0M Sucrose

12.57g 8.85g -3.72g -29.59%

The data above illustrated the change in mass of the potato cylinders depending on the

molarity of the solutions.

5.Result

The molarity, M, was calculated by drawing the best fit line, and setting y value

as 0. The sucrose solution and dialysis bag was in equilibrium in 0.31. The concentration

of mysterious solution was about 0.31M. So, molarity less than 0.31M showed an

example of hypotonic, and molarity higher than 0.31M showed and example of

hypertonic.

Potato core’s molarity, M, was examined by setting the y value as 0. Its molarity

was 0.35. Osmosis worked in the solution with potatoes. In lower concentration solution

than 0.35M, the water molecules went into potato cores, which told that solution was

hypotonic. However, in higher concentration than 0.35M, the water molecules went out

of potato cores, which showed that solution was hypertonic.

6. Conclusion

The difference of the solutions’ concentrations promoted two movements of

molecules between the solution in the beaker and that of the dialysis bag. In the first

experiment, the apparent color change of solution, from opaque white to amber, in the

dialysis tubing demonstrated that Iodine Potassium Iodide (IKI) had entered into the bag.

Conversely, the test strip proved the presence of glucose in the beaker, indicating that

glucose had left the bag.

The color transformation and the test strip confirmed that the molecules of

glucose and IKI were small enough to pass through the selectively permeable membrane.

The glucose and IKI molecules moved to equalize the concentration of each molecule

between the two solutions. IKI molecules presented in the beaker, which contained higher

concentration of IKI, moved into the dialysis tubing that had lower concentration of IKI

molecules. Similarly, glucose molecules moved from the higher concentration, dialysis

tubing, to the lower concentration, beaker, making the solute concentration of the two

solutions equal. Since the starch molecules were too large to pass the permeable

membrane, the color of the beaker remained as brown instead of turning into blue, stating

no reaction between the starch and the IKI. Based on the observation, the smallest size

ranked from IKI molecules, water molecules, glucose molecules, membrane pores, and

starch molecules.

Though the first experiment did not include any quantitative values, by recording

the initial and final percent change in mass of the solution presented in the dialysis

tubing, diffusion of water into the dialysis bag could be verified. Since the mass of

glucose was negligible, the variation of mass in the dialysis tubing indicated the

movements of the water molecules.

Hypothetically, if the experiment started with a glucose and IKI solution inside

the bag and only starch and water outside, the results could be predicted based on the

principles of diffusion and osmosis. Since the size of the molecules were small enough to

pass through the membrane pores, glucose molecules and IKI molecules would move out

of the dialysis tubing into the beaker. Correspondingly the water molecules, which were

smaller than the membrane pores, would move into the dialysis tubing from the baker. On

the contrary, the starch molecules would remain in the beaker because of their too large

molecules’ size to pass through the membrane pores.

The second experiment tested the relationship between the change in mass and the

molarity of sucrose within the dialysis bags. According to the result, the graph

demonstrated that as molarity of solution in the beakers increased, the change in mass

decreased. Knowing the molarity of the unknown solution, 0.31M, the mass of each bag

in this experiment could be predicted when all the bags were placed in a 0.4M sucrose

solution. Since the environment contained higher concentration of solutes, the water

molecules transferred from the potato cylinders to the environment, losing their weight.

Throughout the experiments, the percent change in mass was used rather than the

change in mass due to the different initial and final mass of the six bags. Change in mass

did not measure the amount of change proportional to the solutions. The change in mass

could be same despite the amount of actual difference made between the bags. The

percent change in mass, in fact, calculated the exact mass difference regardless of the

bags’ initial and final mass. The sucrose solution in the beaker would have been

hypertonic to the distilled water in the bag because the concentration of sucrose solution

was higher.

A dialysis bag was filled with distilled water and then placed in a sucrose

solution. The bag’s initial mass in 20g and its final mass was 18g. Then the percent

change of mass could be calculated as following:

Percent change of mass = [(final mass) – (initial mass)]/ initial mass x 100 = [18g (final mass) – 20g (initial mass)]/ 20g (initial mass) x 100 = 10%

Utilizing the concepts of osmosis and diffusion, the lab could be designed to

measure the speed of osmosis. For example, if the solution with dialysis bag was heated

up, the speed could be different. For another instance, if the seaweeds went into the

mysterious sucrose solution, osmosis might not occur because seaweeds came from salty

water.

In the second experiment, percent change in mass and molarity demonstrated that

osmosis worked. Percent change in mass was calculated by using balance, and molarity

was measured in the best fit line equation, which was same as figuring out x-intercept.

The molarity of mysterious sucrose solution was 0.31M. At this molarity condition, it

was isotonic, which meant the amount of flow of water between two solutions was equal.

In the third experiment, potato core’s molarity was calculated in the same way as in the

second experiment. Potato cores had 0.35M, and sucrose solutions below 0.35M were

hypotonic, upper than 0.35M were hypertonic, and sucrose solutions with 0.35M were

isotonic solution, or in equilibrium.

In order to obtain the solution with different molarities, a calculation had to be

done to measure the amount of sucrose needed for 150mL of distilled water. Using the

molarity equation, moles of solute / L of solvent, mass of sucrose could be calculated.

The molecular mass of the sucrose, 342g, was multiplied by 0.15mL to calculate the

amount of sucrose needed to make a 1.0M of sucrose solution. Then, to make 0.2M,

0.4M, 0.6M, and 0.8M, each molarity could be multiplied to 51.3, result of the previous

calculation. Through this calculation, the mass of sucrose needed to make each solution

with different molarities could be calculated.

Though the lab did not include any vagaries that would influence the result, there

were few possible errors. In the first experiment, the dialysis tubing could have leaked or

broke, transforming the solution in the beaker into a dark blue due to the reaction

between starch and IKI. The second experiments, which incorporated six different

molarities of solutions, could not have contained an exact amount of sucrose dissolved.

The different molarity would cause the dialysis tubing to gain or to lose its weight. In the

last experiment, a piece of potato skin might have been left unpeeled, causing erroneous

results. The potato cylinders might not have been perfectly dried when their mass was

weighed, causing additional weight of water to be included.

7. Bibliography

1.http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation

how osmosis works.html

Animation of Osmosis, McGrawhill.com

2.Biology 8th Edition by Campbell, Reece, Urry, Cain, Wasserman, Minorsky, Jackson

3.http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/diffus.html

Diffusion and Osmosis by Carl R. Nave, Physical Science Information Gateway,

4.http://biology.arizona.edu/sciconn/lessons/mccandless/ Diffusion, Osmosis, and Cell

Membranes, An Integrated Science Instructional Unit by John R. MacCandless. Jr

5.http://www.blobs.org/science/article.php?article=20, Diffusion and Osmosis by Tim

Shepard MBBS Bsc

6.Power Point “Osmosis and Diffusion” by Mary Poarch