Osmosis 1

OSMOSIS

By: Shelby Lazorka

Lab Partners: Laurel Miner and Kristi Raible

Biology 120-949

Professor Aguayo

October 31, 2012

Osmosis 2

OSMOSIS

INTRODUCTION:

By definition, osmosis is the diffusion, or dispersion, of water through a selectively

permeable membrane from a higher concentration to a lower concentration (Campbell, pg.132).

There are many different factors that can affect the rate of osmosis such as temperature, particle

size, and the size of the concentration gradient. A higher temperature can cause osmosis to occur

at a much faster rate because the molecules are more likely to pass through the selectively

permeable membrane quicker than they would at a lower temperature due to the molecules’

higher energy. In addition, the size of the particles can affect the rate of diffusion through the

membrane. Without this diffusion, osmosis cannot take place. In the experiment, the sucrose

molecules are too large to pass through the dialysis bags while the water molecules are easily

able to move freely in and out due to their small size. Lastly, the magnitude of concentration

gradient can greatly affect the rate of osmosis due to the fact that the more hypotonic or

hypertonic a solution is, the faster the movement of water molecules and other particles. With

more of the solute in the dialysis bag, the amount of water molecules in the bag as compared

with the amount in water molecules in the beaker of just water is much lower. Water molecules

move from areas of a higher concentration to a lower concentration to move down their

concentration gradient. Because of this, the water molecules will move at a faster rate during

osmosis and diffusion. The magnitude of the concentration gradient will play the biggest role in

this experiment.

A permeable membrane is a type of membrane that only allows certain molecules to pass

through it. The selectively permeable membrane in this experiment was the dialysis bags.

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However, as stated by Hawthorn, a semi-permeable membrane is not absolutely necessary for the

demonstration of osmosis. The concentration within a cell can change merely by the splitting of

larger molecules into smaller ones, or the reverse. (1930) Two objectives in this experiment were

to show the effects of osmosis and to show how one variable can affect the rate at which osmosis

takes place.

METHOD AND MATERIALS:

In this experiment, five dialysis bags were used. My lab partners and I placed clamps on

both ends of each of the tubes to keep the solutions in their bags and in their rightful place. Two

of the dialysis bags contained tap water. The other three solutions inside the bags were 20%

sucrose (at least 10mL), 40% sucrose (at least 10mL), and 60% sucrose (at least 10mL). To

perform this lab, we prepared each of the five bags with their designated sucrose solution. In this

experiment, the dialysis tubing acted as the selectively permeable membrane. The dialysis

tubing allowed the passage of water molecules only because the sucrose molecules are too large

to pass through the membrane. We then placed the bags into five separate beakers. Four of these

beakers contained only tap water and the fifth contained a solution of 40% sucrose.

We filled the first dialysis bag with 10 mL of tap water. Next, we filled the second bag

with 10 mL of the 20% sucrose solution. The third bag was filled with 10 mL of 40% sucrose

solution while the fourth bag was filled with 10mL of 60% sucrose solution. The final bag was

also filled with 10 mL of tap water. We made sure that when filling these bags that we removed

as much air as possible before clamping off the end of each of the bags. In addition, we made

sure that all of the bags were soft and floppy instead of firm to insure that experiment would

work properly. Next, we placed the dialysis bags one through four in there own separate beakers

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filled with tap water. But, our fifth bag full of tap water was placed in a beaker filled with just

enough of the 40% sucrose solution to cover the bag. All of the bags remained in their designated

solutions for 90 minutes. However, all of the bags were quickly removed every ten minutes to

weigh and record each of their masses with the same scale every time. Before weighing the bags

after they were removed from the respective solutions, we had to blot them dry to minimize

error. After all the data was collected, we had to use a corrected cumulative change in weight

equation to fix any variations in our data. For this equation, every time a weight was recorded,

we had to subtract it from the weight of the bag prior to the one being measured.

RESULTS:

The rate of osmosis was greater when there was more of the hypotonic solution in the

bag. On the contrary, the more hypertonic the bag in a sucrose solution, the more the mass of the

bag decreases. The graph shows the differences in the corrected cumulative change in weight in

grams of the four bags over the 80 minutes of time given because our group could not obtain data

for the last 10 minutes of the experiment. The bag containing the 20% sucrose solution gained

the least amount of weight because it had a lower concentration of sucrose. The water diffused

into the bag at the slowest rate. The 40% filled sucrose bag gained more weight than the 20%

sucrose bag because the concentration of sucrose was larger and thus, contained less water then

in its beaker. The water diffused into the bag at a quicker rate than the 20% sucrose solution.

The bag that contained the most weight was the 60% sucrose bag due to the fact that it had the

largest difference in concentration of sucrose to water and the water diffused the quickest across

the membrane.

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DISCUSSION:

Our graph showed that the 20% sucrose bag gained 3.2g of water over the course of our

80-minute experiment. Also, the graph showed that the 40% sucrose bag gained 5.7g of water

over 80 minutes. The 60% sucrose bag gained 9.1g of water and the bag of water in the 40%

sucrose solution lost 4.3g of water. In this experiment, the magnitude of concentration of the

sucrose solution increased as well as the average weight gain of the bag of sucrose concentration.

One change that could be made to make the experiment more accurate and precise would

be to record the mass of the clips and the dialysis bags at the same time. This way, the mass

could be truly precise instead of somewhat correct. The weight of the clamps and the bags could

be subtracted from the total overall weight with the amount of water that travels in and out of

each of the bags, making the measurement of the water more accurate and precise.

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References

Campbell, Reece, Urry, Cain, Wasserman, Minorsky, & Jackson (2008). AP Edition

Biology (8th ed.). San Francisco, CA: Pearson Benjamin Cummings.

Hawthorne. Diffusion, Osmosis and Osmotic Pressure. The Scientific Monthly. (1930) 31.6.

535-542.