Relationship Between Molecular Size and Rate of Diffusion

DIFFUSION AND OSMOSIS

• Minh Thu Vo

INTRODUCTION

The center of this lab states around the diffusion across a cellular membrane, how exactly materials move and diffuse in concentrations. Both diffusion and osmosis are forms of movement that are part of passive transport dealing with cell membranes. Diffusion is where the solutes move from an area of high concentration to a low concentration. Water goes through the cell membranes by diffusion. Osmosis is specifically the movement of water through membranes. Since osmosis and diffusion are both part of passive transport, this means that they do not require energy or pumps. There are different environments created due to diffusion. There are hypotonic, hypertonic, and isotonic environments. Hypotonic is when the solution has a lower solute concentration compared to the water potential. The hypertonic solution has a higher solute concentration and lower water potential. In an isotonic solution, there is no net movement and there is an equal concentration of solutes and water (Veno). In our lab, we modeled diffusion and osmosis with a hospital scenario. It is important for a solution to have sugar sucrose in it so the water and solute can be equal to create an isotonic environment. If there wasn’t, there would either be a hypotonic causing the cell to burst, or there would be hypertonic causing the cell to shrink.

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Purpose of this experience states the relationship between molecular size and the rate of diffusion across a semi-permeable membrane. The osmotis behavior in plant cells and the relationship between the osmolarity of the surrounding solution and this behavior. Each cell type have a unique osmolarity and be able to quantitatively estimate the osmolarity of plant cells experimentally with the use of solution of varying solute concentrations (Lab manual).

We created models of living cells by using dialysis tubing. The dialysis tube represented the cell membrane to act as selectively permeable to water and some solutes. Osmosis is a particular kind of diffusion, because the diffusion happens with water molecules moving from an area of greater concentration to an area of lesser concentration, passing through a selectively permeable membrane. By taking into consideration the movement of two liquids (the iodine and the starch) through a semi-permeable membrane (the dialysis tubing), and bearing in mind the definition of osmosis, we should expect the two substances to mix with each other until the entire content of the test tube appears a homogeneous mix of starch and iodine.

However, we know from the background information that starch molecules are very large carbohydrate molecules, and we also know that selectively permeable membranes only guarantee the passage to small or medium molecules; by knowing this additional information, we can state the following hypotheses.

If the starch molecules are too large to pass through the selectively permeable membrane (the dialysis tubing), then the iodine (which has small molecules) will expand from the space around the dialysis tubing (point of high concentration), and move through it, going to the inside of the tubing (point of low concentration) until equilibrium is gained; while the starch will not manage to diffuse throughout the test tube, and so will remain inside the tubing and never achieve equilibrium.

Instead, if the starch molecules are small enough to pass through the selectively permeable membrane (the dialysis tubing), then the iodine will expand from the space around the dialysis tubing (point of high concentration), and move through it, going to the inside the tubing (point of low concentration) until equilibrium is gained; while the starch will diffuse from inside the tubing (point of high concentration), move through it and diffuse out the test tube (point of low concentration), and so will also achieve equilibrium.

MATERIALS AND METHODS

Part A: Diffusion of molecules through a selectively permeable membrane

Prepare the dialysis bag with the initial solutions of starch and glucose, then tight the bad by rubber band. Pour water into a baker then add several drop of I₂KI to have the color light brown. Place the bag of mix solution in the beaker and wait about 30 minutes to remove this bag into another dry beaker. Pour the solution of beaker into a clean tube, add some drop Benedict’s reagent to tube then heat it.

1) iodine entered the bag, because the solution within the dialysis bag changed from a clear color to a blue/black color. We know because the solution within the bag contains starch that the blue/black color is caused by the diffusion of iodine into the bag, which then reacted with the starch present. Glucose diffused out of the bag, an area of high glucose concentration, into the beaker solution, an area of low glucose concentration. We know this because after the experiment was conducted we tested the beaker solution for glucose using Benedict’s solution. This indicates that glucose molecules are small enough to diffuse through the membrane.

2) The movement of iodine resulted from the relative high concentration of it outside the bag compared the solution within the bag. It also occurred because it is small enough to diffuse through the pores in the membrane. The glucose was in higher concentration in the bag compared to outside the bag resulting in it to diffuse through the bag. Glucose was able to diffuse through the bag because it is small enough, but we know the starch didn’t diffuse, even though there was a concentration gradient, because the outside solution after the experiment was conducted didn’t react to iodine. This means the starch was too large for the membrane

3) I would expect the glucose and IKI molecules the diffuse out of the bag as a result of the higher concentration of the two of them inside the bag in relation to the outside of the bag. When the I2KI diffuses out of the bag I would expect it to react with the starch present outside of the bag and turn the solution blue/black. I would also expect the solution outside the bag to react positively to Benedict’s solution at the end of the experiment due to the glucose that diffused into it. The starch is too large to diffuse, so I would expect it not diffuse at all.

Part B: Osmotic behavior in cells

Prapare a slide of Elodea in sucrose O.5M solution and another slide of Elodea in distilled water.

Part C: Estimating the osmolarity of plant cells

Add 100mL of each solution in to 7 beakers as following order: DI water, sucrose 0.1M, 0.2M, 0.3M, 0.4M, 0.5M, 0.6M. Use sharp blade the get 7 sample of potato, weight each sample then transfer to each beaker solution. Incubate them about 1 hour then remove all sample out of beaker, blot them onto paper to dry then weight each sample.

The experiment we conducted gave the result that when a potato has contact with a sucrose solution, the cells will start to shrink. We did this by cutting up slices of potato and weighed them and recorded their initial weights. We used deionized water and different concentrations of sucrose. We than placed each potato slice in beakers with their designated solutions. We let them soak and incubate for an hour then we removed them from the beakers. Next we measured their new weights and recorded them. The objective of this experiment is to detect diffusion and osmosis in potato cells in sucrose solutions. Diffusion is the spontaneous spread of molecules from an area of high concentration to an area of low concentration. Osmosis is a type of diffusion involving water. The results will either be hypertonic or hypotonic. Hypertonic means that the solution is more salute than water. Hypotonic means less salute and more water. For this lab, we can apply these principles to plant cells. The plant cells have a semi permeable membrane so they can experience osmosis and diffusion as discussed earlier. When it comes to the potato cells, added with sucrose it will experience a reaction changing the form of the cells. The hypothesis of this experiment goes as follows, if we place potato cells in a sucrose solution, then the cells will shrink. They will shrink because when the sucrose is added the water in the potato, where there is high concentration, will move to the outside of the potato where there is low concentration.

RESULTS

Part A

Table 1: Iodine and Benedict’s test

Part C

Graph molarity of sucrose solution vs % change weight potato

DISCUSSION

Part A

The clolor of beaker change to orange after Benedict’s test that indicate glucose molecule are small enogh to disuse through the membrane. Glucose diffused out of the bag, an are of high glucose concentration into the beaker solution where area of low glucose concentration.

1) iodine entered the bag, because the solution within the dialysis bag changed from a clear color to a blue/black color. We know because the solution within the bag contains starch that the blue/black color is caused by the diffusion of iodine into the bag, which then reacted with the starch present. Glucose diffused out of the bag, an area of high glucose concentration, into the beaker solution, an area of low glucose concentration. We know this because after the experiment was conducted we tested the beaker solution for glucose using Benedict’s solution. This indicates that glucose molecules are small enough to diffuse through the membrane.

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2) The movement of iodine resulted from the relative high concentration of it outside the bag compared the solution within the bag. It also occurred because it is small enough to diffuse through the pores in the membrane. The glucose was in higher concentration in the bag compared to outside the bag resulting in it to diffuse through the bag. Glucose was able to diffuse through the bag because it is small enough, but we know the starch didn’t diffuse, even though there was a concentration gradient, because the outside solution after the experiment was conducted didn’t react to iodine. This means the starch was too large for the membrane

3) I would expect the glucose and IKI molecules the diffuse out of the bag as a result of the higher concentration of the two of them inside the bag in relation to the outside of the bag. When the I2KI diffuses out of the bag I would expect it to react with the starch present outside of the bag and turn the solution blue/black. I would also expect the solution outside the bag to react positively to Benedict’s solution at the end of the experiment due to the glucose that diffused into it. The starch is too large to diffuse, so I would expect it not diffuse at all.

Part B

Base on predictions and observation, 0.5M sucrose is hypertonic solution and distilled water is hypotonic solution. Sucrose has the greatest osmolarity. I expect pond water would be hypertonic because it contains compound that make expand to the cell wall (dangerous for cell), in fact pond water is a hypotonic.

Part C

Base on graph, at sucrose molarity 0.2M is the curve cross the zero change line. This information can be used to determine the osmolarity of the potato tissue. Sucrose 0.2M make no change weight potato that means the natural potato sucrose molarity at 0.2M. If solution has higher molarity than potato, it loses weight because water move out of cell. In contast, the solution is lower molarity then it gains weight and water move into the cell. Thus I can estimate the osmolarity of the potato tuber tissue is sucrose 0.2M.

From the results taken, the more concentrated solutions of sugar seemed to draw the water towards it. This supports my hypothesis and also shows that sugar is also a hypertonic solution since it is were there was more water brought into the solution than outside of it.

This could be applied in the real world with people who suffer from dehydration. By raising the sugar level in their body, they are more likely to take in more water into their cells. One thing that could be tried in future experiments could be to add additional types of solutions to the experiment that would simulate more of the human internal cell system. By adding more substances, the chances of seeing how osmosis truly works in the body can be seen that could prove useful for medical purposes (Towle).

REFERENCES

CSULA, lab manual Biol 100b, 2015

“Plasma Membrane” Wikipedia, the Free Encyclopedia. 2009. 8th Nov. 2009.

Towle, Albert, Modern Biology, Holt, Rinehart and Winston Inc., Orlando, Fl, 1993.

Veno, Barbara, slides and take notes biol 100b