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Diffusion

and

OsmosisBy James Marshall

AP Bio 11/2/10

Abstract:

In our lab reports, we take a look at osmosis and diffusion, and the effects it has on a

cell. In experiment 1-A, we look at diffusion. We see that certain molecules pass more easily

than others, if at all. A change in volume is also observed. In 1-B, we look at osmosis. This

experiment was to show how water osmoses when placed in a hypotonic solution. The water

rushes into the dialysis bag to make equilibrium throughout the solution. The change in volume

was observed, as the bag gained weight. In experiment 1-C, water potential of potato cores was

observed. The potato cores had a saturation point, which after that point, would inevitably loseweight. Activity 1-D was about onion cell plasmolysis. In this lab, we studied the effects of an

onion cell being placed in a hyper/hypotonic solution. The cell, when placed in a hypertonic

solution, gets the water sucked out of it. When the cell is placed in a hypotonic solution,

however, the water is rushed into the cell, making it turgid.

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Introduction: What is osmosis?

Osmosis is a simple way of transporting water into and out of a cell. This is always down

the concentration gradient, which means it takes no ATP up. The water molecules are very easy

to move since it is a small molecule and can travel through the hydrophobic core of the cell

membrane with relative ease. The flow of water is dependent on the solution that the cell is

placed in. In a hypotonic solution, water flows into the cell, since there is less solution on the

outside of the cell. When there is more solution in the cell, the water flows into the cell since

the waters concentration gradient is less in the cell. This will make the cell swell, and could

make the cell burst. When placed in a hypertonic solution, the water flows out of the cell. This

is the same reason as the hypotonic solution, as water had a tendency to flow down the

concentration gradient. When water flows out of the cell, the cell may die since it does not

have enough water to facilitate certain functions. When placed in an isotonic solution, water

does not flow. This is due to there not being a concentration gradient, and water does not

osmose unless there is more or less of the solution than the inside of the cell. Osmosis provides

the primary means by which water is transported into and out of the cell. Osmosis may be

opposed by increasing the pressure in the region of high solute concentration with respect to

that in the low solute concentration region. The force per unit area, or pressure, required to

prevent the passage of water through a selectively permeable membrane and into a solution of

greater concentration is equivalent to the osmotic pressure of the solution, or turgor. Osmotic

pressure is a colligative property, meaning that the property depends on the concentration of

the solute, but not on its identity. The osmotic gradient is the difference in concentration

between two solutions on either side of the cell membrane, and is used to tell the difference in

percentages of the concentration of a solute.

Hypothesis:

In lab 1-A, the hypothesis for this experiment can be described as a simple

understanding of diffusion. The hypothesis was that the solution would leave and enter the cell

(dialysis bag) differently and at different rates. When placed in a hypertonic solution, the

solution would move into the cell. And when placed into a hypotonic solution, the opposite

would occur; the solution would pass through the membrane, leaving the cell. In lab 1-B, the

hypothesis was when the bag was placed in a hypertonic solution, the water would leave the

cell; and when placed in a hypotonic solution, it would enter the cell. In lab 1-C, the hypothesis

was that the potatoes would reach a certain point, that water would no longer move into the

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cell. This is because the cell wall allows for a certain amount of water to enter the cell, before it

stops the flow, for the cells benefit. In lab 1-D, the hypothesis was that an onion cell, when

placed into a hypotonic solution, would become flaccid, due to water leaving the cell. When

placed into a hypertonic solution, it would become turgid, due to water entering the cell.

Materials:

In each experiment, different materials were used.

Lab 1-A:

y 1 cup, cleary 1 plastic funnely 15 cm soaked dialysis tubingy 15 mL of 15% Glucose/1% starch solution in medicine cupy 1 plastic Pipety 2 ct. glucose test stripsy 1 mL Starch indicator solution (IKI)y 2 ct. 10-cm pieces of stringy Distilled water

Lab 1-B:

y 6 ct. clear cupsy 6

ct. 15-cm dialysis tubing segmentsy 12 ct. 10-cm string segmentsy 1 plastic medicine cupy 10 mL distilled watery 10 mL 0.2 M sucrose solutiony 10 mL 0.4 M sucrose solutiony 10 mL 0.6 M sucrose solutiony 10 mL 0.8 M sucrose solutiony 10 mL 1.0 M sucrose solutiony Labeling tapey Paper towelsy Balances (shared between groups)y Distilled water (shared between groups)

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Lab 1-C:

y #2 Cork Borer (inner diameter approximately 5mm)y 1 potatoy Knifey Centimeter rulery 6 ct. clear plastic cupsy 6 ct. pieces of plastic wrap/aluminum foily Marking Pencily Labeling tapey 100 mL Distilled watery 100 mL 0.2 M Sucrose Solutiony 100 mL 0.4 M Sucrose Solutiony 100 mL 0.6 M Sucrose Solutiony 100 mL 0.8 M Sucrose Solutiony 100 mL 1.0 M Sucrose Solutiony Paper towelsy Balance (shared between groups)

Lab 1-D:

y 1 microscope cover slidey 1 coverslipy 1 forcepsy 1 pipety Onion Leavesy 15% NaCl solution in beakery Distilled water in beakery Paper towelingy Compound Light Microscope (shared between groups)

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Methods:

Lab 1-A:

1. Fill plastic cup with distilled water to within 1-2 cm of the top.2. Dip a glucose strip into the water in the cup for 1-2 seconds. Run the test strip along the

edge of the cup to remove any excess liquid. Wait approximately 2-3 minutes and

observe any color change on the strip. A positive glucose test is indicated by a greenish

color on the test strip. No color change will occur if the test results are negative. Record

the results in Data Table 1.

3. Using the plastic pipet, carefully add 20-25 drops of the starch indicator solution (IKI)4. Your teacher will provide you with a soaking dialysis tube segment. Gently rub the

tubing between your fingers to open it.

5. Tie one end of the tubing tightly with one piece of string. Make sure this end is tiedtightly enough to prevent any leaks from the end of the bag. Fill the tubing with water

and test it for leaks at a sink. Empty the tubing.

6. Insert the tip of the plastic funnel into the open end of the dialysis bag, and pour the 15mL of 15% gluxose/1% starch solution from the medicine cup into the tubing.

7. Squeeze all the air bubbles out of the tubing and tie the open end shut with anotherpiece of string. Note the color of the starch-glucose solution in the dialysis tubing and

record your observations in Data Table 1.

8. Briefly rinse the outside of the bag under running water. Squeeze the bag gently to besure that there are no leaks. If you find a bag leaking at an end, retie is securely.

9. Completely submerge the model cell into the cup of water and starch indicator solution.Allow osmosis and diffusion to occur for 30 minutes.

10. After 30 minutes, test the water in the cup for sugar content, as in step 2. Note anycolor changes in the dialysis tubing and in the cup. Record these observations in Data

Table 1.

11.Be sure to wash your hands and clean up and dispose of any waster materials asdirected by your teacher.

Lab 1-B

1. Number the plastic cups 1-6 with the pencil and labeling tape.2. Your teacher will provide you with 6 soaking dialysis tube segments. Gently rub

the tubing between your fingers to open it.

3. Tie one end of the tubing tightly with a piece of string. Make sure this end is tiedtightly enough to prevent any leaks from the end of the bag. Fill the tubing with

water and test it for leaks at a sink. Empty the tubing. Repeat for each segment.

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4. Using the medicine cup provided, measure out 10 mL of the appropriatesolution into the dialysis bag as follows:

Rinse the medicine cup between solutions. Be sure to gently squeeze excess air

from each bag. Tie off the other end of each dialysis bag with string.

5. Briefly rinse the outside of each bag under running water. Squeeze each baggently to be sure that there are no leaks. If you find a bag leaking at an end, retie

it securely.

6. Dry the outside of the bag with a paper towel, and obtain the mass of each bag.Record the initial mass of each in Data Table 2.

7. Place the correct model cell into each numbered cup and fill each cup enoughdistilled water to completely cover each model cell. Allow osmosis to occur for

30 minutes.

8. After 30 minutes, remove each bag from its cup. Gently blot excess moisturefrom the surface of each bag with a paper towel and obtain the mass of each

bag. Record the final mass of each bag in Data Table 2.

9. Be sure to wash your hands and clean up and dispose of any waste materials asdirected by your teacher.

10.For each of the solutions, subtract the initial mass of the dialysis bag from thefinal mass of the dialysis bag to obtain the change in mass and record this

information in Data Table 2. Be sure to record a positive result if the bags gained

mass, and a negative result if lost.

11. For each solution, divide the result you got in Step 9 by the initial mass of thebags and multiply by 100. This is the percent change in mass for each bag.

Record your results in data Table 2.

12.Calculate the class average percent change in mass of the bags in each solution,and record your results in data Table 3.

13.Construct a graph using Figure 1, illustrating the percent change in mass for bothyour groups data and the combined class average.

Bag to be placed in cup # Solution

1 Distilled water

2 0.2 M Sucrose

3 0.4 M Sucrose

4 0.6 M Sucrose

5 0.8 M Sucrose

6 1.0M Sucrose

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Lab 1-C

1. Label 6 clear plastic cups with the different solutions used in this lab. Pur 100 mL ofthe appropriate solution into the cups.

2. Push the cork borer completely through the potato. Remove the potato core fromthe borer. Carefully cut off each end of the core where the potato skin is.

3. Lay the core next to a metric ruler. Measure and cut a 3 cm long potato core section.4. Punch a total of 4 potato cores, each 3 cm in length. Obtain the mass of the 4 cores

together to the nearest 0.1 g. If you have to wait to use the balance, be sure to wrap

the cores in a plastic wrap or aluminum foil until the balance is available.

5. Place these 4 cores into one of the labeled plastic cups containing a solution. Recordthe initial mass of the 4 cores for this solution in Data Table 4.

6. Repeat Steps 2-5 with each of the remaining cups. Cover all of the cups with plasticwrap or aluminum foil to keep evaporation at a minimum.

7. Place the cups in a location where they wont be disturbed overnight.8. Remove the cores from one of the cups and carefully place them on a paper towel.

Dab the cores with the paper towel to dry them. Obtain the mass of the 4 cores

together. Record the final mass of the cores in Data Table 4.

9. Repeat step 8 for each of the cups.10.For each of the solution, subtract the initial mass of the cores from the final mass of

the cores to obtain the change in mass. Be sure to record a positive result if the

cores gained mass, negative if lost. Record in data table 4.

11.For each solution, calculate the percent change in mass for the cores as you did forthe dialysis bags in the previous exercise, record the results.

12.Calculate the class average percent change in mass of the cores in each solution indata table 5.

13.You will construct a graph of the percent change in mass for both your groups dataand the combined class averages in Figure 2. Plot the molarity of the solution in the

beakers on the X-axis, and the % change in mass of the potato cores on the Y-axis.

14.Using a ruler, draw a best fit line corresponding to the points on your graph. Thepoint at which your line crosses zero on the Y-axis is an approximation of the molar

concentration of solutes inside the potato tuber cells. This point identifies the

molarity of a sucrose solution that has the same water potential as that of thepotato tuber cells.

15.Be sure to wash your hands and clean up and dispose of any waste materials asdirected by your teacher.

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Lab 1-D

1. Using the forceps, remove a small piece of onion epidermis (onion skin) froman onion leaf. Make a wet mount slide of the epidermis and view at 100X

magnification. Sketch a few of the onion epidermis cells in the analysis section.

2. Without removing the slide from the microscope stage, place 2-4 drops of theNaCl solution at the left edge of the coverslip.

3. While viewing the onion skin through the microscope, hold a piece of papertoweling at the right edge of the coverslip, touching the fluid on the slide. This

will wick the fluid from the left side of the slide to the right. Note any changes

that occur.

4. Now repeat steps 2-3 using distilled water instead of the NaCl solution. Note anychanges that occur in the cells during this process.

Results:

Lab 1-A:

Characteristic Water in cup Solution in TubingInitial Color Yellow Colorless

Final Color Yellow Black

Initial Glucose (+/-) - +Final Glucose (+/-) + +

Initial Starch (+/-) - +

Final Starch (+/-) - +

Initial Solutes Iodine Starch Indicator Glucose, Starch

Final Solutes Iodine Starch Indicator Glucose, Starch

Change in Volume Observed Increase Decrease

Lab 1-B:

Beaker With: Initial Mass (g) Final Mass (g) Change of Mass

(g)

Change of Mass

percentage (g)

Distilled Water 2.2 9.3 7.1 23.6

0.2 Sucrose 0.2 10 9.8 2

0.4 Sucrose 3.4 10.6 7.2 32

0.6 Sucrose 1 10.6 9.6 9.8

0.8 Sucrose 0.8 9.8 9 9.08

1.0 Sucrose 12 11.6 -0.4 1.03

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Lab 1-C

Cup Containing Initial Mass (g) Final Mass (g) Change in Mass (g) % Change in mass

(g)

Distilled Water 4.5 5 0.5 25%

0.2 M Sucrose 5.3 5.6 0.3 5.7%

0.4 M Sucrose 2.6 2.2 -0.4 -18%

0.6 M Sucrose 5.5 3.8 -1.7 -41%

0.8 M Sucrose 5.1 3.5 -1.6 -42%

1.0 M Sucrose 6.1 3.9 -2.2 -47%

Lab 1-D

Prior to Adding Solution After Adding Solution

After flooding with water

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Error Analysis:

Each lab followed a specific trend of data with the rest of the class. There were certain

parts of 1-B that most likely, contained errors. First off, the set of data for the 1.0 solution ofsucrose lost mass, which in turn makes no sense. If it were following the trend of the others, it

wouldve gained mass. This is because when a cell is placed into hypotonic solution, water

rushes into the cell, making the cell gain mass. This is possibly caused by a loose dialysis bag

being tied.

Discussion and Conclusion:

The data in 1-A, it shows that certain solutes move through the cell membrane atdifferent rates. It shows that proteins and hydrophobic materials pass through the membrane

with relative ease. This is because of the core of the plasma membrane being made of

phospholipids. In 1-B, the hypothesis is supported by having each dialysis bag gain mass. The

bags are placed in a hypotonic solution, and the water is rushed into the bag, making it gain

weight. This makes sense as that is the tendency of cells. In 1-C, the hypothesis is once again

supported. The data shows that there is a point in which the potato does not gain any more

mass due to the cell wall. The potato starts to lose mass after being placed into a 0.4 solution

and every solution after that. In 1-D, the hypothesis is once again supported, as the onion cells

bulge when placed into a hypotonic solution. The cells do not change when placed into a watersolution, since there is no solute.