AP Biology Lab 1: Diffusion and Osmosis
Mrs. Dietz
Rachel Butch
Laura Kelly
Period 7
1/7/11
Introduction:
The lab explores the scientific concept of osmosis and other forms of simple diffusion
through a membrane.
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Measure diffusion of molecules through a selectively permeable membrane (dialysis)
o Effect the size of pores has on what can diffuse through
Relationship between concentration of solution and movement of water through
selectively permeable membrane
o Hypertonic , Isotonic, Hypotonic (include definitions)
Water Potential
o Effect on the movement of water
o Physical factors that affect water potential
 Solute potential and its impact on water potential
 Pressure potential and its impact on water potential
 In lab, goal is to isolate water potential from all other factors
Calculation of water potential from experimental data
The effect of highly or lowly concentrated solutions on diffusion and the cells
themselves
o Hypotonic, hypertonic, isotonic
o Plasmolysis
All achieved objectives contribute to the purpose of the lab by demonstrating different
instances of simple diffusion, the conditions that facilitate it (hypotonic, hypertonic, isotonic)
and its potential effects on living cells.
Questions such as the following arose before starting the lab:
What exactly is water potential?
What factors affect the rate of osmosis?
How long can we keep the dialysis tubing out of the water before it dries out and cracks? – a
hole in the tubing would result in skewed results because more than just what is supposed to
would penetrate the selectively permeable membrane.
Methods and Procedure:
Lab 1 A:
1. Obtain 30-cm piece of 2.5cm dialysis tube that has soaked in water and tie off
one end so it can hold a liquid.
2. Test 15% glucose/1% starch solution for glucose with a test strip and record
results.
3. Add 15 mL of glucose/starch solution onto the bag, remove air and tie off other
end of the bag. Record results as well as the color of the solution.
4. Fill a 250mL beaker 2/3rds full of distilled water and add approximately 4mL of
Lugol’s solution…which is iodine.
5. Test beaker contents with test strip for the presence of glucose and record
results as well as the color of the solution.
6. Immerse bag in distilled water/lugol solution for 30 minutes. (we immersed for
20 minutes due to time constraints.)
7. Remove bag from beaker and record color change.
8. Test both fluids for the presence of glucose and record results.
Lab 1 B:
1. Obtain six 30cm strips of presoaked dialysis tubing.
2. Tie a knot in the bottom of each dialysis tube so it can hold fluid.
3. Pour 20mL of each of the following fluids into its own bag: Distilled water, .2M
sucrose, .4M sucrose, .6M sucrose, .8M sucrose, 1.0M sucrose.
4. Remove air from bag and tie shut at the other end leaving room for fluid
expansion.
5. Rinse each bag with distilled water(we used tap water to rinse).
6. Dry outside of bag and weigh and record mass.
7. Fill 6 beakers with distilled water (we used plastic cups) and soak for 30 minutes.
8. Remove bags and blot dry.
9. Weigh and record mass of each bag.
10. Calculate difference in mass and percent change in mass for each concentration.
Lab 1 C:
1. Pour 100mL of each of the following solutions into its own plastic cup (protocol
calls for 250mL beakers).
2. Slice a potato into 3cm thick segments and use a cork borer to create potato
cylinders, four for each beaker.
3.
4.
5.
6.
Determine mass of every four cylinders and allot to their own cup. Record.
Cover cups to prevent evaporation and soak overnight.
Remove cores from solutions and measure mass of every 4 and record.
Calculate change in mass and percent change of mass for each solution for
individual groups and also the entire class.
7. Graph data.
8. Our group ran this lab twice because of a potential mixup of the potato cores.
Exercise 1 D:
1. Do requested calculations.
Exercise 1 E:
1. Prepare a wet slide of onion epidermis and observe and draw under 100x
magnification. Observe, sketch and describe.
2. Remove coverslip and add 2 or 3 drops of 15% NaCl solution to cells on slide.
(protocol calls for adding drops on edge of the coverslip and drawing solution
across the cell with a papertowel, however we found that dropping the solution
directly on the cells was more effective, probably due to the lack of effective
paper towels). Replace coverslip and observe under 100x magnification. Sketch,
describe and explain what has happened to the cells.
3. Remove coverslip and flood cells with distilled water. Replace coverslip and
observe under 100x magnification. Sketch, describe and explain what has
happened to the cells.
For AP Biology Lab 1, we conducted four different experimental protocols to observe
osmosis and diffusion. In lab exercise A we utilized dialysis tubing, a 15% glucose and 1% starch
solution, distilled water, glucose test strips and Lugol’s solution. We began with filling a section
of dialysis tubing with the uncolored 15% glucose and 1% starch solution (tested positive for
glucose) and then sealing the bag off. We then filled a 250mL beaker with distilled water and
4mL of Lugol’s solution that turned the solution a deep amber. This solution tested negative for
the presence of glucose. Then, the bag dialysis bag, with glucose and starch solution inside, was
floated in the distilled water and lugol solution for twenty minutes. The lab protocol called for a
half-hour but we ran out of time in class. After twenty minutes’ time, we removed the bag from
the solution and observed the solution within to have changed to a dark-purple-to-black color.
What did the outside solution test as?
In exercise B, we investigated how concentration gradients across a membrane effect osmosis.
We filled six dialysis tubes with 20 mL of a sucrose solution. One bag of each: distilled water,
.2M sucrose, .4M sucrose, .6M sucrose, .8M sucrose, 1.0M sucrose. We then recorded the mass
of each of the bags before immersing them each in distilled water in their own beakers. Thirty
minutes later we removed the bags from the distilled water, blotted them dry and recorded
their mass, the difference in mass before and after soaking and the percent change in mass.
Exercise C is intended to further investigate the effects of concentration gradients of solutes on
water potential. First, a potato was cut into 3cm thick segments an then cut with a corer into
cylinders. Then, 100mL of each of the following solutions were poured into plastic cups:
distilled water, .2M sucrose, .4M sucrose, .6M sucrose, .8M sucrose, 1.0M sucrose. The lab
protocol calls for using beakers to contain the liquid, however plastic cups were used. Then,
four potato cylinders were chosen for each cup, massed and then immersed into their
respective solutions, covered to prevent evaporation and left to soak overnight. The next day,
the potato cylinders were removed from the solutions, and weighed. The differences between
their mass before and after were calculated as well as the percent change in mass for each
group.
In Exercise E, plasmolysis was observed. A wet mount of onion cells were observed under 100x
magnification. Then, the coverslip was removed and a few (2-4) drops of a 15% NaCl solution
were added to the solution over the cells. The coverslip was then replaced and the slide was
observed under 100x magnification. The protocol says to draw the NaCl solution over the cells
without removing the coverslip by using a paper towel, however dropping the fluid directly on
works better. Finally, the coverslip was again removed and the cells were washed over with
distilled water. The coverslip was replaced and then the slide was observed under 100x
magnification.
Results (Charts and Graphs):
The main finding of the lab is that the more hypertonic the internal environment
(dialysis bag, potato core, or onion cell) is compared to the external (beaker or cup holding it),
the more water will diffuse into the internal environment, and vice versa.
In Table 1.1, the data shows that in Exercise 1A, the placing of the substances in either
the external or internal environment affected direction of diffusion across the dialysis
membrane; if the H2O and IKI were inside the bag and the glucose and starch were in the
beaker, the external substance would have been the one to change color. As it is, since the
glucose and starch were inside the bag, water and IKI flowed into it, the IKI detected starch and
turned the inside of the bag purple. It is important to note that after exercise was completed,
there was no purple coloring in the external environment, nor was glucose detected.
As shown in the individual data collected for Exercise 1B in Table 1.2, the molarity of the
substance inside the dialysis bag affected the percent change in mass: except for in the case of
the 0.2 M sucrose solution bag, the greater the molarity of the sucrose solution inside, the
greater the change in mass of the bag after thirty minutes.
Though the class average percentages of changes in mass of the dialysis bags were
different than the individual data, it is important to note that in Table 1.3, the same pattern
occurred. The greater the molarity of the solution inside the bag, or the more hypertonic it is
compared to the water surrounding it in the beaker, the greater the percent change in mass
was. For example, the 0.4 M Sucrose dialysis bag increased in mass by 9.46% after thirty
minutes, while the 1.0 M Sucrose dialysis bag increased in mass by 18.29% - almost double the
0.4 M bag’s change.
Graph 1.1: Percent change in Mass of Dialysis Bag vs. Molarity of Sucrose Solution
further supports the data found in Tables 1.2 and 1.3. In this graph it is easier to clearly see the
relationship between the two variables.
In Table 1.4, the individual data for Exercise 1C are included. In this exercise there is a
relationship between the molarity of the contents in the beaker and the percent change in
mass of the potato cores. Contrary to the results of the previous exercise, the greater the
molarity of the sucrose solution in the beaker, the more negative the change in mass. For
example, the beaker containing 0.0 M Sucrose caused a change in mass of the potato cores of
23.53%. The cores increase in mass. However, the beaker containing 1.0 M Sucrose caused a
change in mass of the potato cores of -28.1%. The cores greatly decreased in mass.
Table 1.5 illustrates the group data for Exercise 1D, and further supports the individual
findings in Table 1.4. For each of the other lab groups in the class, the lower molarities of
sucrose in the beakers brought about increases in mass, while the higher molarities caused
negative increases in mass of the potato pieces.
Graph 1.2 combines Tables 1.4 and 1.5 visually. It demonstrates the decrease in the
mass of the potato cores as the molarities within the beakers increase. It is important to note
that the line representing the individual data crosses the X-axis, showing a 0% change, at 2.0
molarity of sucrose. The line representing group data crosses the X-axis nearer to a 0.32 M.
Graph 1.3…………..come back to once we figure out water potential
The illustration of Exercise 1E (1) shows how the onion cells normally look in freshwater,
an isotonic solution to the solution inside the cell. The vacuoles are full, but not bursting. The
cells look healthy and normal.
The illustration of Exercise 1E (2) shows that when the onion cells were placed in salt
solution, which was hypertonic to the solution inside the cell vacuole. Plasmolysis occurred:
the vacuole and cytoplasm shrunk.
The illustration of Exercise 1E (3) shows that when the onion cells were flooded once
again with freshwater, an isotonic solution, the vacuoles swelled again to a normal size, as did
the cytoplasm.