Investigating Osmosis

Portfolio Template for AP Biology A, Unit 6
I had a volunteer carry out an experiment in my kitchen to study osmosis, using the following
procedure: (You do not have to complete the procedure.)

  1. Set out four similar plastic containers. Label these A – D.
  2. Prepare sucrose solutions.
    a. Use a glass measuring cup to measure 2 cups of distilled water and pour it into
    the first container. Repeat for the other three containers.
    b. Add nothing to container A, and add the label dH2O (distilled water).
    c. Use a digital kitchen balance (scale) to measure 25 g of sucrose (table sugar).
    Add this to container B, and add the label 25 g. Stir until no more crystals of
    sugar are visible. This creates a sucrose solution with an approximate
    concentration of 0.154 M.
    d. Add 50 g of sucrose to container C and label it, using the same procedure as in
    step b. This creates a sucrose solution with an approximate concentration of
    0.308 M.
    e. Add 75 g of sucrose to container C and label it, using the same procedure as in
    step b. This creates a sucrose solution with an approximate concentration of
    0.462 M.
  3. Obtain and wash one large russet potato (these are the “Idaho potatoes” usually used to
    make baked potatoes).
  4. Create four similar groups of “potato chunks.”
    a. Peel potato and remove any dark spots or bruises.
    b. Chop potato into large chunks. These should be vaguely “block” shape, and
    about 1 – 2 cm on each side.
    c. Place a large piece of plastic wrap on the balance and tare it (that is, set the
    mass to 0 g).
    d. Use the scale with plastic wrap to ensure that the mass of each chunk is no
    smaller than 5 g and no larger than 9 g.
    e. Combine the potato chunks into four groups, switching them around until each
    group has approximately the same total mass (about 50 g). There will be 6 or 7
    potato chunks in each group.
  5. Take a more exact initial mass:
    a. Set the first group of potato chunks on the balance and wait about two seconds
    for the reading to “settle.” Note the mass.
    b. Use the plastic wrap to pick up the potatoes and lift them off the balance; wait for
    about five seconds. Set them back down on the balance, wait for the reading to
    settle, and note the mass again.
    c. Repeat step b in order to take a third initial mass.
    d. Average the three masses to the nearest 0.1 g. Record this as the initial mass of
    that potato group.
  6. Put the group of potato chunks into container A. Record the time.
  7. Repeat steps 10 – 11 with the other three groups of potato chunks, placing them in
    containers B – D.
  8. Repeat steps 3 to 7 with a large sweet potato.
    a. Sweet potatoes have a layer inside the peel that’s a different color than the
    “main” sweet potato. Remove as much of this as reasonably possible.
    b. Use the same procedure to cut, group, and measure the chunks of sweet potato.
    c. Place the sweet potato chunks in the same containers as the potato chunks.
    (Note: potatoes and sweet potatoes are different colors, so they’re easy to
    visually tell apart.)
    d. My volunteer observed that the russet potato chunks immediately sank to the
    bottom of the container, but the sweet potato chunks floated at the top of the
    water in all containers.
  9. Place lids on the containers. Leave at room temperature overnight.
  10. The next day, measure final masses of the potato chunks.
    a. Set a new piece of plastic wrap on the digital kitchen balance and tare it (set it to
    0) as before.
    b. Remove the lid from container A. Use a clean spoon to remove each chunk of
    potato (not sweet potato). Let most water drip off the spoon. Set the chunks on
    a folded paper towel.
    c. Use a second paper towel to pick up the first potato chunk. Firmly pat it dry
    without damaging it. Set the potato chunk aside.
    d. Once all potato chunks in the group are dry, set them on the balance. Use the
    procedure described in step 5 to take and record the final mass of group A.
    e. Record the time.
    f. Repeat this process with the potato chunks in groups B – D.
  11. Repeat step 10 to measure and record the final masses of the sweet potato chunks.
    Analyzing the experiment: 6 pts
  12. Suggest a possible scientific question that this experiment would help us investigate.
    0.5 pts
  13. What are the TWO independent variables in this experiment? What is the dependent
    variable? 1 pt
  14. Write a hypothesis to predict an outcome specifically for the potato portion of the
    experiment. Your prediction could relate to one specific group (such as group A, in the
    distilled water) or to the overall trend (such as “increasing sucrose amount”), but be
    specific enough that we can clearly tell which you’re using. This should be in
    if/then/because format — if (what we’re doing or what changes), then (our predicted
    result), because (relevant and plausible scientific reason). 1 pt
  15. Write a second hypothesis to predict how the results for potatoes and sweet potatoes will
    compare to each other. This should also be in if/then/because format. 1 pt
  16. Write a null hypothesis that parallels either of the first two hypotheses that you wrote.
    (Remember, a null hypothesis is our “default” — it predicts that our independent variable
    will have no impact on the outcome.) This does not need a “because” section. 0.5 pts
  17. Identify two things that were done in this experiment to control other potential variables
    or factors. (Remember that controlled variables are the same as constants; they are not
    either the independent or dependent variables. Also, something that is carefully
    measured or constrained, but is different at any point in the experiment, is not a
    controlled variable.) 1 pt
    Independent:
    Dependent:
  18. Identify one uncontrolled variable in this experiment. An “uncontrolled variable” is
    something that was not fully controlled (kept constant), but that should have been. In
    what way did it vary, or what potential effect could that variation have? 1 pt
    Raw Data
    Here are the average initial and final masses for each group of potatoes and sweet potatoes,
    along with the conditions used and the times that the data was recorded:
    Data Calculations and Presentation: 5 pts
    Russet
    Potato
    Group
    Concentration of
    sucrose
    Initial Time Initial mass of
    potato chunks
    Final Time Final mass of
    potato chunks
    A 0 M 5:20 PM 51.04 g 3:01 PM 57.72 g
    B 0.154 M 5:22 PM 50.82 g 3:06 PM 53.06 g
    C 0.308 M 5: 24 PM 50.97 g 3:11 PM 49.53 g
    D 0.462 M 5:27 PM 50.15 g 3:14 PM 42.05 g
    Sweet
    Potato
    Group
    Concentration of
    sucrose
    Initial Time Initial mass of
    potato chunks
    Final Time Final mass of
    potato chunks
    A 0 M 6:07 PM 51.52 g 3:16 PM 68.26 g
    B 0.154 M 6:09 PM 50.94 g 3:19 PM 58.68 g
    C 0.308 M 6:11 PM 50.94 g 3:21 PM 58.85 g
    D 0.462 M 6:12 PM 50.52 g 3:24 PM 53.33 g
    Use the raw data to complete the chart below. 2 pts
  • “Change” in data is always final – initial. Therefore, if a potato lost mass, then this will be
    negative.
  • Percent change is always Change divided by Initial, then converted to a percentage
    (multiply by 100, or use a “percent” setting in a spreadsheet program).
    Graph the data on one set of axes: 3 pts
  • Use an appropriate computer program, such as Microsoft Excel or Google Sheets.
  • If you don’t think you have access to any such program, please let me know,
    including telling me what program(s) you use for other schoolwork.
  • Note: right-clicking, using Chart Editor, or something similar will be helpful for
    many of the steps below. If you aren’t familiar with the program you’re using, you
    may wish to find and view a quick tutorial before you start. Otherwise, you can
    use the Help features within the program.
  • If you did not already use this program to calculate the above, then type in the
    concentration of sucrose and the percent changes.
  • Use the russet potato data to create a Scatter graph with dots; these dots should not be
    connected with a line.
  • The data for group A should be directly on the y-axis, at the far-left side of the
    graph. If it isn’t, then take another look to make sure that the type of graph is
    correct and that the data is on the correct axes.
  • Remember that the independent variable is always on the x-axis.
  • Add the sweet potato data to the same graph. In most programs, you’ll use Add Series.
  • The two sets of data should be different colors, shapes, or some other distinction
    to tell them apart.
    Group Concentration of
    sucrose
    Change in Mass Percent Change
    Russet Potato A 0 M
    Russet Potato B 0.154 M
    Russet Potato C 0.308 M
    Russet Potato D 0.462 M
    Sweet Potato A 0 M
    Sweet Potato B 0.154 M
    Sweet Potato C 0.308 M
    Sweet Potato D 0.462 M
  • If possible, include a legend to tell us which is which. If not, include that info in
    the graph’s title. (This is possible, but strangely difficult, to do in Google Sheets.)
  • Add a trendline to each data series. This will likely be done through right-clicking or
    through editing the data series itself. The trendline should be linear, which is usually the
    default.
  • Include axis labels so that we know what the numbers on each axis mean, including
    units.
  • Include a specific title for your graph so that we understand what this graph is showing
    us. (If there’s no legend to tell us which data set is which, be sure the title includes this
    as well as a more general description.)
    Paste your graph below, or attach it as a separate file, or include the Google Sheets link (in
    which case, be sure it’s set to “anyone with the link can view”).
    Analysis and Conclusions: 10 pts
  1. Remember that the terms hypertonic, hypotonic, and isotonic are all relative — that is, a
    solution or a potato isn’t hypertonic by itself; it’s hypertonic compared to something else.
    The thing you’re comparing it to will be hypotonic. For instance, if a cell with a lot of
    solute is in a solution that has much less solute than the cell does, then the cell is
    hypertonic to the solution, which is hypotonic to the cell. If both things being compared
    have the same amount of solute, then they are each isotonic to the other. This relative
    comparison is called “tonicity.”
    Based on the above and on the data, write a statement to compare the tonicity of each
    potato group listed below to the solution that it was in. These may or may not all be the
    same — determine that based on the data!) 2 pts
    Sweet potato A:
    Russet potato B:
    Sweet potato C:
    Russet potato D:
  2. Write 4 – 6 sentences to fully explain why both Sweet potato group C and Russet potato
    group D had the changes in mass that they did. Be sure to focus on the process of
    osmosis, including what’s happening on the molecular level in each group. 2 pts
  3. No matter how carefully an experiment is done, there are limitations to its accuracy, and
    sometimes, mistakes may be made and not noticed until it’s too late — if ever. Other
    times, “real-world” biological systems simply don’t follow the expected patterns. Take
    another look at the graph you constructed, focusing on the data points and the
    trendlines.
    One data point clearly doesn’t fit the overall pattern seen for each type of potato;
    we don’t know why, since no mistakes or differences were observed for this group. (In
    statistics, points like this are called outliers.) Which group / data point was that? How
    does that unusual data point affect the trendline that was created for that type of potato,
    or how could it affect the predictions that we might make from our data? Suggest an
    approximate value that would fit our expectations, based on the overall data trends. 1 pt
  4. Remember that all of the russet potato chunks were cut from the same russet potato,
    which we can assume had the same concentration of solutes throughout. What was that
    concentration of solutes? (Hint: use your graph!) 1 pt
  5. Did the sweet potato have a higher or lower concentration of solutes than the russet
    potato? How can you tell? 1 pt
  6. Predict what the results would have been for both russet potatoes and sweet potatoes if
    they were in a “group E” that had 100 g of sugar added to the water, for a final
    concentration of 0.616 M of sucrose. Be as specific as you can, and explain your
    answer. 1 pt
  7. Evaluate each of your three hypotheses. Remind us what you predicted, tell us the
    relevant data, and then identify whether that data supports the hypothesis. (More
    explanation might be needed, depending on how you wrote it.)
    Keep in mind that data supports or does not support a hypothesis. We don’t have nearly
    enough data to declare it correct or incorrect, and we never “prove” a hypothesis. 2 pts
    Water Potential (5 pts)
    Water potential is the potential energy of water, and it lets us determine the direction that water
    will flow in different conditions.
    Note: The official College Board curriculum includes water potential and these
    calculations, so they may be on the national AP Exam in May. Potential resources for learning
    more details include Bozeman Science, Khan Academy, and Section 36.2 of your textbook. For
    this portfolio, the information you need to know is below.
    Water always flows from a region of higher water potential to a region of lower water
    potential — though the actual numbers can seem counterintuitive, since water potential is often
    negative. For example, if a cell with water potential of -5 bars is in a solution with water potential
    of -10 bars, the water will move from the cell (-5 bars, higher) to the solution (-10 bars, lower).
    Water potential has two parts: solute potential, based on the solute concentration, and
    pressure potential, related to physical pressure placed on a system. Systems of solutions in an
    open beaker are considered to have 0 pressure potential, so their water potential is only based
    on solute concentration. In plant cells, this pressure may be produced by the water pushing
    outward as the cell walls push inward; this is called turgor pressure.
    If a plant cell is in equilibrium with its surroundings, with no net movement of water, that
    could mean two things — first, it could mean that the concentration of solutes inside and outside
    the cell is identical, and there is no turgor pressure. Second, it could mean that the turgor
    pressure potential is positive, but equal in magnitude to the solute potential (which is always
    negative).
    We have two mathematical equations to measure water
    potential:
    Notice that R is a constant, so it always has the same
    value. Its strange units are set so that the units of molar
    concentration and temperature cancel out, leaving bars as
    the unit for solute potential.
    Ionization constant i is the number of ions that the substance breaks into when it dissolves.
    Molecular compounds that don’t break apart when they dissolve — like sucrose — have an
    ionization constant of 1.
    Remember to convert all Celsius temperatures to Kelvin, simply by adding 273.
  8. Calculate the solute potential of a 0.20 M NaCl solution at 25° C. Remember from
    chemistry that NaCl breaks into two ions when it dissolves in water. 1 pt
  9. If the concentration of NaCl inside the plant cell is 0.25 M, which way will the water
    diffuse if the cell is placed inside the 0.20 M NaCl solution? 0.5 pt
  10. What must the turgor pressure equal if the solution and the cell from question 2 are at
    equilibrium, so there is no net diffusion between them? 1 pt
  11. In the experiment we examined, solution B contained sucrose at a concentration of
    0.308 M. The experiment was carried out at approximately 24° C. Calculate the solute
    potential of solution B. 1 pt
  12. Was the water potential of each potato higher or lower than your answer for Q4? How
    do you know? 0.5 pt
  13. In the experimental analysis, question 4, we estimated the solute concentration inside
    the russet potatoes. How could this further understanding about water potential change
    or complicate your earlier estimation?

Sample Solution

ACED ESSAYS