BACKGROUND (continued)
Water potential (Ψ) is a measure of water’s potential to do work. In order to do work, an object must be able
to apply enough force to another object to cause displacement. In order for water to displace another object,
water must be moving. The largest water potential any volume of water can have, if only standard atmospheric
pressure is being applied to that volume of water, is defined as 0. This is the water potential for distilled water.
Distilled water has the greatest potential to move, and thus displace another object.
The combined effects of solute concentration and physical pressure can be used to calculate water potential.
= s + p
water potential = solute potential + pressure potential
Solute Potential (s)
Solute potential is also called the osmotic
potential because solutes affect the direction
of osmosis.
The s of any solution at atmospheric pressure
is always negative because there are less free
water molecules available to do work if they
are tied up in hydration shells around a solute.
Pressure Potential (p)
p is the physical pressure on a solution. It
can be: NEGATIVE: if it pulls on a solution (i.e. water tension/transpiration), or POSITIVE: if it pushes on a
solution (i.e. turgor pressure from cell wall).
The standard for measuring water potential is pure (distilled) water, which in an open container has a
water potential () of zero at one atmospheric pressure.
Notice that adding solute DECREASES
water potential…be sure you can explain
why!
Notice that adding a positive pressure
increases water potential…be sure you
can explain why!
Notice that adding a negative pressure
decreases water potential…be sure you
can explain why!
BACKGROUND (continued) A cell must exchange materials with its surroundings, a process
controlled by the plasma membrane. Plasma membranes are
selectively permeable, regulating the cell’s molecular traffic:
• Small, uncharged polar molecules and small
nonpolar molecules, such as N2, freely pass
across the membrane.
• Hydrophilic substances such as large polar
molecules and ions move across the membrane
through embedded channel and transport
proteins.
• Water moves across membranes through channel
proteins called aquaporins.
In plants, water pressure against the cell wall provides turgor pressure. An isotonic solution in plants generally
promotes limp (flaccid) cells. A lack of water in plant cells causes shrinking of the cytoplasm away from the
cell wall, a process referred to as plasmolysis.
Different types of cells fair best in different types
of solutions:
a) ANIMAL CELL – fares best in an
isotonic environment…unless it has
special adaptations to offset the osmotic
uptake or loss of water.
b) PLANT CELL – turgid and generally fair
best in a hypotonic
environment…tendency for water uptake
is balanced by the elastic wall pushing
back on the cell.
ARROW INDICATES WATER
MOVEMENT WHEN CELL IS FIRST
PLACED IN THE SOLUTION!!!
PRELAB QUESTIONS: To be answered using background information. Show All Work! 1. If a cell’s ΨP = 3 bars and its ΨS = -4.5 bars, what is the resulting Ψ?
2. The cell from question #1 is placed in a beaker of sugar water with ΨS = -4.0 bars. In which direction will
the net flow of water be?
3. The original cell from question # 1 is placed in a beaker of sugar water with ΨS = -0.15 MPa. We know that
1 MPa = 10 bars. In which direction will the net flow of water be?
4. The value for Ψ in root tissue was found to be -3.3 bars. If you take the root tissue and place it in a 0.1 M
solution of sucrose at 20°C in an open beaker, what is the Ψ of the solution, and in which direction would
the net flow of water be?
5. NaCl dissociates into 2 particles in water: Na+ and Cl
-. If the solution in question 4 contained 0.1M NaCl
instead of 0.1M sucrose, what is the Ψ of the solution, and in which direction would the net flow of water be?
6. A plant cell with a Ψs of -7.5 bars keeps a constant volume when immersed in an open-beaker solution that
has a Ψs of -4 bars. What is the cell’s ΨP?
7. At 20°C, a cell containing 0.6M glucose is in equilibrium with its surrounding solution containing 0.5M
glucose in an open container. What is the cell’s ΨP?
8. At 20°C, a cell with ΨP of 3 bars is in equilibrium with the surrounding 0.4M solution of sucrose in an open
beaker. What is the molar concentration of sucrose in the cell?
Use this key to answer all the problem below.
T = TRUE F = FALSE NEI = NOT ENOUGH INFORMATION
PROBLEM: The initial molar concentration of the cytoplasm inside a cell is 2M and the cell is placed in a
solution with a concentration of 2.5M.
9. Initially, free energy is greater inside the cell than outside
10. It is possible that this cell is already in equilibrium with its surroundings.
11. Initially, solute concentration is greater outside the cell than inside.
12. Water will enter the cell because solute potential is lower inside the cell than outside.
13. The cell will become flaccid because the pressure potential is greater outside the cell than inside.
14. The cell is already in equilibrium with its surroundings because of the combination of pressure potential and
solute potential inside and outside the cell.
15. Initially, the cytoplasm is hypertonic to the surrounding solution.
16. Initially, the numerical value of the solute potential is more negative inside the cell than outside.
17. Net diffusion of water will be from inside the cell to outside the cell.
18. At equilibrium, the molarity of the cytoplasm will have increased.
19. At equilibrium, the pressure potential inside the cell will have increased.
20. What does increasing SOLUTE potential do to water potential? Explain.
21. What does increasing PRESSURE potential do to water potential? Explain.
PART A – STRUCTURED INQUIRY: Observing Osmosis & Plasmolysis
TEACHER DEMONSTRATION
Materials List:
Celery stick soaked in water, celery stick soaked in saltwater
Video Clip: plasmolysis in Elodea cells, Video Clip: plasmolysis in Onion cells
Colored Pencils
Procedure
1. Observe the celery stick that was soaked in water. Record your observations in the table below.
2. Break the celery stick that was soaked in water. Record your observations in the table below.
3. Observe the celery stick that was soaked in saltwater. Record your observations in the table below.
4. Break the celery stick that was soaked in saltwater. Record your observations in the table below.
Observation of Celery Stick Reaction to Breaking Sketch the Movement of H2O
Soaked in
Water
Soaked in
Saltwater
Analysis of Results: Part A
1. When you drink a glass of water, most of it is absorbed by osmosis through cells lining your small intestine.
Drinking seawater can actually dehydrate the body. Explain how this might occur.
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2. A marine clam is mistakenly added to a freshwater aquarium. What will happen to the clam and why?
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3. What hypothesis was this experiment designed to test?
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Procedure
4. Observe the following video clip: http://www.youtube.com/watch?v=W_0qlQzN3V0. What do the Elodea
cells look like before the solution is added? Sketch and record your observations in the table below. What
do the Elodea cells look like after the solution is added? Sketch and record your observations in the table
below.
5. Observe the following video clip: http://www.youtube.com/watch?v=x11mkGnOc8g. What do the onion
cells look like before the solution is added? Sketch and record your observations in the table below. What
do the onion cells look like after the solution is added? Sketch and record your observations in the table
below.
Analysis of Results: Part A
6. On the basis of your observations, explain the processes that are occurring in both the Elodea and onion
cells throughout the course of the video animations.
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Elodea Cells Before/After Treatment
Onion Cells Before/After Treatment
PART B – STRUCTURED INQUIRY: Observing Diffusion TEACHER DEMONSTRATION
Materials List:
Dialysis tubing, plastic cup, glucose/starch solution, distilled water, iodine-potassium iodide (IKI) solution,
dropping pipet, glucose test strips, funnel.
In this activity, you will explore the diffusion of different molecules through dialysis tubing, a semipermeable
membrane. You will use glucose test strips to check for the presence of glucose and IKI solution to test for the
presence of starch. As you probably know, IKI reacts with starch to give a dark blue, almost black color. When
IKI reacts with starch, it becomes a part of the starch molecule and is removed from solution.
Procedure
1. Pour 160-170 mL of distilled water into a plastic cup. Add approximately 4 mL of IKI solution to the water
and mix well. Record the initial solution color in Table 1.
2. Dip a glucose test strip into the solution and record the initial glucose test results in Table 1. Use a +
symbol to indicate a positive glucose test result and a – symbol to indicate a negative glucose test result.
Discard the glucose test strip.
3. Obtain a piece of dialysis tubing that has been soaked in water. Touch the dialysis tubing ONLY AT the
ends so that the oils from your fingers do not clog the pores in the tubing. The tubing should be soft and
pliable. Close one end of the tubing by tying it into a knot. Roll the other end of the tubing between your
thumb and index finger to open it.
4. Using a small funnel, pour 15 mL of glucose/starch solution in the dialysis bag. Smooth out the top of the
bag, gently running it between your thumb and index finger to expel the air. Tie off the open end of the bag.
Leave enough room in the bag to allow for expansion.
5. Record the initial color of the glucose/starch solution in Table 1.
6. Immerse the dialysis bag in the solution in the cup. Make sure it is completely covered by the solution in
the cup.
7. Wait 30 minutes. While waiting, complete the following exercise:
Use the space provided on the next page to make your
predictions and explanations about the movement of
molecules in the diagram above.
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8. After 30 minutes, remove the bag from the cup. Blot it with a paper towel. Cut a slit in the bag large
enough to insert a glucose test strip. Fill in the final columns of Table 1.
Analysis of Results: Part B
1. Does this activity account for the diffusion of all molecules that you listed in Figure 2? If not, what data
could have been collected to show the net diffusion of this molecule or molecules?
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2. What does your data tell you about the sizes of the molecules relative to the pore size of the dialysis tubing?
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PART C – STRUCTURED INQUIRY: Modeling Cellular Environments TEACHER DEMONSTRATION
Materials List:
Balance, graduated cylinder, beaker 1000 mL, funnel, 20 cm dialysis tubing, 1M sucrose solution, 1M salt
solution, 1M glucose solution, 5% albumin (protein) solution, distilled water.
In this activity, you will construct and simulate model cells in an external environment, to relate solutes passing
through a semi-permeable membrane in hypertonic, hypotonic, and isotonic solutions. The pores in dialysis
tubing allow some molecules to freely diffuse across the membrane and some to be restricted. In this lab you
will use dialysis tubing as a model cell membrane.
Procedure
1. Obtain 5 pieces of pre-soaked dialysis tubing. Tie a knot in one end of each piece of tubing.
2. Measure and pour 10 mL of each of the four prepared solutions into separate graduated cylinders. The
solutions are: salt, glucose, sucrose, and protein.
3. Open the dialysis tubing and use a funnel to pour 10 mL of prepared solution into the tubing. Tie a knot in
the open end to form a model closed cell membrane (similar to a bag). Be sure to leave enough space in the
bag for expansion. Minimize air enclosed in the tubing.
4. Fill beakers with about 100 mL of the solutions (water or salt) to be paired with your model cells. See data
table for pairings.
5. Determine the initial weight of each “cell” and record this data in the data table below.
6. Completely immerse the model cells in their pairing solutions in the beaker. Start your timer.
7. Given what you know about solute concentration, predict whether each “cell” volume will grow, shrink, or
remain constant. Record your predictions below: -
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8. Allow the “cells” to soak for 30 minutes. When 30 minutes has passed, remove the model cells from the
solution, pat them dry, and determine the final weight of each. Record this data in the data table.
9. Calculate the percent change in weight and record your results in the data table.
Modeling Cells Data Table
Cell Weight (g)
% Change in Mass*
Paired Extra-Cellular Solution (in beaker) Start Time 0 End Time 30 min.
Cell 1 (protein) Salt
Cell 2 (sucrose) Water
Cell 3 (water) Water
Cell 4 (glucose) Salt
Cell 5 (salt) Water * (Final Mass - Initial Mass) x 100 (Initial Mass)
Analysis of Results: Part C
1. Examine the initial and final weights of the model cells. What causes the mass of the dialysis bags to
change? Was there more or less water in the dialysis bags at the conclusion of the experiment? Explain.
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2. From your results, which solutes, if any, diffused across the membrane, and which, if any, were restricted?
Explain why you think this occurred.
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3. How is dialysis tubing different from a cell membrane? How is it similar?
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4. List three variables that could influence the outcome of this experiment. Briefly describe a method of
control that could be used for each of these variables.
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PART D – STRUCTURED INQUIRY: Water Potential in Plant Cells TEACHER DEMONSTRATION
Materials List:
Plastic cups, distilled water, sucrose solutions, cork borer, potato cores, plastic wrap, paper towels, balance.
In this activity, you will investigate water potential by immersing potato cores in sucrose solutions and
determining the change in mass, if any, of the cores. You will graph your data and use the graph to determine a
value for C. Using the experimentally determined value for C, you will then calculate a value for s.
Procedure
Continue to step 9 after the potato cores have been in the sucrose solution overnight.
Analysis of Results: Part D
1. Graph your percent change in mass of potato cores in different solutions. Gridlines are provided on the next
page.
2. On your completed graph, find the point where the line of your data crosses the 0 line (x-axis) of the grid.
This is the equilibrium point; at this point there is no net gain or loss of water from the potato cells.
3. Read the corresponding value of sucrose molarity for this point. This is the molar concentration of sucrose
that produces equilibrium. Below, record this concentration of sucrose as your experimentally determined
value for C. Convert ambient temperature from C to K.
4. Review the information on water potential provided in the introduction at the beginning of this lab. Using
the formula s = -iCRT, calculate the solute potential at equilibrium. Show your calculations in the space
below.
5. Using the formula = p + s, give the following:
6. Imagine that you are an agri-science consultant to a large corporation that raises 7,000 acres of wheat on
desert land adjoining the Mediterranean Sea. Just before the wheat matures, all the wells used for irrigation
run dry. The farm manager wants to irrigate the fields with water drawn from the Mediterranean. From
previous tests, you know that the average solute potential of root tissue taken from wheat fields is -11.13
bars. You test the seawater and determine its solute potential to be -24.26 bars. What will you advise the
farm manager and why?
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PART E – STRUCTURED INQUIRY: Surface Area to Volume Ratio in Cells
TEACHER DEMONSTRATION
Materials List:
3 Phenolphthalein agar cubes: 3 x 3 cm, 2 x 2 cm, and 1 x 1 cm, 1 plastic spoon, 1 plastic cup, 1 metric ruler
6”, 100 mL of white vinegar, timer.
The agar cubes have been prepared with 1% phenolphthalein, which is a pH indicator. The chart below
indicates a color scale of pH for phenolphthalein. The blocks are pink because the agar blocks were soaked in
0.01% sodium hydroxide.
Phenolphthalein Color Indicator Chart Color pH Acid or Base
Colorless 0 – 8.2 Acidic or slightly neutral
Pink to Red 8.2 – 12.0 Basic
Procedure
1. Obtain agar cubes in a plastic cup from your teacher. Be careful not to scratch any surface of the cubes.
2. Using the metric ruler, measure the dimensions of each agar cube and record the measurements in the table
below.
3. Place the three cubes carefully in a plastic cup. Add white vinegar (acetic acid) until the cubes are
submerged. Using a plastic spoon, keep the cubes submerged for 10 minutes turning them as needed.
Be careful not to scratch any surface of the cubes.
Be sure to start the timer once the cubes are submerged.
4. As the cubes soak, calculate the surface area, volume, and surface area to volume ratio for each agar cube.
Record this data in the table below.
Cube Size (length,
width, and height of
each side in cm)
Surface Area (cm2) Volume (cm
3) Surface Area/Volume
Ratio (cm2:cm
3 or 1:cm)
Formulas
Surface Area = length x width x # of sides
Volume = length x width x height
Surface Area/Volume Ratio = surface area / volume
Extent of Diffusion = (total cube volume - volume of cube that has not changed color) x 100
total cube volume
5. After 10 minutes has elapsed, use the spoon to remove the agar cubes and carefully blot them on dry paper
towel. DO NOT CUT THE AGAR CUBES UNLESS EXPLICITLY TOLD TO DO SO BY YOUR TEACHER.
6. Using a metric ruler, measure the distance in centimeters (cm) that the white vinegar diffused into each cube
(see Figure below). Record this as the distance from the surface in the table below.
7. Calculate the rate of diffusion for each cube in centimeters per minute (cm/min). Record your calculations
in the table below.
8. Calculate the volume of the portion of each cube which has not changed color (in other words, the portion of
the cube that is still pink). Record your calculations in the table below.
9. Calculate the extent of diffusion into each cube as a percent of the total volume. Record your calculations in
the table below.
10. Graph the rate of diffusion (cm/min, Y-axis) relative to the surface area to cell volume ratio (1/cm, X-axis).
11. Graph the extent of diffusion (Y-axis) relative to cell volume and surface area (X-axis).
Show all Calculations: Cube Size
(length,
width, and
height of each
side in cm)
Total Volume
of Cube (cm3)
Distance
from
Surface
(cm)
Rate of Diffusion
(cm/min)
Volume of Undiffused Area
(Still Pink)
(subtract distance from surface
– both sides from length of 1
side to get 2. Now cube this
number to get area of pink)
Volume of Diffused Area
(White Area)
(subtract volume still pink
from total volume of cube)
Extent of Diffusion
(divide volume of white area by total volume of
cube and multiply by 100)
For each cube, measure the penetration of
vinegar into the agar (distance in cm from edge
of white edge of cube to edge of pink in the
cube).
cm
Analysis of Results: Part E
1. Examine your data. What dimensions supported the fastest rate of diffusion? Why?
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2. What dimensions supported the greatest diffusion percent total volume? Why?
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3. Compare the rate of diffusion for each cell size to the extent of diffusion for each cell size. Discuss the
significance of the data.
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4. The size of some human cells is 0.01mm. Using the formulas in this activity, calculate the surface to
volume ratio of such a cell (assume 0.01 mm cube). Describe the extent of diffusion into this living cells as
compared to the smallest agar cube. Explain.
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PART F – OPEN INQUIRY: Diffusion & Osmosis STUDENT-DESIGNED INVESTIGATION
Suggested Materials List:
Plastic cups, graduated cylinders, paper towels, balance, dialysis tubing, red mystery solution, orange
mystery solution, yellow mystery solution, green mystery solution, purple mystery solution, blue mystery
solution, china markers, masking tape, timers, pipettes.
Problem:
A laboratory assistant prepared six solutions of 1.0 M, 0.8 M, 0.6 M, 0.4 M, 0.2 M and 0.0 M sucrose, but forgot
to label them. After realizing the error, the assistant randomly added color to each of the flasks containing these
six unknown solutions as red, orange, yellow, green, blue, purple, and clear.
Challenge:
1. Design an experiment, based on the principles of diffusion and osmosis that the assistant could use to
determine which of the flasks contains each of the five unknown solutions. Use the following steps when
designing your experiment:
Describe the background information, including that which was discovered in previous experiments.
Include this information in your inquiry lab NB.
Define the question. Include this information in your inquiry lab NB.
State a testable hypothesis. Include this information in your inquiry lab NB.
Describe the experiment design with controls, variables, and observations. Discuss the materials &
methodology used to collect data. Include this information in your inquiry lab NB.
Sketch the experimental setup. Label appropriate components. Include this information in your
inquiry lab NB.
Describe the possible results and how they will be interpreted. Include this information in your
inquiry lab NB.
After the plan is approved by your teacher:
2. Perform your experiment and determine the molar concentration of each of the mystery solutions.
Outcomes of the step-by-step procedure should be documented in your inquiry lab NB. This includes
recording the calculations of concentrations, etc., as well as the weights and volumes used.
3. Data tables should be included and used throughout your experiment.
4. The results should be recorded (including drawings, photos, data print-outs, etc.).
5. Graphs of the results should be included.
6. The analysis of the results should be recorded. Draw conclusions based on how the results compared to the
predictions.
7. A statistical analysis IS NOT REQUIRED for this inquiry investigation because there is no baseline data.
8. Limitations of the conclusions should be discussed, including thoughts about improving the experiment
design, statistical significance and uncontrolled variables.
9. Further study direction should be considered.