By the end of this lab, you will be able to:

  • Recognize the difference between qualitative and quantitative data
  • Apply the terms osmosis, hypertonic, hypotonic & isotonic to experimental results
  • Determine the independent, dependent, and controlled variables within an experiment
  • Create a high-quality graph based on data, state graph trends, and explain results
  • Explain the biology and chemistry behind hydrating and dehydrating cells and organisms

Required prior knowledge and textbook readings:

Introduction:

Consider this: We drink water and water our plants for hydration, but if we drank saltwater (or if plants were watered with saltwater) this causes dehydration. Why is this the case? What is happening to the cells?

Osmosis is the diffusion of water across the plasma membranes of cells, and is driven by concentration gradients. When there is a difference in solute concentration inside vs. outside of a cell, water moves by osmosis towards the higher solute concentration/lower water concentration. This can cause cells to shrink as they lose water, also known as crenation, or swell as they gain water and potentially burst, known as lysis.

[INSERT H5P ACTIVITY FOR REVIEW OF THEORY]

In this lab, you will qualitatively examine osmosis across the plasma membranes of potato cells to investigate what causes these cells to gain and lose water (hydration and dehydration).

Before beginning, ensure you have the following materials ready:

Materials:

  • 1 potato (if you don’t have a potato, use another starchy vegetable such as a sweet potato or turnip)
  • 1 knife
  • 1 cutting board
  • 1 ruler
  • 3 cups
  • Tap water
  • 2 teaspoons of salt
  • Measuring spoon (teaspoon)
  • Measuring cup (250mL)
  • Tape
  • Marker
  • Stopwatch
  • Paper or cloth towel
  • Plastic wrap (or anything similar)

Qualitative Potato Osmosis Experiment

Begin by considering the following hypothesis: The texture and firmness of potato pieces is affected by the solute and water concentration of the surrounding solution.

You will then test this hypothesis using water, salt, and potatoes. You will cut three identical pieces of potato and place one in water, one in saltwater, and one in no solution, covering them all with plastic wrap. After one hour, you will remove each potato piece and observe their firmness to see if water moved in or out of the potato. This will be a qualitative investigation.

Method:

1. Before beginning, make a prediction for each experimental condition. Consider what texture and firmness differences you expect to see between the potatoes in the three experimental treatments and WHY?

[INSERT H5P ACTIVITY RELATING TO PREDICTIONS]

2. Using tape and a marker, label one cup “Water”, one cup “Saltwater”, and one cup “No solution”.

3. Use your measuring cup to measure 250mL of tap water, and pour this into your “Water” cup. Repeat for your “Saltwater” cup. Do not add water to your “No solution” cup.

4. In the “Saltwater” cup, add two teaspoons of salt and stir until completely dissolved (no visible grains are left).

5. Using your knife, cutting board, and ruler, carefully cut the potato into three small pieces that are the same size and shape. The potato pieces should be longer than they are wide, for example: 5cm x 0.5cm x 0.5cm. It’s recommended to cut the potato in half lengthwise, then again lengthwise, then into 2cm slices that can be trimmed down to 0.5cm, and finally adjust the lengths to 5cm. Make sure your pieces do not have peel on them! **BE CAREFUL USING YOUR KNIFE**

Note: It’s important that the potato pieces are the same size! Remember the saying: “Measure twice, cut once“.

6. Before proceeding, make some brief “before” observations regarding the potato pieces. What is their size? Texture? Firmness? Make a note of your observations for each piece.

7. Place one potato piece in the “Water” cup, one potato piece in the “Saltwater” cup, and one potato piece in the “No solution” cup. Cover each cup with plastic wrap and set a timer for one hour.

8. After incubating for one hour, remove the potato pieces from their cups and place them onto the paper towel. Make sure not to mix them up! Consider using your marker to label “Water”, “Saltwater”, and “No solution” directly on the paper towel.

9. Make some brief “after” observations regarding the potato pieces. What is their size? Texture? Firmness? What changes did you notice from the “before” observations? Make a note of your observations for each piece.

[INSERT H5P ACTIVITY ABOUT WATER GAIN OR LOSS FROM THE DIFFERENT PIECES, CONNECTING TO CRENATION, LYSIS, HYPERTONIC, HYPOTONIC, ISOTONIC etc.]

10. Take a photo of your potato pieces after incubation.

Assignment:

If required by your instructor, you can download the assignment for the Osmosis lab here. Note that the file download works best using Microsoft Edge or Mozilla Firefox web browsers (not Google Chrome). Once you have completed the assignment, submit the file to your instructor as per their instructions.