1.2 Applications and skills

1.2.4 Practical 2: Osmolarity of plant tissue

Osmolarity is a measurement of the concentration of dissolved salts in a solution. It is difficult to measure osmolarity accurately in living cells, but a very good estimate can be made by immersing cells in solutions of known concentration.

Effects of osmosis on cells in different tonic environments Figure 1.2.4a – Effects of osmosis on cells in different tonic environments 

  • In a hypertonic environment, the concentration of solute is higher outside than inside the cell. Water moves out by osmosis. The cell shrinks (animals) or is plasmolysed (plants).
  • In a hypotonic environment, the concentration of solute is lower outside than inside the cell. Water moves in by osmosis. The cell swells and becomes turgid (plants) or may undergo lysis (animal cells).
  • In an isotonic environment, the osmolarity is equal inside and outside the cell. There is no net water movement. The cell stays the same size and shape.

In which of the three environments is the osmolarity of a cell’s interior higher than the environment? Lower than the environment? Equal to the environment?

Activity 1: Plasmolysis and turgidity in red onion cells

Water moves across a plasma membrane relatively quickly. You can easily observe plasmolysis and turgidity using a microscope and red onion epidermal cells.

  • Prepare a wet mount of a thin layer of red onion cells and a drop of distilled water. Observe your slide on medium or high power.
  • Add a drop of 10% salt solution to the edge of the slide, and use a small piece of paper towel to pull the solution through the specimen. Replace your slide on the microscope. What has happened to the cells?

Activity 2 - Design an experiment: Osmolarity of potato tissues

In this activity, you will bathe potato chips in solutions of varying concentrations in order to estimate the osmolarity of potato cells.

  • You should modify the method so that you are taking repeated, accurate measurements.
  • You may need to do a pre-lab trial to determine an appropriate range of solution concentrations for this experiment, making sure to have solutions of varying tonicities with respect to the potato cells.

You will need:

  • A potato chip cutter (or knife)
  • A knife and cutting board
  • Potatoes
  • Distilled water
  • A solute (table salt or white sugar are appropriate)
  • An electronic balance
  • Test tubes
  • A test tube rack

Instructions

  1. Prepare solutions of known concentrations using distilled water and the solute of your choice.
  2. Cut potato chips of equal length, width and height.
  3. Measure the mass and volume of the chips.
  4. Add an equal volume of solution to the test tubes.
  5. Place one chip in each test tube.
  6. After one or two days, remove the chips from the test tubes. Measure the mass and volume of each chip.
  7. Graph your results to determine the osmolarity of potato cells.

Analysis and conclusions

  • How will you graph your data in order to come up with an estimate of osmolarity?
  • The unit of osmolarity is osmoles of solute / litre of solution. You probably do not know the number of osmoles of solute that you used to make your solutions. What is an appropriate unit to use?

Discussion and extension

  • If you used a different solute, would you get the same results? Why/why not?
  • Does temperature affect osmolarity? How could you modify the method to find out?

Nature of Science

Experimental design: Accurate quantitative measurements are essential in experiments on osmosis.

red onion epidermisFigure 1.2.4b – Red onion epidermis
Which of these micrographs (top or bottom) shows cells in a hypertonic environment? A hypotonic environment?

Try it

Do you remember the rules for biological drawings and how to calculate magnification of diagram? (See Page 1.2.2) 

  • Draw a diagram of one red onion cell you have observed.
  • Label the diagram and include a scale bar to indicate its size.

Potato cutterFigure 1.2.4c – Potato cutter
A potato chip cutter ensures all of your chips have equal height and width.

Electronic balanceFigure 1.2.4d – Electronic balance
This electronic balance measures mass with accuracy of +/–0.01g.

In the lab

Accuracy and precision: In the discussion section of a lab report, you should include a consideration of these concepts.

  • Precision is the degree to which the results of multiple experiments agree with each other. You can increase the precision of your experiment by performing multiple trials. You can determine the precision of your results by calculating the mean and standard deviation.
  • Accuracy refers to how close your experimental value is to the actual value (i.e. of osmolarity). You can increase the accuracy of your results by being careful and checking equipment for error. You can determine the accuracy of your results by comparing them with published data.

Consider this

You can see in Figure 1.2.4a that hypertonic and hypotonic solutions damage cells. For this reason, tissues or organs that are used in medical procedures must be bathed in a solution with the same osmolarity as human cytoplasm. Do some research to find out how this principle applies to different medical procedures.