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Onion cell plasmolysis experiment

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  • SL Core /
  • Cells: Activities for Learning /
  • Onion cell plasmolysis experiment

Do sweet and strong onions have different cytoplasm solute concentrations?

This experiment is a good introduction to data processing and illustrates skills needed for success in data analysis questions in IB exams. Students use techniques learned earlier in the topic to prepare slides of onion cells. They collect data about onion cell plasmolysis in different solute concentrations, and use a scatter graph of the results to identify the concentration for 50% plasmolysis in each type of onion

Lesson Description

Guiding Question

Do sweet onions have a more dilute cytoplasm than strong onions?

Activity 1

Carry out the Experiment using the instructions on the Student Worksheet for the onion experiment below.
To get an idea of what the microscope slides would look like watch this Onion cell plasmolysis - video

Activity 2

Complete a written report of your experiment.
There is a little guidance on the second page of the worksheet.

Teachers notes

The idea of this experiment is to give students a chance to observe cells and the effects of osmosis under the microscope in a setting which can be can provide some training in the skills needed for an investigation in the Internal Assessment.

The question to find out if the cytoplasm concentration is different in two different types of onion cells makes the experiment analytical not simply descriptive. Two similar varieties of Onion will not give such a good difference as Garlic or Spring onions will when compared to big yellow onions.

It is possible to complete this experiment in a 1 hour lesson.

This Lab technician prep sheet may also be useful.

Alternative Method

As an alternative method the cells could be mounted on slides in water, and the slides washed with salt solutions of different concentrations. Students would then be able to see the cells plasmolising in the higher concentrations.