IB Biology IA idea · Water, osmosis and membranes
IB Biology IA idea: find the water potential of onion cells by plasmolysis
Under the microscope, cells shrink away from their walls in strong sugar. Find the concentration at which half have started to give.
These ideas are starting points, not an IA to copy. Never copy an idea blindly: make the question, method and analysis your own, and check your plan with your teacher.
Example research question
How does sucrose concentration (0, 0.2, 0.3, 0.4, 0.6 and 0.8 mol dm⁻³) affect the percent of red onion epidermal cells plasmolysed, counted in five fields of view after 15 minutes?
Change the system, the range or the method so the question is yours. Check it against the research question checklist in our Biology IA guide.
Variables
- You change
- sucrose concentration, 6 or more values from 0 to 0.8 mol dm-3
- You measure
- percent of cells plasmolysed in a field of view (or incipient plasmolysis point)
- You keep the same
- same onion and tissue layer
- same soak time
- same slide and cover slip method
- same counting rule for a plasmolysed cell
Method
Peel the thin inner epidermis of red onion, soak pieces in each sugar solution and count plasmolysed cells in several fields. Take the concentration at which half the cells are plasmolysed.
Data analysis
Percent plasmolysed against concentration with a sigmoid curve, mean and standard deviation of fields, estimate of tissue water potential with uncertainty. Compare with the potato method.
How it works
You change Sucrose concentration (mol dm⁻³) and measure Cells plasmolysed (%) in Red onion epidermis, keeping Soak time, Tissue region, Field size, Onion bulb the same. Expect a graph of Cells plasmolysed / % against Sucrose concentration / mol dm⁻³: Look for the point where half the cells have shrunk.
Make it yours
Compare onion varieties, leaf ages, or salt against sugar.
Safety
Take care with: sharp tools.
Use a scalpel on a tile with care or tear the tissue by hand. Wash hands.
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