Osmosis in potato strips
Soak potato strips in sugar solutions of different strengths, weigh them, and find the strength at which they neither gain nor lose water.
Core concept
Water moves by osmosis through a partially permeable membrane from a dilute solution to a more concentrated one
Topic: Cells
Osmosis is the movement of water through a partially permeable membrane, such as a cell: The tiny building block that every living thing is made of. All key words membrane, from a dilute solution (more water) to a more concentrated solution (less water). A potato is made of living cells full of a weak solution called cell sap. In pure water, water moves into the cells, so they swell and become firm (turgid). In a strong sugar solution, water moves out, so the cells shrink, the cytoplasm pulls away from the cell wall (plasmolysis) and the strip goes floppy. In a solution of the same strength as the cell sap there is no overall movement.
What you will learn
- Measure the change in mass of potato strips in different solutions.
- Explain the results in terms of osmosis.
- Find the concentration of the cell sap from a graph.
Before you start
- Know that cells have a membrane and that plant cells have a cell wall.
Materials
- Potato, sugar, distilled water, paper towels
Equipment
- Beakers, knife or cork borer, tile, top-pan balance
Procedure
- Cut equal potato strips and weigh them: each is 5.00 g.
- Place one strip in each sugar solution, from 0 M (pure water) to 1.0 M.
- After 120 minutes, blot each strip dry, weigh it and record.
- Record the strip in pure water after 30 and 60 minutes too, to see how fast it changes.
- Repeat in pure water with a boiled potato strip.
How this simulation is simplified
- The cell sap is as concentrated as 0.3 M sugar; at balance the mass changes by 30% for each 1 M difference.
- The strip approaches balance with a time constant of 40 minutes; boiling leaves a tenth of the effect.