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Grades 9 and 10 Intermediate About 20 min Version 1 Awaiting review Premium

Osmosis in potato strips

Osmosis is the movement of water through a partially permeable membrane, such as a cell 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

  1. Cut equal potato strips and weigh them: each is 5.00 g.
  2. Place one strip in each sugar solution, from 0 M (pure water) to 1.0 M.
  3. After 120 minutes, blot each strip dry, weigh it and record.
  4. Record the strip in pure water after 30 and 60 minutes too, to see how fast it changes.
  5. 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.