Osmosis in potato tissue

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Kenneth Blake

PLANNING

Aim:

  • I want to find out how the concentration of a sucrose solution affects the mass of several pieces of potato in different molarities, through the process of Osmosis.
  • Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration.

Factors:

  • Temperature: This may affect the rate of osmosis as water moves quicker when warmed causing the rate of osmosis to increase. Likewise if the water is cooled the water will move slower so the rate of the movement of water will decrease.

  • The surface area of Potato: The rate of osmosis may be affected as the bigger the surface area, the more likely water will move through the selectively permeable membrane, as there are more holes in the membrane for it to pass through. Whereas a small surface area will less likely allow water to move through the membrane, as there are less gaps.

  • The total volume of the Solution: The rate of osmosis may be influenced by the total amount of solution in each boiling tube, because if you have more volume in one boiling tube compared to another it is more likely that osmosis will occur more in the greater volume than in the lesser volume. Also the inequality of the total volumes will make the test unfair, as this factor must be kept equal.

  • The size/length of the potato: The rate of osmosis may be affected as the bigger the size, the more likely water will move through the selectively permeable membrane, as there are more openings in the membrane for it to pass through. Whereas a small size will less likely allow water to move through the membrane, as there are less gaps.

  • The Molarity of the Solution: If the molarity is strong in the solution then osmosis will make the potato expel water to allow both the concentration in the potato and the concentration inside the solution equal. Whereas if there is a low molarity then osmosis will occur but in the opposite way. Instead of expelling water, the potato will absorb water molecules from the solution as to make the two concentrations equal. 

A Fair Test:

  • To make the investigation a fair test only one variable will change. This changing variable will be the molarity of the solution. As the investigation is carried out the ratio of water to sucrose will change as slowly the total solution will go from pure distilled water, to completely sucrose solution.
  • All of the other variables will be kept constant to the best of my ability. These are:
  1. The Volume of the Solution
  2. Temperature
  3. The surface area of Potato
  4. The size/length of the potato
  5. The Molarity of the Solution

Prediction:

  • I predict that as the concentration (molar) of the sucrose solution increases, the percentage change in mass (grams) will decrease, so much that the potato pieces should loose mass at high concentrations but slightly gain mass at low concentrations. This on a graph should look like this, with a negative gradient:

  • Scientific Proof and Background information:

  • All living cells are surrounded membranes. These are very thin sheets of material which contain tiny holes. This allows particles that are smaller than the holes to pass through easily (e.g. a solvent like particle of water), but other bigger particles are too large to pass through the holes (e.g. dissolving substances like sugar).
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  • Water Particles can get through these holes, so that if a membrane separates a strong solution from a weak solution, the water may pass through the membrane to dilute the strong solution until both solutions are the same concentration. Diffusion of water molecules across a partially permeable membrane like this is called OSMOSIS.

  • The diagram below shows Osmosis at work. The water molecules move through the partially permeable membrane from a high water concentration to a lower water concentration until, both is equal. As the water moves from a high water concentration solution to a low water ...

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