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Osmosis

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Author: Janice VanCleave

So You Want to Do a Project about Osmosis!

Let's Explore

Purpose

To demonstrate osmosis.

Materials

  • 2 small bowls
  • tap water
  • vegetable knife (requires adult help)
  • potato
  • adult helper

Procedure

  1. Fill one of the bowls half full with water.
  2. Ask your adult helper to cut four slices about inch (0.63 em) thick from the potato.
  3. Test the firmness of each potato slice by holding it in your hands and trying to bend it. Do not break the slices.
  4. Place two potato slices in each bowl.
  5. After 30 minutes, pick up the potato slices one at a time from the water with your fingers. Again test their firmness by trying to bend them.
  6. Repeat step 5, using the potato slices that were not placed in water.
  7. How does the firmness of the potato slices soaked in water compare to the firmness of those not soaked in water?

Results

The slices not soaked in water do not change or decrease only slightly in firmness. The potato slices soaked in water are firmer and do not bend easily.

In and Out

Why?

Osmosis is the movement of water through a type of membrane (thin layer of animal or plant tissue) called a semipermeable membrane. This membrane selectively allows materials, such as water, to pass through. Each cell in the potato has a semipermeable membrane around it.

Inside each potato cell is a solution, which is a mixture of a solute (the substance that is dissolved) and a solvent (the substance that does the dissolving). When a solute dissolves, it is broken into small particles and mixes thoroughly with a solvent. The cell solution is made of water (the solvent) with different solutes dissolved in it. Tap water is a solution with a very low concentration (the amount of solute in a solution) of solutes. In this investigation, the solution outside the cell (tap water) has a lower concentration of solute and a higher concentration of water than the solution inside the cell, therefore the tap water is called a hypotonic solution. When a cell is placed in a hypotonic solution, water moves into the cell. This increases the pressure inside the cell. Water moves through a semipermeable membrane from the side with a low solute concentration to the side with a high solute concentration.

The pressure inside a plant cell due to the presence of water is called turgor pressure. As the water inside the cells increases, the turgor pressure increases, and the water-soaked potato slices feel firm. The turgor pressure of the potato slices not placed in water may have decreased slightly due to a loss of water as the water from inside the potato cells moved out of the cells.

For Further Investigation

If the solution outside the potato cells has a higher concentration of solute and a lower water concentration than the solution inside the cells, the outer solution is called a hypertonic solution. A project question might be, How does soaking potato cells in a hypertonic solution affect turgor pressure?

Clues for Your Investigation

  1. Repeat the investigation, adding salt to the water in the bowl. Since you do not know the concentration of the solute inside the potato, you can have several bowls of water and use varying amounts of salt in each.
  2. Design a way to measure the results, such as how far the slice will bend before breaking.

In and Out

References and Project Books

Bonnet, Robert L., and G. Daniel Keen. Botany: 49More Science Fair Projects. Blue Ridge Summit, Pa.: Tab Books, 1991.

Chisholm, Jane. Introduction to Biology. London: Usborne, 1984.

Hershey, David R. Plant Biology Science Projects. New York: Wiley, 1995.

VanCleave, Janice.]anice VanCleave's Biology for Every Kid. New York: Wiley, 1990.

____.Janice VanCleave's Plants. New York: Wiley, 1996.

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