Osmosis Lab

Background:

Human blood, at 0.9% salt concentration, is a little less salty than seawater, which has a salt concentration of about 35 parts per thousand (3.5%).

If we take seawater as an example of a solution, the salt is called the solute (the particles that are dissolved) and the water is the solvent (the liquid that dissolves the particles).

Osmosis is the movement of a solvent across a semi-permeable membrane from an area of lower solute concentration to an area of higher solute concentration.

The water (the solvent) can move across the membrane but the dissolved solutes (the sodium and chloride ions that form salt) cannot.

In such situations, water will move across the membrane to balance the concentration of the solutes on both sides. Cells tend to lose water (their solvent) in hypertonic environments (where there are more solutes outside than inside the cell) and gain water in hypotonic environments (where there are fewer solutes outside than inside the cell). When solute concentrations are the same on both sides of the cell, there is no net water movement, and the cell is said to be in an isotonic environment.

In this lab we will test samples of potato tissue to see how much water they absorb or release in salt solutions of varying concentrations. This gives us an indirect way to measure the osmotic concentration within living cells.

Hypo=under, iso=equal, hyper=over

Problem/Purpose: What is the concentration of salt in a potato cell?

Hypothesis:

Materials:

• • electronic balance (0.01 g range)

  • metric ruler with mm scale

  • metric measuring cups

  • 6 cereal bowls or shallow pans

  • a small piece of raw potato to cut into six ~5 mm cubes (this square is 5 x 5 mm)

  • single edged razor or knife

  • paper towels

  • watch or clock

  • table salt, distilled or tap water

  • 6 beakers (250 ml or larger) or cups

Methods: General Hints

• 1. Pre-mix beakers of salt solutions in distilled water. You can use this solution calculator to help you make your solutions. Just enter the water volume of your container and the percentage of salt you want and it will tell you how many grams of salt to add. A 1% salt solution is 1 part salt to 100 parts water. To make a 1% salt solution, you could use a 100 ml bottle, add exactly 1 gram of salt (use your electronic balance) to your bottle, and bring the water volume up to 100 ml. To make a 0.1% solution, add 1 gram of salt to 1000 ml of water (or add 0.1 g salt to 100 ml of water). If you have more water than you need, just stir well and then discard the excess.