Potato and Osmosis Lab-Sailor and the Ocean
Principle(s) Investigated: The concentration of solute in a solution will affect the movement of water across potato cell membranes.
Chemistry: 6. Solutions are homogeneous mixtures of two or more substances. As a basis for understanding this concept:
a. Students know the definitions of solute and solvent.
1. The fundamental life processes of plants and animals depend on a variety of chemical reactions that occur in specialized areas of the organism’s cells. As a basis for understanding this concept:
a. Students know cells are enclosed within semipermeable membranes that regulate their interaction with their surroundings.
Introduction: A shipwrecked sailor is stranded on a small desert island with no fresh water to drink. She knows she could last without food for up to a month, but if she didn't have water to drink she would be dead within a week. Hoping to postpone the inevitable, her thirst drove her to drink the salty seawater. She was dead in two days. Why do you think drinking seawater killed the sailor faster than not drinking any water at all? Today we explore the cause of the sailor's death. We'll prepare solutions of salt water to represent the sea, and we'll cut up slices of potato to represent the sailor. Potatoes are made of cells, as is the sailor!
The human body is about as salty as seawater. If we take seawater as an example of a solution, the salt is called the solute and the water is the solvent. Osmosis is the movement of water across a membrane from an area of lower solute concentration to an area of higher solute concentration. 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.
• electronic balance (0.01 mg range)
• 6 dishes or pans
• potatoes (1 or 2 per class)
• single edged razor or knife
• paper towels
• watch or clock
• table salt, tap water
6 beakers (250 ml or larger)
1) Pre-mix 6 beakers of salt solutions (0.0001%, 0.001%, 0.01%, 0.1%, 1%, 10%) in water.
2) Prepare six potato slices that are the same thickness (approximately 5 mm cubes) and blot them dry on a paper towel.
3) Mass (weigh) each to the nearest 0.1 grams, keeping them separate, and record as initial mass.
4) Fill each pan with one of the 6 stock solutions, keeping track of which is which!
5) Leave one of the potato slices in each of the salt solutions for at least 15 minutes so that they may gain (or lose) water by osmosis (Keep them all in the salt water the same amount of time).
6) Remove the slices, blot them dry on a paper towel, carefully re-weigh them and record in the data table as final mass.
Student prior knowledge: What prior concepts do students need to understand this activity?
Explanation: Give a thorough explanation of the experiment or demonstration. Your explanation should be written to give your fellow teachers a solid understanding and include greater detail than what you might provide for your secondary students. Make certain to include equations whenever pertinent.
Questions & Answers: Give three thought-provoking questions and provide detailed answers.
Applications to Everyday Life: Explain (don't just list) three instances where this principle can be used to explain other phenomenon.
Photographs: Include a photograph of you or students performing the experiment/demonstration, and a close-up, easy to interpret photograph of the activity --these can be included later.