Objectives: 1. Describe the direction of the movement of water molecules across a differentially permeable membrane and the condition that brings this about. 2. Observe the effects of osmosis in living cells. Materials: - 2 pcs transparent glass/plastic cup - 4 strips of raw potato (peeled and cut into the same length and width) - 2 teaspoons salt - Water - Ruler marker Procedure: 1. Measure the dimensions of the potato strips. Record these data in the table below. 2. Fill the cups with water (height just enough to cover the potato strip). Label the first cup as A and the second cup as B. Place 2 teaspoons of salt into container B. Stir the contents in container B until salt is fully dissolved. Mark the location of the top water level in each container. 3. Soak the potato strips for 30 to 45 minutes in the container, two in each container. 4. Take the strips out of the container and measure the dimensions of the soaked strips. Also observe the level of water in each container. Are there changes? Record this data on the table below. | Container | Solution | Sample | BEFORE SOAKING | AFTER SOAKING | | | |-----------|---------------|--------|----------------|---------------|------|------| | | | | Length | Width | Length | Width | | A | Pure/Tap water| 1 | | | | | | | | 2 | | | | | | B | Salt solution | 1 | | | | | | | | 2 | | | | | Questions to Answer: 1. What happens to the potato strips in container A after soaking? 2. What happens to the potato strips in container B after soaking? 3. What may have caused these to happen to potato strip in A? in B?
Answer (1)
To understand the student's inquiry, we need to explain the principles of osmosis and how it applies to the experiment with the potato strips.
Osmosis and Water Movement : Osmosis is the movement of water molecules across a differentially permeable (or semi-permeable) membrane. In this case, the membrane is the potato cell walls. Water moves from an area of low solute concentration (more water) to an area of high solute concentration (less water) to achieve equilibrium.
Experiment Procedure and Observations :
Container A - Pure/Tap Water : The potato strips in container A are soaked in pure or tap water. Here, the concentration of water outside the potato is higher than inside the cells, causing water to move into the potato strips by osmosis. This may cause the strips to swell or become turgid as they absorb water.
Container B - Salt Solution : The strips in container B are soaked in saltwater, which has a high solute concentration compared to the inside of the potato cells. In this case, water will move out of the potato strips into the salt solution to balance the concentration levels. This movement causes the potato strips to lose water and they may become limp or shrink.
Reasons for Changes :
In Container A : The potato strips likely become firmer and may increase in size slightly as they gain water in an attempt to equalize water concentration inside and outside the cell membrane.
In Container B : The potato strips are expected to become limp or decrease in size as they lose water to the hypertonic (high salt concentration) environment. The salt solution draws water out of the potato, leading to plasmolysis where the cell membrane pulls away from the cell wall.
This experiment demonstrates osmosis, a fundamental biological process, in living cells and highlights how different environmental conditions affect cell water content.
