Lesson Four: Demonstration of Osmosis in Living Cells using Potato

• Outcome
• Starter activity ( 5 minutes)
• Main Activity (15 minutes)
• Experiment to Demonstrate Osmosis
• Suggested Assessment Task

Outcome

• Practically demonstrate how osmosis takes place in living cells.
• Observe the effect of osmosis on living cells.
• Explain the effects of osmosis in living cells using potato cubes.

Materials required: Potato tubers (cylinders), razor /blade/, salt solution, distilled water, Petri dishes, ruler, absorbent tissue. A chart illustrating osmosis.

Starter activity ( 5 minutes)

The teacher may use different ways of introducing the lesson. He/ she may use the method of activating background knowledge.  For example he/she can review the previous lesson by asking the following questions.

• What is diffusion ? 
• What are the transport mechanisms in living system  or  in our cells?
• Why do we need the transport of substances?

The above questions help to engage students and they stimulate for active participation.  The teacher may expect possible answers as follows.

Diffusion is the movement of molecules from a region of high to a low concentration until equilibrium is reached. Substances move down concentration gradient in diffusion. Diffusion is a passive process (does not require energy). There are different mechanisms of transport of substances in living organisms or in cells. These are simple diffusion, osmosis, facilitated diffusion and active transport. We need to transport substances for cells’ metabolic activities and regulation of the internal environment. Cells need oxygen and different nutrients for sustaining life.  Conversely, metabolic waste products are not needed by cells and they should be removed from cells.

Main Activity (25 minutes)

The teacher should organize the learners into groups and guide learners. The learners are required to set up the experiment on osmosis using the procedure below.

Experiment to Demonstrate Osmosis

Procedure

• Cut the potato cylinder into 2 cm pieces.
• Place 4 pieces in each of the salt solution and distilled water.
• Observe after 20 minutes.
• Measure the length of potato cubes in each container (salt solution and distilled water).

Complete the calculations in figure below using the formula provided.

% Change in length = (change in length × 100) / Initial length

Change in length = Final length - Initial length

Note: The change in length may be negative in some cases whenever there is a loss of water. 

Fig 4.1 Plant Cells Surrounded by Different Solutions

Note: Osmosis is solely used for describing movement of water molecules only.  Sometimes students described osmosis as the movement of water potential rather than the movement of water molecules.

Always remember that water potential is a measure of the tendency of water molecules to move from one place to another. A dilute solution contains more water molecules per unit volume than a concentrated solution so it has a higher water potential than a concentrated solution

Suggested Assessment Task

The teacher is expected to evaluate the success of the lesson.  As this lesson is a practical activity, students are expected to prepare a report of their practical investigation for the next class.  Actually, the following questions can be formulated to enable students draw conclusions from their experiment.

• In which potato cube did you observe water gain by the cell? Why?
• For which potato cubes did you observe the highest loss of water? Why? 
• In which of the experimental set up did you notice turgidity of potato cubes?
• What condition do you think is the 50% NaCl? (hypertonic, isotonic , hypotonic)
• If a red blood cell with internal concentration of 10% NaCl  is placed in a 90% NaCl solution, what would happen to the cell? Why?