Osmosis in Living Cells

Activity 3: Pre-Assessment

1. Saline solutions are given to patients in an IV. Why might the salt concentration in the solution need to be the same as that in the blood cells?
2. Discuss the answer to question 1 with your group.

Activity 3: Osmosis in Living Cells^[1]

A cell lacking a cell wall is affected greatly by the tonicity of the environment. In a hypertonic solution where the concentration of dissolved solute is high, water will be drawn out of the cell. In a hypotonic solution where the concentration of dissolved solute is lower than the interior of the cell, the cell will be under great osmotic pressure from the environmental water moving in and can rupture (Figure 6.5).

Plants have rigid cell walls composed of cellulose. These cell walls permit for maintenance of cellular integrity when the external environment is hypotonic (less dissolved substances). In this situation, the water moves into the cell. Without the cell wall, the cell would burst open from the excessive water pressure entering the cell. This state of swelling is referred to as turgid, resulting from turgor pressure (Figure 6.6).

The inside and the outside of a cell are mainly composed of water with dissolved solutes. The differences in solute concentration direct the movement of water across the plasma membrane of the cell. This difference between the two solutions (i.e., the cytoplasm and the extracellular fluid) is known as tonicity. Solutions are said to be isotonic if they both have equal concentrations of solute. The extracellular fluid is hypertonic to the intracellular fluid if it has a higher solute concentration. On the other hand, a lower solute concentration in the extracellular fluid compared to the inside of the cell would mean that the outside of the cell is hypotonic to the inside.

We can observe tonicity in Elodea leaves by placing them into different solutions with various solute concentrations.

Safety Precautions

• Be careful handling glass slides; the edges may be sharp.
• Observe proper use of the microscope; avoid handling the electric cord with wet hands.
• Do not use the coarse adjustment knob of the microscope at higher magnifications.
• Inform your teacher immediately of any broken glassware, as it could cause injuries.
• Clean up any spilled water or other fluids to prevent other people from slipping.
• Handle all chemicals safely.

For this activity, you will need the following:

• Two glass slides
• Elodea leaves
• Two coverslips
• Distilled water
• 10 percent NaCl solution
• 30 percent NaCl solution
• Microscope
• Potato cubes
• Electronic balance
• Weigh boat
• Three 50 mL beakers

For this activity, you will work in groups of four.

Structured Inquiry

Step 1: Obtain 2 glass slides and 2 cover slips. On 1 slide, place an Elodea leaf and put 1 drop of distilled water. Make a second slide with an Elodea leaf but put a drop of 10% NaCl. Allow these to sit for 2–3 minutes.

Step 2: Hypothesize/Predict: Predict what will happen with the leaf that has been placed into NaCl. What do you think will happen to the leaf in water? Record your predictions.

Step 3: Student-led planning: Determine what happens to the Elodea leaf cells in both solutions by observing them under the microscope. Record your observations for each. Draw pictures that demonstrate what you observe.

Step 4: Critical Analysis: Are the predictions you made in step 2 supported by your data? Why or why not? What methods could you use to improve your results? Discuss with your group and then write your answers in your notebook.

Guided Inquiry

Step 1: Hypothesize/Predict: What happens to a cell in an isotonic solution? Hypertonic? Hypotonic? Predict what might happen to a potato in these solutions. Will it gain, lose, or maintain its weight? Write your ideas in your notebook.

Step 2: Student-led planning: Design a study that would test your hypothesis in step 1. Plan which solutions you will use and how to determine if osmosis happened in each. Show your design to a teacher for approval, then create data tables that you would need to test your hypothesis. Write your results in your notebook and create a graph or table to summarize your results.

Step 3: Critical Analysis: Discuss your results with your group. Are your results what you expected? How can you improve your experiment? Write your ideas in your notebook.

Assessments

1. Have you ever seen a slug hanging out on the steps? Oftentimes, people use salt to get rid of them. What do you think this does to the slug in terms of osmosis?
2. How can you predict whether osmosis will occur into or out of a cell?
3. Describe what is meant when we say that a cell membrane is selectively permeable. What types of molecules might have a difficult time crossing the membrane?