Effects of Solution Concentration on Cells

Aim

This experiment investigates the effect of immersing plant tissue (red onion or Rhoeo discolor epidermis, which show a clear colour change) in solutions of different concentration, and compares this with the expected effect on animal cells (such as red blood cells) using diagrams or prepared slides, since fresh blood is not handled directly in a school laboratory.

Variables

• Manipulated variable, the concentration of the external solution (distilled water and at least three different sucrose or salt concentrations).
• Responding variable, the condition of the plant cells, observed and counted under the microscope as turgid, flaccid, or plasmolysed; and, for comparison, the condition of animal cells described using diagrams as normal, haemolysed (burst), or crenated (shrunken).
• Controlled variables, the immersion time, the temperature of the solutions, the type and size of the tissue sample, and the volume of solution used.

Materials and Apparatus

• a red onion or a Rhoeo discolor leaf
• distilled water
• sucrose solutions of at least three different concentrations
• forceps
• a scalpel
• microscope slides and cover slips
• a light microscope
• a stopwatch
• prepared diagrams or slides of red blood cells in different solutions, for the animal-cell comparison

Procedure

1. Using forceps, peel thin strips of epidermis from the leaf or onion.
2. Place one strip of epidermis into each solution, including distilled water as one of the conditions, for a fixed time such as 15 to 20 minutes.
3. Mount each strip on a separate slide in a drop of the same solution it was immersed in, and add a cover slip.
4. Observe each slide under the microscope and record the proportion of cells that appear turgid, flaccid, or plasmolysed.
5. Compare these observations with the diagrams of red blood cells behaving in a similar way in each type of solution, to relate the plant-cell results to the expected animal-cell response.

Expected Results

The condition of the plant cells is expected to change with the concentration of the external solution, and the diagrams predict a similar pattern for animal cells, though the specific terms used differ between the two cell types.

Conclusion

Cells placed in a dilute (hypotonic) solution gain water by osmosis: plant cells become turgid, while animal cells would swell and may burst. Cells placed in a concentrated (hypertonic) solution lose water by osmosis: plant cells plasmolyse, while animal cells would shrink and become crenated.

The cell wall allows plant cells to survive in dilute solutions that would burst an animal cell, since it resists the pressure that builds up as the cell takes in water.

Safety

Common Mistakes

Paper 3-style questions

Question 1 (hypothesis). A student places strips of red onion epidermis in distilled water and in a concentrated sucrose solution. State a hypothesis for what will happen to the cells, and the result that would support it.

Model answer. Hypothesis: cells in the concentrated sucrose solution will become plasmolysed, while cells in distilled water stay turgid. It is supported if, under the microscope, a larger proportion of cells in the concentrated solution show the cytoplasm pulled away from the cell wall.

Question 2 (variables and fair test). State the manipulated and responding variables and one variable that must be controlled to keep the comparison fair.

Model answer. The manipulated variable is the concentration of the external solution; the responding variable is the proportion of cells that are turgid, flaccid or plasmolysed. A controlled variable is the immersion time, kept the same for every strip so the comparison is fair.

Question 3 (tabulation and graph). The student counts plasmolysed cells at four sucrose concentrations. Suggest how to record the results and what a graph would show.

Model answer. Results are recorded in a table with concentration of sucrose against percentage of plasmolysed cells. A graph of percentage plasmolysed (y-axis) against concentration (x-axis) rises as concentration increases, showing more plasmolysis in more concentrated solutions.

Question 4 (inference). In the most concentrated solution nearly all the plant cells are plasmolysed. What inference can be made about water movement?

Model answer. The inference is that the cells have lost water by osmosis to the surrounding solution, because the external solution is more concentrated (hypertonic) than the cell sap. Water moves out of the cells down the water potential gradient, so the cytoplasm shrinks and pulls away from the cell wall.

Source:SRC-DSKP-EN

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