Water Concentration and Change in Turgor Pressure in Plant Cells
The stiffness of plant stems, roots, and leaves is due to the presence of water in their cells. Plants exhibit turgor when they stand erect and return to their original position after being bent This rigidity in plants is the result of the firmness of each water-filled cell.
In this project, you will determine the changes in turgor pressure in plants as a result of increases and decreases of water concentration in a plant's cells. Factors affecting the absorption of water into cells, such as variations in cell types, temperature, and permeability of the cell membrane, will be determined. You will also study the effect of turgor pressure on plant movement
Purpose: To demonstrate the effects of turgor pressure on an animal cell membrane.
- Baby food jar
- White vinegar
- Raw egg in shell
- 1-cup (250-ml) measuring cup
- Distilled water
CAUTION: Always wash your hands after touching an uncooked egg. It may contain harmful bacteria.
- Fill the jar with vinegar.
- Stand the egg in the jar of vinegar with the small end of the egg below the surface of the vinegar (see Figure 7.1).
- Put the jar and the egg in the refrigerator to prevent the egg from spoiling.
- After 24 hours, remove the egg and discard the vinegar.
- Carefully place the egg into the measuring cup without cracking the eggshell.
- Fill the cup with distilled water.
- Put the cup in the refrigerator.
- Observe the egg for seven days.
The membrane exposed by the vinegar swells and finally ruptures. Cracks in the shell starting at the edge of the exposed membrane form and extend across the egg. (See Figure 7.2.)
The egg is a single cell surrounded by a cell membrane. This membrane—the shell membrane—surrounds and controls the passage of materials into and out of the egg.
Membranes that are selective in what passes through them are called semipermeable membranes. Pores in the membranes are large enough to allow the easy passage of water molecules, but they are too small to allow larger molecules such as fats and proteins to get through. The movement of water through a cell membrane is called osmosis and occurs when there is a difference in the concentration of water on either side of the membrane.
The swollen shell membrane ruptures when placed into a hypotonic solution (a solution with a higher water concentration than that of the area to which it is compared). The water in the cup (100% water) is hypotonic to the fluid content of the egg. As more water moves into the egg through the membrane, the cell becomes crowded with excess molecules, which results in a buildup of pressure. This pressure caused by excess water is called turgor pressure. As the fluid content of the egg continues to increase, the pressure of the expanding shell membrane breaks the hard eggshell. The thin, unprotective shell membrane stretches under the pressure, creating a bulge that ultimately ruptures.
Try New Approaches
- How do the results change when the egg is placed into a hypertonic solution (a solution with a lower water concentration than that of the area to which it is compared)? Repeat the experiment replacing the distilled water with a salt solution made with 1 cup (250 ml) of water and 1 tablespoon (15 ml) of table salt (sodium chloride). Science Fair Hint: Use a data table to record written descriptions and diagrams of observations made of eggs placed into hypertonic and hypotonic solutions.
- If more of the membrane is exposed, does the egg continue to swell and rupture when placed into a hypotonic solution? Repeat the original experiment removing the entire shell from the egg by covering the egg with white vinegar for 24 hours. Measure the circumference of the egg before placing it into the vinegar (mixture of acetic acid and water) and before placing it into the water. After placing it in the water, measure it daily for seven days or until the egg breaks (if it does). Use these measurements to determine the change in size of the cell due to osmosis and whether the water continues to enter the cell at an even rate each day.