Cell Homesostasis: A Steady State
The survival of living cells greatly depends on their capacity to achieve homeostasis. The ability of a cell to regulate a stable internal environment is accomplished by controlling the movement of materials through the cell membrane. When this delicate balance is lost, the cell can be injured and even die.
In this project, you will study osmosis and determine how placing cells into hypotonic and hypertonic solutions affects osmosis. The rate of water movement through the cell membrane of an egg will be calculated, and factors affecting osmotic and turgor pressure will be studied.
Purpose: To determine the effect of placing a cell into a hypotonic solution (a solution with a high concentration of water as compared to that in the cell).
- baby food jar with small mouth
- distilled water
- raw egg in shell
- metal spoon
- drinking straw (clear or transparent)
- 8-inch (20-cm) candle (or longer)
CAUTION: Always wash your hands after touching an uncooked egg. It may contain harmful bacteria.
- Fill the baby food jar three-fourths full with distilled water.
- Gently tap the rounded end of the egg's shell with the edge of the spoon.
- Use your fingernails to carefully pull away several small pieces of the eggshell. Note: You want to remove a small section of the shell about the size of the tip of your finger. Be careful not to pierce the thin membrane under the shell.
- On the opposite end of the egg, break a hole in the shell large enough to insert the end of the straw.
- Stand the egg in the jar of water, rounded end down.
- Insert about 1 inch (2.5 cm) of one end of the straw into the hole in the shell and through the cell membrane.
- Light the candle and hold the unlit end.
- Allow the melted wax to drip around the base of the straw until the space between the straw and the eggshell is sealed (see Figure 6.1).
- Allow the egg and attached straw to stand undisturbed overnight.
- Observe any movement of liquid.
A clear, watery liquid moves up the drinking straw.
Cell homeostasis is the ability of a cell to achieve a stable internal environment by regulating the passage of fluids through the cell membrane. Removing the eggshell exposes the semipermeable membrane of the cell (a hen's egg is a single cell). A semipermeable membrane is a membrane through which some substances can pass but others cannot. Water molecules are small enough to move through the membrane, but larger molecules inside the cell are too large to move out of the cell. The movement of water through the membrane is called osmosis.
Osmosis occurs when there is a difference in the concentration of water on either side of the membrane. The water concentration in the jar is 100%. The water concentration inside the egg is less than 100% because of the dissolved fats, proteins, and other materials. Water moves through any cell membrane from an area of high water concentration to an area of low water concentration. Thus, the water in the jar moves through the exposed membrane of the egg and into the egg. As water enters the egg, the volume of fluid increases and the extra molecules are forced up the straw. This force is produced by osmotic pressure (the pressure of water diffusing through the semipermeable membrane). The greater the difference in water concentration on either side of the membrane, the greater the osmotic pressure.