Homeostasis is the ability or tendency to maintain internal stability in an organism (cells) to compensate for environmental changes. An example of homeostasis is the human body keeping an average temperature of 98.6 degrees. Cellular homeostasis involves maintaining a balance of water and ions. This allows cells to maintain a higher concentration of sodium ions out the outside of the cell and a higher concentration of potassium ions inside. The membrane also allows for water to be pumped in or out, which affects the concentration of the cytoplasm.

Concentration gradient- movement from a higher concentration of molecules to a lower concentration of molecules. We talk about the concentration of molecules other than water (salt, sugar, etc.) and the concentration of water molecules.

Remember:

Hypotonic solutions contain a high concentration of water (solution) molecules and a low concentration of molecules other than water, like salt (solutes).

Hypertonic solutions contain a high concentration of molecules other than water (solutes like salts) and a low concentration of water molecules.

See picture below to review Isotonic, hypertonic, and hypotonic solutions.

Check Your Understanding

1. Why is maintaining homeostasis important to a cell?

2. How does movement of substances across the cell membrane maintain homeostasis?

3. How can you determine if a type of transport requires energy?

4. Why does passive transport not require energy?

5. Free Response: An amoeba lives in a freshwater lake with an isotonic environment to its cells. Pollution from runoff is changing the concentration of solutes in the lake and the water environment is becoming hypertonic to the amoeba's cells. Draw a diagram illustrating the amoeba's interaction with the initial environment and the new environment, use arrows to show the movement of water.