Here's why:
* Natural tendency of matter: Matter naturally moves from areas of high concentration to areas of low concentration (this is called diffusion).
* Sodium gradient: Inside a cell, the concentration of sodium is typically lower than outside the cell. Therefore, the natural tendency would be for sodium to move *into* the cell.
* Active transport: To move sodium *out* of the cell, the cell needs to use energy. This is because it's working against the natural concentration gradient. This energy-requiring process is called active transport.
Key takeaway: Active transport is essential for maintaining the proper balance of ions within cells, which is crucial for various cellular functions.
