Final answer:
The cell membrane controls what enters and leaves the cell, maintaining a stable internal environment by allowing some substances to pass through while keeping others out.
Step-by-step explanation:
The cell membrane, also known as the plasma membrane, controls what enters and leaves the cell. It is selectively permeable, allowing some substances to pass through while keeping others out. This helps to maintain homeostasis, or a stable internal environment, within the cell. For example, the membrane allows helpful molecules like nutrients to enter the cell, while blocking unwanted or dangerous substances from entering.