What is the primary function of the cell membrane?

The primary function of the cell membrane is to regulate the entry and exit of substances in and out of the cell. This selectively permeable barrier is critical for maintaining the cell's internal environment, known as homeostasis. It controls which molecules can enter or leave the cell, allowing essential nutrients in while keeping harmful substances out. The cell membrane achieves this through various mechanisms, including passive transport, active transport, and facilitated diffusion.

In contrast, energy production primarily occurs in the mitochondria of the cell, while cell division involves processes separate from the function of the membrane, such as mitosis. Protein synthesis takes place in the ribosomes, where the genetic information is translated into proteins, and is not directly related to the membrane's functions. Understanding the role of the cell membrane is fundamental in the study of cell biology and physiology, as it underpins many cellular processes that are critical for life.