Admission Test for Dental Hygiene (ATDH) Practice

The cell membrane, also known as the plasma membrane, is the component responsible for defining the external boundary of a cell. It acts as a selective barrier that regulates the movement of substances in and out of the cell, maintaining the internal environment necessary for cellular function. The structure of the cell membrane consists of a phospholipid bilayer with embedded proteins, which allows for both protection and communication with the external environment. This boundary is crucial for maintaining homeostasis, allowing the cell to maintain the necessary conditions for processes such as metabolism and signaling.

While the cytoskeleton provides structural support and helps in maintaining the shape of the cell, it does not serve as the boundary. The nucleus holds genetic material and controls cellular activities but does not define the outer edge of the cell. Ribosomes are involved in protein synthesis and are not related to the formation of the cell's external boundary.