Which structure controls what enters and exits the cell?

The cell membrane, also known as the plasma membrane, plays a crucial role in controlling what enters and exits the cell, maintaining homeostasis. This semi-permeable membrane is composed of a phospholipid bilayer with embedded proteins that facilitate the transport of substances in and out of the cell. Its selective permeability allows it to regulate the internal environment of the cell, enabling the passage of essential nutrients while keeping harmful substances out. Additionally, the membrane also has receptor sites that can interact with signaling molecules, influencing various cellular processes.

In contrast, the nucleus is responsible for housing the cell's genetic material and regulating gene expression but does not directly control transport across the cell boundary. Mitochondria are primarily involved in energy production through cellular respiration, and the cytoplasm provides a medium for biochemical reactions and organelle suspension. However, neither of these structures has a direct role in regulating the movement of substances across the cell membrane.