What part of the cell controls what enters and exits the cell?

The correct answer is the cell membrane, which plays a crucial role in maintaining the internal environment of the cell. The cell membrane, also known as the plasma membrane, is a selectively permeable barrier. This means it regulates the passage of substances in and out of the cell, ensuring that essential nutrients can enter while waste products and harmful substances are kept out or expelled.

The structure of the cell membrane is primarily composed of a phospholipid bilayer with embedded proteins that assist in transport. This arrangement allows the cell membrane to selectively allow certain molecules, such as water, oxygen, and glucose, to pass through while blocking others, maintaining homeostasis within the cell.

In contrast, other components like the cell wall provide structure and protection in plant cells but do not control substance movement in the same selective way. The nucleus is responsible for storing genetic information and regulating cellular activities, while the cytoplasm is the jelly-like substance surrounding organelles within the cell, facilitating cellular processes but not specifically controlling what enters or exits the cell.