Which part of the cell is responsible for controlling what enters and exits?

The cell membrane serves as a selective barrier that controls the movement of substances into and out of the cell. It is primarily composed of a phospholipid bilayer with embedded proteins that facilitate transportation. This structure enables the cell to maintain homeostasis by allowing essential nutrients and molecules to enter while keeping out potentially harmful substances. The cell membrane's selective permeability is crucial for the cell’s functionality and survival, enabling processes such as nutrient uptake, waste removal, and communication with the external environment. In contrast, the mitochondria are involved in energy production, the nucleus contains genetic material and regulates gene expression, and ribosomes are responsible for protein synthesis, none of which directly control entry and exit of substances across the cellular boundary.