What structure is responsible for controlling the movement of substances into and out of a cell?

The cell membrane, also known as the plasma membrane, is a critical structure that regulates the movement of substances into and out of a cell. It is composed of a phospholipid bilayer with embedded proteins, which creates a semi-permeable barrier. This means that the cell membrane allows certain molecules to enter or exit the cell while keeping others out. The embedded proteins play various roles, such as acting as channels or transporters for specific substances, receptors for signaling molecules, and enzymes that facilitate chemical reactions.

Additionally, the cell membrane is involved in vital processes like endocytosis and exocytosis, which allow the cell to uptake larger molecules or expel waste products. This selective permeability is essential for maintaining homeostasis within the cell, managing the internal environment, and responding to changes in the external environment.

Other structures mentioned, like the nucleus, cytoplasm, and ribosome, have different functions that do not directly pertain to the movement of substances across the membrane. The nucleus contains genetic material and regulates cellular activities, the cytoplasm is the fluid that supports cellular components, and ribosomes are involved in protein synthesis. None of these structures control the entry and exit of materials as effectively as the cell membrane.