What structure primarily facilitates the movement of substances in and out of the cell?

The cell membrane is the structure that primarily facilitates the movement of substances in and out of the cell. This semi-permeable membrane surrounds the cell and acts as a selective barrier, allowing certain molecules to pass through while blocking others. It is composed of a lipid bilayer with embedded proteins that help transport substances. These proteins function as channels or transporters, enabling essential nutrients to enter the cell and waste products to exit.

Additionally, the cell membrane plays a crucial role in communications within the organism and helps maintain the cell's homeostasis by regulating the internal environment against external changes. Its dynamic nature is critical for processes like endocytosis and exocytosis, which are essential for the uptake of larger molecules and the secretion of substances. This makes the cell membrane integral to the overall functioning of the cell in terms of substance exchange.