What controls the flow of substances in and out of a cell?

The plasma membrane (also called the cell membrane) controls the flow of substances in and out of the cell. This thin, flexible boundary consists of a phospholipid bilayer embedded with proteins, cholesterol, and carbohydrates, creating a selectively permeable barrier that regulates molecular traffic. The membrane's structure allows it to maintain the cell's internal environment while facilitating necessary exchanges with the external surroundings.

Control mechanisms include passive processes like diffusion and osmosis for small or non-polar molecules, and active processes requiring energy for moving substances against concentration gradients. Specialized transport proteins act as gates or carriers for specific molecules, while receptor proteins respond to chemical signals to trigger controlled entry or exit of substances. This regulatory function is vital for maintaining proper concentrations of ions, nutrients, and waste products, enabling the cell to respond to environmental changes, and ensuring cellular homeostasis essential for survival.