The structure that controls the passage of substances into and out of a cell is the cell membrane , also known as the plasma membrane. It is a selectively permeable barrier that surrounds the cell, separating its interior from the external environment and regulating the movement of molecules and ions based on size, charge, and lipid solubility
. The cell membrane is primarily composed of a phospholipid bilayer , with hydrophilic (water-attracting) heads facing outward and hydrophobic (water- repelling) tails facing inward. This arrangement creates a barrier that allows small, nonpolar molecules like oxygen and carbon dioxide to diffuse freely, but blocks larger or charged molecules
. Embedded within this bilayer are various membrane proteins that facilitate the transport of substances that cannot easily cross the lipid bilayer on their own. These include:
- Channel proteins that form pores allowing ions to diffuse directly.
- Carrier proteins (facilitators) that help specific molecules cross.
- Pumps that actively transport substances against their concentration gradients using energy
Additionally, the membrane can engulf large particles via endocytosis or expel them via exocytosis , processes involving membrane fusion and reformation
. In summary, the cell membrane's phospholipid bilayer combined with specific transport proteins controls the selective passage of substances into and out of the cell, maintaining cellular homeostasis and enabling communication with the environment
