The cell membrane acts as a barrier that allows certain substances to pass freely, but blocks the passage of others. Large molecules, such as proteins, polysaccharides, and nucleic acids are too large and too charged or polar to slip easily through the plasma membrane. If a cell takes them in, or releases them, it does so by the processes of endocytosis or exocytosis. In endocytosis, the cell's membrane surrounds a part of the exterior environment and buds off as an internal vesicle. In exocytosis, an internal vesicle fuses with the plasma membrane and thereby releases its contents to the outside of the cell.
Endocytosis and exocytosis are important mechanisms for bringing substances into and out of a cell.
Cells use exocytosis to secrete proteins. Many of the proteins made on the rough endoplasmic reticulum and processed by the Golgi apparatus are sorted into vesicles destined for the plasma membrane. The vesicles fuse with the plasma membrane, causing the contents of the vesicles to spill outside the cell. The proteins embedded in the vesicle's membrane become part of the plasma membrane.
Three types of endocytosis bring substances into the cell. Using phagocytosis, certain protists and white blood cells are capable of engulfing very large particles. Using pinocytosis, cells continually take in dissolved substances and liquids from the outside. Using receptor-mediated endocytosis, animal cells take in specific molecules that can bind to certain receptors on their plasma membranes.
Receptor-mediated endocytosis is an important mechanism the body uses to remove cholesterol from the bloodstream. Persons with the inherited disease hypercholesterolemia lack normal LDL receptors and, as a consequence, have dangerously high levels of circulating cholesterol. Their livers cannot remove the circulating LDL particles, and so the load of cholesterol tends to accumulate on the walls of their arteries, which eventually blocks blood flow.