Endocytosis is the process of bringing something into a cell without its passing through the cell membrane via transport. This is particularly important for polar or very large molecules. There are three generally accepted mechanisms for endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis.
All of the processes involve vesicle formation, a “bubble” inside the cell, created from the plasma membrane as the external material is engulfed and brought into the cell. How this vesicle, called an endosome, forms is the difference in the mechanisms.
Macrophages are probably the most well-known cells that practice phagocytosis. This is the process by which foreign material and pathogens are “eaten” and digested as the body battles or prevents disease. The cell membrane sends out projections called pseudopodia (‘false feet’). A ligand-receptor interaction tells the cell if it found something that should be engulfed. The pseudopodia extend and the membrane folds around the material. When the two projections meet, the membrane fuses to fully enclose it within the intracellular vesicle, which then pinches off from the main membrane. See an animation of phagocytosis.
The endosome in this case is called a phagosome, or vacuole if very large. The process occurs only in specialized cells. The contents of the phagosome are delivered to lysosomes, vesicles containing enzymes that digest the material.
This mechanism is very similar to phagocytosis, except the material being endocytosed is liquid. Instead of pseudopodia, the membrane forms a depression. The material to be engulfed gathers in the cavity, which closes off when the edges of its opening meet, resulting in membrane fusion. The intracellular vesicle is pinched off from the main membrane.
This process occurs continuously in almost all cells and brings in extracellular fluid. A subtype of this process involves caveolae, also known as lipid rafts. They implicate endocytosis in cell signaling and have been associated with cardiovascular functions, among others.
A well studied application of this process in the human body is cholesterol (specifically LDL) entry into cells, but iron transport also occurs via this process. Basically, a molecule is recognized by a specific receptor on the cell surface, triggering an intracellular vesicle to fold around the material and pinch off from the membrane. See an animation of receptor-mediated endocytosis.
A subtype of this process is clathrin-mediated endocytosis. Cell surface receptors may be embedded in clathrin-coated pits. Clathrin is an intracellular protein attached to the membrane in regions where the receptors are found. When enough receptors are bound, the vesicle forms, similarly to pinocytosis, but is coated with the clathrin proteins. Therefore, this mechanism of endocytosis results in coated vesicles.
Summary of the cell entry mechanism
Endocytosis is an essential process by which certain materials are brought into the cells. It occurs by recognition of the material and then formation of an intracellular vesicle from the cell membrane. What is done with the material once it is inside the cell depends on the process used to bring it in. Not all cells are capable of every mechanism, though every cell is capable of some form of endocytosis.
See additional animations of the processes by McGraw-Hill.