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.
Particles or macromolecules that are too large and too charged or polar to pass through biological membranes are taken into cells by a process called endocytosis. Phagocytosis is one of three types of endocytosis.
In phagocytosis, which means "cellular eating," part of the cell membrane engulfs the particle. Unicellular protists use phagocytosis for feeding, and some white blood cells use it for defending the body, engulfing foreign cells or substances. The resulting vesicle, called a phagosome, fuses with a lysosome, and the contents are digested.
In another type of endocytosis, called pinocytosis, small vesicles pinch off from the cell membrane, bringing small dissolved substances and fluids into the cell. Pinocytosis means "cellular drinking."
A type of endocytosis called receptor-mediated endocytosis is used by animal cells to capture specific macromolecules from the cell's environment. For example, most mammalian cells use this method to take up cholesterol packaged in low-density lipoprotein particles.
The cell uses receptors that specifically recognize and bind to the LDL particle. The receptors are clustered together in a reinforced membrane structure called a coated pit.
An LDL particle contains one thousand or more cholesterol molecules at its core. A monolayer of phospholipids surrounds the cholesterol core and is embedded with proteins. These proteins are specifically recognized by the receptors in the cell's membrane.
The receptors in the coated pit bind to the proteins on the LDL particle. The pit is reinforced by a lattice-like network of proteins called clathrin. Additional clathrin molecules then add to the lattice, which eventually pinches off a part of the membrane with the attached LDL particle inside.
Inside the cell, the vesicle soon becomes uncoated as the clathrin molecules dissociate from the vesicle and from each other. Although not shown here, another vesicle then fuses with the LDL-containing vesicle, causing the pH inside to drop and forcing the receptors to dissociate from the LDL particle.
After the receptors are released from the LDL particle, the vesicle fuses a lysosome. A lysosome carries enzymes capable of digesting the LDL particle into amino acids and fatty acids. The cholesterol is released into the cytosol for use by the cell.
Meanwhile, the cell's LDL receptors are returned to the cell membrane. Once in the cell membrane, they can take in more LDL particles by receptor-mediated endocytosis.
In contrast to endocytosis, exocytosis is the process by which a variety of substances leave the cell. For example, newly made proteins destined for secretion are packaged into vesicles inside the cell and transported to the cell membrane.
The vesicle membrane and cell membrane fuse together. The contents of the vesicle spill outside the cell while the vesicle membrane is smoothly incorporated into the cell membrane.
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.