- Explain the process of endocytosis and exocytosis.
A few materials the cell must take in or expel are too big to cross the cell membrane via active or passive transport so the cell uses a method of transport that does not require the materials to pass through the phospholipid bilayer. The cell membrane can fold in on itself to create a bubble-like sac called a vesicle which it uses to engulf or expel large materials.
When the cell membrane folds inward trapping and enclosing a small amount of matter from the extracellular fluid, the process is called endocytosis (endo meaning "within"). There are three forms of endocytosis:
- Pinocytosis: Cell "drinking" involves the intake of a small droplet of extracellular fluid with any dissolved subtances or small particles it may contain. This process occurs in nearly all cell types nearly all of the time.
- Phagocytosis: Cell "eating" involves the intake of a large droplet of extracellular fluid, often including particulate matter such as bacteria or bits of organic matter. This process occurs only in specialized cells such as amoeba or the bacteria-eating cells of the human immune system (macrophages) and only when they encounter something suitable for engulfing.
- Receptor-assisted endocytosis: Animal cells bring in cholesterol using receptor-assisted endocytosis. To ensure that the membranes of cells and of structures within cells have enough cholesterol, the liver manufactures the cholesterol the cells need from natural lipids in the diet. However, being a lipid, cholesterol cannot dissolve directly in the blood or extracellular fluid, thus cholesterol cannot cross the cell membrane by pinocytosis. Cholesterol molecules are transported in the blood and extracellular fluid inside vesicles with protein tags on their membranes which can only be recognized by matching receptors on the cell membrane. Once the tag and receptor connect, the surrounding membrane folds inward, forming a double vesicle with cholesterol inside. The vesicles pinch in two making two separate vesicles one with the cholesterol and the other with the now-empty receptors. The cholesterol is released in the cell and the receptors are returned to the cell membrane.
The reverse of endocytosis is exocytosis in which a vesicle from inside the cell fuses with the cell membrane and releases its contents into the extracellular fluid. Exocytosis is especially important in cells that specialize in secretion of various cell products. For example, specialized cells in the human pancreas secrete the hormone insulin by means of exocytosis.