In this picture, Endocytosis is a form of active transport in which the cell transports molecules (such as protein) into a cell by engulfing them in an energy using process. It is also the taking in of matter by a living cell by invagination of its membrane to form a vacuole. Endocytosis and its counterpart, exocytosis, are used by all cells because most of the chemical substances that are are important to them are large polar molecules that can not pass through the hydrophobic plasma or cell membrane by passive means. Eukaryotic cells internalize fluid, large and small molecules and even other cells from their surroundings by the process called endocytosis. During endocytisis, The plasma membrane of a cell forms a pocket around the material to be internalized. The pocket closes then separates from the inside surface of the plasma membrane to form a membrane-enclosed bubble or vesicle containing the ingested material.
Endoytosis is the ability to internalize material from outside of the cell and it is important for several cellular processes including the ingestion of the essential nutrients, the removal of dead or damaged cells from the body, and defense against the macro organisms. Endocytosis pathways can be sub divided into 4 categories such as Receptor Mediated Endocytosis, Caveolae, Macropinocytosis, and Phagocytosis. Receptor Mediated Endocytosis is mediated by the production of small vesicles that have a morphologically characteristic cot made up of cytosolic protein clathrin. Clathrin coated vesicles (CCV's) are found in virtually all cells and form domains of the plasma membrane termed clathrin coated pits. Coated pits can concentrate large extracellular molecules that have different receptors responsible for the receptors mediated endocytosis of ligands, low density lipoproteins, transferrin, growth factors, anti bodies, and many other more. Caveolaes are the most common reported non clathrin coated plasma membrane buds, which can exist on the surface of many but not all cells type. they consist of cholesterol binding protein caveolin with a bilayer enriched in cholesterol and glycolipids. Caveolae are small flasked shaped pits in the membrane that resemble the shape of a cave. They can constitute up to a third of the plasma membrane of the cells of some tissues being especially abundant in smooth muscle, type 1 pneumocytes, fibroblast, adipocytes, and endothelial cells. Uptake of extracellular molecules is also believed to be specifically mediated to the receptors in caveolae. Macropinocytosis which usually occurs from highly ruffled regions of the plasma membrane, is the invagination of the cell membrane to form a pocket, which the pinches off into the cell to form a vesicle filled with a large volume of extracellular fluid and molecules within it. The filling of the pocket occurs in a non specific manner. The vesicles then travels into the cytosol and then fuses with other vesicles such as endosomes and lysosomes. Phagocytosis si the process by which cells bind and internalize particulate matter larger than around 0.75um in diameter, such as small sized dust particles, cell debris, micro organisms and apoptotic cells. These processes involve the uptake of larger membrane areas then clathrin mediated endocytosis and caveolae pathways. The endocytic pathways of mammalian cells consists of distinct membrane compartments which internalize molecules from plasma membrane and recycle them back to the surface (as in early endosomes and recycling endosomes), or sort them to degration (as in late endosomes and lysosomes).