The word ‘endocytosis’ is derived from the Greek words ‘endon’, meaning ‘within’, ‘kytos’, meaning ‘cell’, and ‘-osis’, meaning ‘process’. The process was first proposed and described by cytologist Christian De Duve in 1963. Examples of Substances Transported Using the Process Substances like fluids, electrolytes, proteins, and other macromolecules are taken up inside the cell using endocytosis.
Steps Involved in Phagocytosis
Example Immune cells such as white blood cells take up antigens such as viruses, bacteria, or dead cells, digest the content and move the digested material outside the cell.
2. Pinocytosis
Also known as cell drinking, it is the process by which small particles such as fluids and dissolved nutrients are taken up within the cell. Pinocytosis follows the same basic steps of endocytosis involving vesicle formation and transport. Once inside, the vesicle either combines with the lysosome or the cell membrane. Lysosomes then degrade the trapped molecules within the vesicles, thus releasing them into the cytoplasm. The cell undergoing pinocytosis is called a pinocyte. Example Absorption of fat droplets in cells lining the small intestine.
3. Receptor-mediated endocytosis (Clathrin-mediated endocytosis)
It is a selective process used by all cell types to take up specific molecules with the help of membrane receptor proteins present in the cell membrane. The membrane receptors are coated with a special type of protein called clathrin that helps to trap the substance by forming a vesicle around it. The vesicle then fuses with membrane-bound sacs called endosome that removes the clathrin coating from the vesicle, thus releasing the contents into the cytoplasm. Example Uptake of cholesterol attached to low-density lipoprotein (LDL) inside the cell.
4. Caveolae
It is located in the cell membrane of specific cell types such as smooth muscles, adipocytes, fibroblasts, and endothelial cells. Caveolae are small flask-shaped pits resembling tiny caves. In this process, the cells take up molecules using a cholesterol-binding protein called caveolin present in the cell membrane. The remaining procedure is similar to receptor-mediated endocytosis. Example Internalization of the insulin receptor in adipocyte cells.
