Cytosis - Endocytosis

Endocytosis

Endocytosis is when a cell absorbs molecules, such as proteins, from outside the cell by engulfing it with the cell membrane. It is used by most cells, because many critical substances are large polar molecules that cannot pass through the cell membrane. The two major types of endocytosis are pinocytosis and phagocytosis.

Pinocytosis

Pinocytosis, also known as cell drinking, is the absorption of small aqueous particles along with the membrane receptors that recognize them. It is an example of fluid phase endocytosis and is usually a continuous process within the cell. The particles are absorbed through the use of clathrin coated pits. These clathrin coated pits are short lived and serve only to form a vesicle for transfer of particles to the lysosome. The clathrin coated pit invaginates into the cytosol and forms a clathrin coated vesicle. The clathrin proteins will then dissociate. What is left is known as an early endosome. The early endosome merges with a late endosome. This is the vesicle that allows the particles that were endocytosed to be transported into the lysosome. Here there are hydrolytic enzymes that will degrade the contents of the late endosome. Sometimes rather than being degraded, the receptors that were endocytosed along with the ligand are then returned to the plasma membrane to continue the process of endocytosis.

Receptor-mediated endocytosis

Receptor-mediated endocytosis is a mode of pinocytosis. Proteins in the clathrin coat on the plasma membrane have propensity to bind and trap macromolecules or ligands. However, it is not the receptors in the pit that caused the pinocytosis. The vesicles would have formed regardless of whether or not the receptors and ligand were there. This is why it is still a continuous non-triggered event, unlike phagocytosis which is explained below.

Phagocytosis

Phagocytosis is also known as cell eating, is the absorption of larger particles into the cytosol including things like bacterium. In smaller single celled organisms this is how it feeds. In larger multicellular organisms it is a way of destroying old or damaged cells or ingesting microbial invaders. In the case of ingesting a bacterium, the bacterium will be bound by antibodies in the aqueous environment. When this antibody runs into a receptor on the surface of a cell, the plasma membrane responds by extending itself to surround the bacterium. Thus, phagocytosis is not a randomly occurring event. It is triggered by a ligand binding to a receptor.

Some cells are specially designed to phagocytize. These cells include Natural Killer cells, macrophages, and neutrophils. All of these are involved in the immune response and serve to degrade foreign or antigenic material