What Types Of Cells Use Endocytosis

In this what types of cells use endocytosis post we have briefly explained about pinocytosis, phagocytosis, receptor mediated endocytosis.

Definition

Endocytosis is defined as a transport mechanism by which the macromolecules enter the cell. Macromolecules (substances with larger molecules) cannot pass through the cell membrane either by active or by passive transport mechanism. Such substances are transported into the cell by endocytosis.

Pinocytosis

Pinocytosis function is a process by which macromolecules like bacteria and antigens are taken into the cells. Pinocytosis function is otherwise called the cell drinking.

Mechanism

Macromolecules (in the form of droplets of fluid) bind to the outer surface of the cell membrane. Now, the cell membrane evaginates around the droplets. Droplets are engulfed by the membrane. Engulfed droplets are converted into vesicles and vacuoles, which are called endosomes. Endosome travels into the interior of the cell.

Endocytosis

Pinocytosis Function

Primary lysosome in the cytoplasm fuses with endosome and forms secondary lysosome. Now, hydrolytic enzymes present in the secondary lysosome are activated resulting in digestion and degradation of the endosomal contents.

Phagocytosis

Phagocytosis process which particles larger than the macromolecules are engulfed into the cells.

Phagocytosis process is also called cell eating. Larger bacteria, larger antigens and other larger foreign bodies are taken inside the cell by means of phagocytosis.

Only few cells in the body like neutrophils, monocytes and the tissue macrophages show phagocytosis. Among these cells, the macrophages are the largest phagocytic cells.

Mechanism

Endocytosis

When bacteria or foreign body enters the body, first the phagocytic cell sends cytoplasmic extension (pseudopodium) around bacteria or foreign body.

Then, these particles are engulfed and are converted into endosome like vacuole. Vacuole is very large and it is usually called the phagosome.

Phagosome travels into the interior of cell. Primary lysosome fuses with this phagosome and forms secondary lysosome.

Hydrolytic enzymes present in the secondary lysosome are activated resulting in digestion and degradation of the phagosomal contents.

Receptor Mediated Endocytosis

Receptor-mediated endocytosis is the transport of macromolecules with the help of a receptor protein.

Surface of cell membrane has some pits which contain a receptor protein called clathrin. Together with a receptor protein (clathrin), each pit is called receptor-coated pit. These receptor-coated pits are involved in the receptor mediated endocytosis.

Endocytosis

Mechanism

Receptor-mediated endocytosis is induced by substances like ligands. Ligand molecules approach the cell and bind to receptors in the coated pits and form ligand receptor complex.

Ligand-receptor complex gets aggregated in the coated pits. Then, the pit is detached from cell membrane and becomes the coated vesicle. This coated vesicle forms the endosome.

Endosome travels into the interior of the cell. Primary lysosome in the cytoplasm fuses with endosome and forms secondary lysosome.

Now, the hydrolytic enzymes present in secondary lysosome are activated resulting in release of ligands into the cytoplasm. Receptor may move to a new pit of the cell membrane.

What Types of Cells Use Endocytosis

Growth hormone, thyroid stimulating hormone, luteinizing hormone, prolactin, insulin, glucagon, calcitonin and catecholamines.

Lipids: Cholesterol and low-density lipoproteins (LDL).  Growth factors (GF): Nerve GF, epidermal GF, platelet-derived GF, interferon.

Cholera toxin, diphtheria toxin, pseudomonas toxin, recin and concanavalin A v. Viruses: Rous sarcoma virus, semliki forest virus, vesicular stomatitis virus and adenovirus.

Transferrin and transcobalamine. Antibodies: IgE, polymeric IgG and maternal IgG.

Some of the receptor-coated pits in cell membrane are coated with another protein called caveolin instead of clathrin. Caveolin-coated pits are concerned with the transport of vitamins into the cell.

Further Readings

Reference