SPECIAL TYPES OF ACTIVE TRANSPORT

SPECIAL TYPES OF ACTIVE TRANSPORT

In addition to primary and secondary active transport systems, there are some special categories of active transport which are generally called the vesicular transport.

Special categories of active transport:

1. Endocytosis

2. Exocytosis

3. Transcytosis.

ENDOCYTOSIS

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. Endocytosis is of three types:

1. Pinocytosis

2. Phagocytosis

3. Receptor-mediated endocytosis.

1. Pinocytosis

Pinocytosis is a process by which macromolecules like bacteria and antigens are taken into the cells. It is

otherwise called the cell drinking.

Mechanism of pinocytosis

Pinocytosis involves following events:

i. Macromolecules (in the form of droplets of fluid) bind to the outer surface of the cell membrane

ii. Now, the cell membrane evaginates around the droplets

iii. Droplets are engulfed by the membrane

iv. Engulfed droplets are converted into vesicles and vacuoles, which are called endosomes

v. Endosome travels into the interior of the cell

vi. Primary lysosome in the cytoplasm fuses with endosome and forms secondary lysosome

vii. Now, hydrolytic enzymes present in the secondary lysosome are activated resulting in

digestion and degradation of the endosomal contents.

2. Phagocytosis

Phagocytosis is the process by which particles larger than the macromolecules are engulfed into the cells. It 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 of phagocytosis

i. When bacteria or foreign body enters the body, first the phagocytic cell sends cytoplasmic

extension (pseudopodium) around bacteria or foreign body

ii. Then, these particles are engulfed and are converted into endosome like vacuole.

Vacuole is very large and it is usually called the phagosome

iii. Phagosome travels into the interior of cell

iv. Primary lysosome fuses with this phagosome and forms secondary lysosome

v. Hydrolytic enzymes present in the secondary lysosome are activated resulting in digestion

and degradation of the phagosomal contents.

3. 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 receptormediated endocytosis .

Mechanism of receptor-mediated endocytosis

i. Receptor-mediated endocytosis is induced by

substances like ligands

ii. Ligand molecules approach the cell and bind to receptors in the coated pits and form ligandreceptor

complex

iii. Ligand-receptor complex gets aggregated in the coated pits. Then, the pit is detached from cell

membrane and becomes the coated vesicle.

iv. Endosome travels into the interior of the cell. Primary lysosome in the cytoplasm fuses with

endosome and forms secondary lysosome.

v. Now, the hydrolytic enzymes present in secondary lysosome are activated resulting in release

of ligands into the cytoplasm .

vi. Receptor may move to a new pit of the cell

membrane .

Receptor-mediated endocytosis play an important role in the transport of several types of macromolecules into the cells, viz.

i. Hormones: Growth hormone, thyroid stimulating hormone, luteinizing hormone, prolactin, insulin,

glucagon, calcitonin and catecholamines

ii. Lipids: Cholesterol and low-density lipoproteins (LDL)

iii. Growth factors (GF): Nerve GF, epidermal GF, platelet-derived GF, interferon

iv. Toxins and bacteria: Cholera toxin, diphtheria toxin, pseudomonas toxin, recin and

concanavalin A

v. Viruses: Rous sarcoma virus, semliki forest virus, vesicular stomatitis virus and adenovirus

vi. Transport proteins: Transferrin and transcobalamine

vii. 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.

EXOCYTOSIS

Exocytosis is the process by which the substances are expelled from the cell. In this process, the substances are extruded from cell without passing through the cell membrane. This is the reverse of endocytosis.

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