Download to read offline
HANNAH . J - ENDOCYTOSIS AND EXOCYTOSIS..pptx
- 1. ENDOCYTOSIS AND EXOCYTOSIS FROM: HANNAH. J BSc.CND 2nd SEM HUMAN PHYSIOLOGY SEMINAR PPT SUBMITTED TO: Dr. DHANALAKSHMI
- 2. INDRODUCTION : ● Inaddition to primary and secondary active transport system, there are some special categories of active transport. The substances are transported through cell membrane in the form of vesicles. So, these types of transport mechanism are generally called the vesicular transport. ● Special categories of active transport are : 1. Endocytosis 2. Exocytosis
- 4. ENDOCYTOSIS : ● Endocytosisis defined as a transport mechanism by which the macromolecules enter the cell. Macro- molecules (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.
- 6. 1. PINOCYTOSIS : Pinocytosisis 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 .
- 7. iii. Droplets areengulfed 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.
- 9. PHAGOCYTOSIS : ● Phagocytosisis 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.
- 10. 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.
- 11. iv. Primary lysosomefuses 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 .
- 13. RECEPTOR-MEDIATED ENDOCYTOSIS: ● Receptor-mediatedendocytosis 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
- 14. Mechanism of receptor-mediatedendocytosis: 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 ligand- receptor complex. iii. 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.
- 15. iv. Endosome travelsinto 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 .
- 16. Macromolecules transported byreceptor-mediated endocytosis : ● 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.
- 17. v. Viruses: Roussarcoma virus, semliki forest virus, vesicular stomatitis virus and adenovirus. vi.Transport proteins: Transferrin and transcobalamin. 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.
- 18. EXOCYTOSIS : ● Exocytosisis 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.
- 19. Mechanism of Exocytosis: ●Exocytosis is involved in the release of secretory substances from cells. ● Secretory substances of the cell are stored in the form of secretory vesicles in the cytoplasm. ● When required, the vesicles approach the cell membrane and get fused with the cell membrane. ● Later, the contents of the vesicles are released out of the cell
- 21. Role of Calciumin Exocytosis: ● Calcium ions play an important role during the release of some secretory substances such as neurotransmitters. ● The calcium ions enter the cell and cause exocytosis. ● However, the exact mechanism of exocytosis is not clear.
- 23. REFERENCE : ● FROMBOOK OF ESSENTIALS OF MEDICAL PHYSIOLOGY , BY K. SEMBULINGAM AND PREMA SEMBULINGAM , 6TH EDITION.