Sunday, August 5, 2007

EXOCYTOSIS








EXOCYTOIS

+ is the process by which a cell directs secretory vesicles to the cell membrane. These membrane-bound vesicles contain soluble proteins to be secreted to the extracellular environment, as well as membrane proteins and lipids that are sent to become components of the cell membrane.


TYPES

In multicellular organisms there are two types of exocytosis: 1) Ca2+ triggered non-constitutive and 2) non Ca2+ triggered constitutive. Exocytosis in neuronal chemical synapses is Ca2+ triggered and serves interneuronal signalling. Constitutive exocytosis is performed by all cells and serves the release of components of the extracellular matrix, or just delivery of newly-synthesized membrane proteins that are incorporated in the plasma membrane after the fusion of the transport vesicle


FUNCTION

Exocytosis is needed by cells for secretion of proteins like enzymes, peptide hormones and antibodies from cells, turnover of plasma membrane, release of neurotransmitter from presynaptic neurons, placement of integral membrane proteins, acrosome reaction during fertilization, antigen presentation during the immune response and recycling of plasma membrane bound receptors.
Exocytosis is important in cellular signaling. In neuronal communication both chemical and electrical information needs to be sent throughout the cell. Exocytosis sends and converts the electrical information into chemical information. Within the neural cells, the information is electrical. In the synapse, after exocytosis has occurred, the neurotransmitters are released, and the information is chemical. The release of the neurotransmitter can either be excitatory (causing activity from the target cell) or inhibitory (preventing activity by the target cell).


PROCESS

Membrane-bound vesicles move to the cell surface where they fuse with the plasma membrane. This accomplishes three things:
It restores the normal amount of plasma membrane.
Any molecules dissolved in the fluid contents of these vesicles are discharged into the extracellular fluid - this is called secretion.Example: the various components of the extracellular matrix are secreted by exocytosis.
Any integral membrane proteins exposed to the interior surface of the vesicles will now be displayed at the cell surface because the vesicles turn inside out as they fuse with the plasma membrane. Thus exocytosis does not simply replace plasma membrane but ensures that the plasma membrane will display its characteristic cell-surface proteins.

Exocytic vesicles are created from several sources:
Some are simply endosomes traversing the cell.
Others are pinched off from endosomes before they fuse with lysosomes.
Others bud off from the endoplasmic reticulum and Golgi apparatus taking their products to the surface of the cell.
The exocytosis of lysosomes supplies the membrane needed to repair wounds in the plasma membrane.



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