Exocytosis is the reverse of endocytosis. It is a membrane- enclosed vesicle that move to cell surface where they fuse with the plasma membrane. It accomplish three things; restores normal amount of plasma membrane, any molecules dissolved in the fluid contents of these vesicles are discharged into the extracellular fluid called secretion, and 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 plasma membrane. Exocytosis does not simple replace plasma membrane but ensures that the plasma membrane will display its characteristic cell surface proteins. Exocytic vesicles are created from several sources like endosomes traversing the cell, others are pinched off from the endosomes before they fuse with the lysosome, and others bud off from the endoplasmic reticulum and Golgi apparatus taking their products to the surface of cell. Exocytosis of lysosome supplies the membrane the needed to repair wounds in plasma membrane. Exocytosis is the process by which a cell transports secretory products through the cytoplasm to the plasma membrane. The secretory products are packaged into transport vesicles. Examples of these secretory products are secreted proteins (enzymes, peptide hormones, and antibodies), neurotransmitters from nerve cells, plasma membrane proteins, and antigens. Exocytosis can either be calcium dependent or calcium dependent.
Calcium dependent is when an influx of calcium into the cell will stimulate secretion. It happens when protein is only secreted when needed. Transport vesicles travel from the Golgi vesicles to the plasma membrane with its cargo. The vesicle stops at the plasma membrane and waits for the secretion signal. This is called regulated exocytosis because secretion from the vesicle is controlled.
In calcium independent occurs constantly. It also transports vesicle and its cellular products travel through the cell to the plasma membrane. The vesicles do not wait at plasma membrane and it fuses with plasma membrane. The secretion of vesicles contents into the extracellular environment happens automatically. This is called constitutive exocytosis, which is when secretion from the vesicles is not controlled.
Calcium dependent is when an influx of calcium into the cell will stimulate secretion. It happens when protein is only secreted when needed. Transport vesicles travel from the Golgi vesicles to the plasma membrane with its cargo. The vesicle stops at the plasma membrane and waits for the secretion signal. This is called regulated exocytosis because secretion from the vesicle is controlled.
In calcium independent occurs constantly. It also transports vesicle and its cellular products travel through the cell to the plasma membrane. The vesicles do not wait at plasma membrane and it fuses with plasma membrane. The secretion of vesicles contents into the extracellular environment happens automatically. This is called constitutive exocytosis, which is when secretion from the vesicles is not controlled.
Exocytosis, the transport of macromolecules and large particles outside the cell, is the reverse of endocytosis. In exocytosis, materials inside the cell are packed in a vesicle, which fuses to the plasma membrane.
Some vesicles contain structural proteins and polysaccharides, whereas other vesicles, such as vacuoles, contain digestive enzymes. Each vesicle attaches to the plasma membrane. The site where it attaches opens, and the materials in the vesicle are dumped out of the cell. The vacated vesicle straightens and becomes a part of the plasma membrane. A problem arises if vesicles continually fuse with the plasma membrane, because large amounts of new membrane being added can double the amount of plasma membrane every thirty minutes, and the plasma membrane has little room to expand in cells with rigid cell walls. Nevertheless, plants cells regularly recycle excess plasma membrane via endocytosis. |