What is exocytosis? Please give some breif explanation?

Like a cell factory, cells make products. Some of these stay in the cell and others are exported. Cellular products are shipped out of the cell using a process called exocytosis.

Exocytosis is a process by which a cell transports secretory products through cytoplasm to plasma membrane. Secretory products are packaged into transport vesicles. Examples of cellular secretory products:

Secreted protein – enzymes, peptide hormones and antibodies

Neurotransmitters from nerve cells

Plasma membrane proteins

Antigens – pieces of bacteria which stimulate immune response.

Exocytosis can be either calcium-dependent or calcium-independent. In calcium-dependent exocytosis, an influx of calcium into the cell will stimulate secretion. This happens when a protein is only secreted when needed. Transport vesicles with their cargo will travel from golgi vesicle (protein-packaging organelles) to plasma membrane. The vesicle then docks at plasma membrane and waits for a secretion signal. This is also called regulated exocytosis as secretion from vesicles is controlled.

In contrast, calcium-dependent exocytosis occurs constantly. Just like with regulated secretion, transport vesicle and its cellular product will travel through cell to plasma membrane. Unlike regulated secretion, vesicle does not wait at plasma membrane. Instead, fusion with plasma membrane and secretion of vesicle contents into extracellular environment happens automatically. This constant flow of secretory product is also called constitutive exocytosis because secretion from vesicles is not controlled.

Exocytosis is the reverse of endocytosis. 

In 30 minutes, an active cell like a macrophage can endocytose an amount of plasmsa membrane equal to its complete plasma membrane.

The cell must have a mechanism to restore normal amount of plasma membrane. Exocytosis is that mechanism.

Membrane-enclosed vesicles move to cell surface where they fuse with plasma membrane. This accomplishes three things:

It restores normal amount of plasma membrane.

Any molecules dissolved in fluid contents of these vesicles are discharged into extracellular fluid – this is called secretion.

Example: various components of extracellular matrix are secreted by exocytosis.

Any integral membrane proteins exposed to interior surface of vesicles will now be displayed at cell surface because vesicles turn inside out as they fuse with plasma membrane. Thus, exocytosis does not simply replace plasma membrane but ensures that the plasma membrane will display its charcteristis cell-surface proteins.

Some are simply endosomes traversing the cell.

Others are pinched off from endosomes before they fuse with lysosomes.

Others bud off from endoplasmic reticulum and Golgi apparatus taking their products to the surface of the cell.

Exocytosis of lysosomes supplies the membrane needed to repair wounds in the plasma membrane.

Some cells specialize in secretion. In cells which secrete large amounts of protein, for example, protein accumulates in specialized secretory granules formed by Golgi apparatus. These move to cell surface and discharge their contents to the outside.

Exocrine cells in pancreas synthesize and secrete pancreatic digestive enzymes. Lumen where their apical surfaces meet leads to pancreatic duct draining into the small intestine. Spherical bodies contain precursors of digestive enzymes. One is discharging its content into lumen by exocytosis.

Cells lining our intestine synthesize tiny droplets of fat and discharge them into lacteals by exocytosis.

The process described above involves fusion of exocytotic vesicle with the plasma membrane.

 In some cells, like at synapses, a second type of exocytosis also takes place:

Vesicles make a brief contact at the plasma membrane

Release their contents to the exterior and

Then retreat back into the cytosol.

This ‘kiss-and-run’ version of exocytosis does not restore plasma membrane to the cell.

A third type of exocytosis is found in some cells.

This involves endosomes themselves invaginating their membrane. As invaginations break off, they produce vesicles within vesicles, called multivesicular bodies.

When these fuse with cell’s plasma membrane, these tiny internal vesicles-called exosomes-are secreted.

Exosomes are produced in abundance by dendritic cells and B-cells and enhance their antigen-presenting function.