Why is endocytosis and exocytosis important processes to cells

In reality, this is a process that takes in molecules, including water, which the cell needs from the extracellular fluid. Pinocytosis results in a much smaller vesicle than does phagocytosis, and the vesicle does not need to merge with a lysosome Figure 2. A variation of pinocytosis is called potocytosis. This process uses a coating protein, called caveolin, on the cytoplasmic side of the plasma membrane, which performs a similar function to clathrin. The cavities in the plasma membrane that form the vacuoles have membrane receptors and lipid rafts in addition to caveolin.

The vacuoles or vesicles formed in caveolae singular caveola are smaller than those in pinocytosis. Potocytosis is used to bring small molecules into the cell and to transport these molecules through the cell for their release on the other side of the cell, a process called transcytosis.

Figure 3. In receptor-mediated endocytosis, uptake of substances by the cell is targeted to a single type of substance that binds to the receptor on the external surface of the cell membrane.

A targeted variation of endocytosis employs receptor proteins in the plasma membrane that have a specific binding affinity for certain substances Figure 3. In receptor-mediated endocytosis, as in phagocytosis, clathrin is attached to the cytoplasmic side of the plasma membrane.

If uptake of a compound is dependent on receptor-mediated endocytosis and the process is ineffective, the material will not be removed from the tissue fluids or blood.

Instead, it will stay in those fluids and increase in concentration. Some human diseases are caused by the failure of receptor-mediated endocytosis. In the human genetic disease familial hypercholesterolemia, the LDL receptors are defective or missing entirely.

People with this condition have life-threatening levels of cholesterol in their blood, because their cells cannot clear LDL particles from their blood.

Although receptor-mediated endocytosis is designed to bring specific substances that are normally found in the extracellular fluid into the cell, other substances may gain entry into the cell at the same site. Flu viruses, diphtheria, and cholera toxin all have sites that cross-react with normal receptor-binding sites and gain entry into cells. The reverse process of moving material into a cell is the process of exocytosis.

Exocytosis is the opposite of the processes discussed in the last section in that its purpose is to expel material from the cell into the extracellular fluid. Waste material is enveloped in a membrane and fuses with the interior of the plasma membrane.

The uncoated vesicle fuses with an early endosome to form a sorting vesicle i. Sorting vesicles separate imported content from the receptors that are recycled to the membrane. In the vesicle that remains, now a lysosome , digestive enzymes catalyze hydrolysis of the vesicle contents. The digest products are then released for cellular use. A well-known example of receptor-mediated endocytosis is the uptake of cholesterol bound to low density lipoprotein LDL , a complex of phospholipid, protein and cholesterol illustrated below.

A single LDL complex carries as many as 15, molecules of cholesterol. Maintaining cell size or volume seems to be a built-in component of the machinery of receptor-mediated endocytosis that balances endocytosis with membrane recycling.

However, exocytosis is also necessary to restore plasma membrane internalized by pinocytosis and phagocytosis, and for eliminating cellular waste products. Exocytosis is also the end-point of a complex process of packaging proteins destined for secretion or for insertion into the membrane themselves. The pathways of exocytosis and endocytosis share common features, as illustrated on the next page.

Note that the formation of both lysosomes and secretion vesicles begins in the rough endoplasmic reticulum, followed by passage and maturation through Golgi vesicles. The table on the next page lists some representative proteins packaged for secretion or destined to live in cell membranes.

As we have seen, many secretory and membrane proteins are glycoproteins, to which sugars are covalently attached starting in the rough endoplasmic reticulum. As we have also seen, their glycosylation begins in the RER. Check the link below to see the process again. Next we consider how cells target proteins to their different intracellular and extracellular destinations.

Endocytosis Different forms of endocytosis are illustrated on the next page. How are they different? Why is endocytosis important to cells? Is the white blood cells disposing of a worn-out red blood cell carried out through What are some examples of situations where a cell might use phagocytosis or exocytosis?

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