Endocytosis: Definition, Types and Functions of this Molecular Mechanism

It is the process of actively transporting molecules to the cell by enveloping it with its membrane.

Endocytosis and exocytosis are used by all cells to transport molecules that cannot pass through the membrane passively.

Exocytosis provides the opposite function and pushes the molecules out of the cell. Like all systems in the human body, the need for homeostasis allows for an equal flow of molecules into and out of the cell.

This means that the number of molecules that enter the cell by endocytosis is equal to the number of molecules that leave the cell by exocytosis. The two processes combined ensure that there is a balance of nutrients and wastes for life and cell function.

The necessary components in the endocytic pathway are early endosomes, late endosomes, and lysosomes .

The first endosomes are responsible for receiving vesicles on the cell surface. They classify the received molecules into the other components en route through trans vesicular compartments such as endosomal carrier vesicles or multivesicular bodies .

The late endosomes receive the molecules of the early endosomes. They begin the degradation of the molecules and can also receive molecules from the trans-Golgi network or from phagosomes.

The final endosomes then deliver the molecules to the lysosomes. Lysosomes contain enzymes that break down carbohydrates, proteins, fats, and other cellular waste products into smaller, simpler components.

These components are then returned to the cytoplasm to be used as building materials throughout the cell.

Types of endocytosis

There are four different types, or pathways, of endocytosis:

  • Caveolae.
  • Macropinocitosis.
  • Receptor-mediated endocytosis.
  • Phagocytosis.

Each path has a different way of bringing encapsulated molecules. Caveolae are non-clathrin coated shoots that form and localize to the plasma membrane, and are composed of caveolin, an integral membrane protein.

Caveolin activates, forms and maintains the formation of “caves” in the cell membrane or caveolae. They serve as “collection” wells that gather specific molecules for cell signaling and metabolic pathways.

The next three endocytosis operations use the clathrin system; Clathrins are proteins that accumulate inside the cell membrane when endocytosis begins.

Due to their structure, clathrins, when stimulated, naturally bind together to form cages around ingested molecules.

Macropinocytosis is the process of ingesting larger molecules, and it is activated when the cell membrane undergoes a puckering of the cell surface or structural reshaping of the membrane.

Physical stimulation causes vesicles to form; these are internalized later. This has made macropinocytosis less selective and more efficient than other types of endocytic processes.

Unlike macropinocytosis, receptor-mediated endocytosis relies on cell surface receptors to distinguish certain molecules and therefore can only deliver molecules in a one-to-one ratio.

The molecules will accumulate on the surface of the cell and signal the membrane to begin ingestion.

Once the concentration is high enough, invagination begins with the recruitment of clathrin, forming a coat or cage around the particle.

Phagocytosis engulfs molecules by manipulating the cell membrane to surround and trap molecules, creating a vesicle called phagosomes. Phagocytosis is unique in that it specializes in the destruction and removal of waste.

Role of endocytosis

Endocytosis is used for receptor signaling, nutrient uptake, membrane remodeling, pathogen entry, and neurotransmission, as well as modulation of cell signaling responses.

In developing tissues, endocytosis has been found to aid in cell migration. Toxins, pathogens, and foreign debris have also been found to exploit different endocytic pathways to gain entry into the cell.

The particles will either recruit clathrins (proteins necessary to form and form vesicles) or initiate the first steps of the pathway to begin the process of entering the cell.

Example of endocytosis

Cholesterol is a very necessary component in the cell that is present in the plasma membrane and is also used as a hormonal precursor. A complex of lipoproteins (such as LDL or low-density lipoproteins) is then used to transport the cholesterol to other cells in the body.

On the surface of the cell there is an LDL receptor that binds to the LDL complex to start the endocytic process.

Receptors were synthesized in the Endoplasmic Reticulum (ER) and were then transported and processed in the Golgi.

Once the receptor binds to the complex on the cell surface, clathrins are recruited along with other proteins that help in the process.

Receptors clump together to form clathrin-coated pits. The lined pits are pinched, forming endocytic vesicles, and then they are uncoated.

After removing the coating, the gallbladder is delivered to an endosome. The low pH in the endosome causes a conformational change that releases the LDL particles. These particles are then directed to the lysosome for degradation, releasing cholesterol into the cell.

The purpose of the immune system is to rid the body of any pathogens or foreign particles that can cause disease.

These molecules can be harmful to the body, so it is imperative that the invading pathogen is removed quickly.

Signal proteins within the body alert phagocytes, or immune cells, to travel to localized pathogens at infected sites.

Embedded on the surface of phagocytes are cellular receptors, called Toll-Like Receptors (TLRs), which bind to specific bacteria; different types of bacteria will bind to different TLRs.

Once a bacterium binds to the receptor, a signal cascade will initiate endocytosis. The cell membrane will begin to envelop the bacteria and create a phagosome or phagocytic vesicle. The phagosome will then transport the bacteria to the lysosome, where they fuse to form a phagolysosome.