Characteristics of exocytosis and endocytosis

Movement of molecules or particles via the plasma membrane is the most important process in a cell.

This results in the transportation of essential molecules across the cell membrane, such as simple nutrients, hormones, proteins for cellular processes, pathogens, etc.

Since cell membranes are semi-permeable, small molecules and ions passively diffuse through them, and other small molecules move through carrier proteins or channels on the membrane. But large molecules can pass only in the form of vesicles and require energy to get transported.

Therefore, macromolecules are transported inside the vesicles, and vesicle transport occurs in two ways based on the direction of movement of particles. This includes exocytosis and endocytosis.

Vesicle-mediated transportation is required for the movement of large polar molecules that cannot pass through the hydrophobic portion of the membrane in a passive way.

The macromolecules are usually transported across the membrane by the active mechanism, which requires cellular energy.

This mechanism helps in the uptake of molecules that are present in low concentrations in the extracellular fluid.

Such mechanisms are required for the uptake of essential molecules such as LDL cholesterol, growth factor EGF, and iron transport protein transferring from the blood.

It also triggers opsonization mechanisms when the virus binds to these receptors and is transported to the endosomes for pathogen destruction. This is common for influenza and adenoviruses.

The mechanisms help in the transport of a large number of molecules.

Further, they relay the transducing signals from periphery to the nucleus, especially to trigger a hormonal response.

They are also known to transport the signaling complexes to the nucleus to speed up signal transduction.

Exocytosis
Exocytosis is the process of expulsion of material from the cell into the extracellular fluid.

The material to be expelled is usually enveloped in a membrane forming a vesicle which fuses with the interior of the cell membrane and is finally released into the extracellular space.

It is chiefly used to eliminate waste products such as carbon dioxide and water, the byproducts of cellular respiration, aid in cellular communication, and facilitate cell membrane growth, repair, signaling, and migration.

Examples of materials to be dismissed by exocytosis include secretion of proteins into ECM; neurotransmitters released into synaptic clefts by synaptic vesicles.

Endocytosis
Endocytosis is a process where large molecules enter into the cell. Here, cell membrane invaginates and forms a pocket around the target/foreign particle.

Then the pocket pinches off, and the particle is carried in an intracellular vesicle formed from the membrane.

These molecules slowly get budded off in the form of vesicles from the plasma membrane, with the help of fission proteins such as dynamin.

Eventually, these vesicles are uncoated and fused with the endosome for sorting, degradation, trafficking, and recycling of molecules.

When the vesicle fuses with the endosome, multiple compartments are formed where the ligands are separated from the receptors.

Gradually early endosomes transform into late endosomes where segregation of the molecules occurs.

The molecule-containing endosomes further fuse with the lysosome and undergo degradation.

The principal components of endocytosis include early and late endosomes and lysosomes.

This process can even engulf an entire cell into a cell. There are three types of endocytosis based on the mechanism of uptake.