PINOCYTOSIS: PROCESS, FUNCTIONS AND DIFFERENCE WITH PHAGOCYTOSIS - BIOLOGY - 2025

The pinocytosis is a cellular process of ingestion of medium particles, usually small and in soluble form through the formation of small vesicles in the plasma membrane of the cell. This process is basically considered as the cellular action of “drinking”. The vesicles will be released after a process of invagination of the cell membrane within it.

This liquid material capture process includes dissolved molecules or suspended microparticles. It is one of the various means of incorporating extracellular material or endocytosis, which the cell uses for its energy maintenance.

Source: Mariana Ruiz Villarreal derivative work: Gregor_0492

Other processes in which the cell transports extracellular material include the use of transporter proteins and channel proteins across the phospholipid bilayer of the cytoplasmic membrane. However, in pinocytosis the trapped material is surrounded by a portion of the membrane.

Types of pinocytosis

This endocytosis process can be generated in two different ways: “fluid pinocytosis” and “adsorptive pinocytosis”. Both differ in the way that the particles or substances in suspension are incorporated into the cytoplasm.

In fluid pinocytosis the substances are absorbed soluble in the fluid. The rate of entry of these solutes into the cell is proportional to their concentration in the extracellular environment and also depends on the ability of the cell to form pinocytic vesicles.

In contrast, the rate of entry of a "molecule" by absorptive pinocytosis is given by the concentration of the molecule in the external environment as well as the number, affinity and function of the receptors of said molecules located on the surface of the cell membrane. The latter process conforms to the Michaelis-Menten enzyme kinetics.

All things being equal (concentration of the molecules to be absorbed), absorptive pinositosis would be 100 to 1000 times faster than fluid, and also more efficient in absorbing liquids (less amount).

Process

Pinocytosis is a very common process in eukaryotic cells. It consists of the movement of particles from the outside of the cell through the formation of a pinocytic vesicle, an invagination of the cell membrane, which ends up being detached from the latter, to form part of the cytoplasm.

In general, most endocytic vesicles originating from the cell membrane follow the route of pinocytosis. These vesicles are primarily destined for endosomes that will then be transferred to lysosomes, cellular organelles responsible for cellular digestion.

Receptor-mediated endocytosis or absorptive pinocytosis

It is the best studied form of pinocytosis. In this case the mechanism allows the selective entry of defined macromolecules. Macromolecules found in the extracellular environment will by default bind to specific receptors on the plasma membrane.

Generally, specialized receptors are found clustered in sectors of the membrane known as "clathrin-coated depressions." At this point, the pinocytic vesicles formed in these regions will have a coating of this protein (clathrin) and will also contain the receptor and the ligand (usually lipoproteins).

Once the coated vesicles are already in the cytoplasm, they fuse with the early endosomes, that is, those closest to the cell membrane.

From this point on, several complex processes can occur, including the exit of recycling vesicles towards the cell membrane and the Golgi apparatus (which carry membrane receptors and other materials) or vesicles or multivesicular bodies that follow the process of transport of material to the lysosomes.

How many receivers are there?

There are more than 20 different receptors that selectively introduce macromolecules into the cell. During this process, fluid other than the cytoplasmic medium is also non-selectively incorporated, which is called "fluid phase endocytosis".

In each clathrin-lined depression or cavity present in the cell membrane, there is not a single type of receptor; instead, there are various receptors that are simultaneously internalized into the cell with the formation of a single vesicle.

In this process and in the formation of recycling vesicles that travel back to the membrane to be reintegrated, the presence of a receptor complex or its ligands (received molecules) affects in some way the presence of other receptors and molecules.

Fluid pinocytosis

In this case, it is a non-selective process, in which the molecules or particles are actively captured. Vesicles formed from the cell wall are not coated by clathrin but by proteins such as caveolin. In some cases, this process is known as potocytosis.

Features

During the process, many materials are incorporated into the cell, either selectively with the formation of clathrin-coated vesicles or non-selectively through uncoated vesicles.

Absorptive Pinocytocis

Various receptors that recognize hormones, growth factors, carrier proteins, as well as other proteins and lipoproteins can accumulate in clathrin-coated hollows of the plasma membrane.

One of the best evaluated processes is the capture of cholesterol in mammalian cells, which is mediated by the presence of specific receptors on the cell membrane.

Cholesterol is generally carried in the bloodstream in the form of lipoproteins, the most common being low-density lipoprotein (LDL).

Once the coated vesicle is in the cytoplasm, the receptors are recycled back to the membrane and cholesterol in the form of LDC is transported to the lysosomes to be processed and used by the cell.

Other metabolites trapped in absorptive pinocytes

This process is also used to capture a series of metabolites of great importance in cellular activity. Some of them are vitamin B12 and iron that the cell cannot obtain through active transport processes through the membrane.

These two metabolites are essential in the synthesis of hemoglobin, which is the major protein present in red blood cells in the bloodstream.

On the other hand, many of the receptors present in the cell membrane that are not recycled are absorbed in this way and transported to the lysosomes to be digested by a great variety of enzymes.

Unfortunately, through this pathway (receptor-mediated pinocytosis), many viruses such as influenza and HIV enter the cell.

Pinocytosis of vesicles not covered by clathrin

When pinocytosis occurs by other pathways in which clathrin-coated vesicles do not form, the process turns out to be particularly dynamic and highly efficient.

For example, in endothelial cells that are part of blood vessels, the formed vesicles must mobilize large amounts of solutes from the bloodstream to the intracellular space.

Pinocytosis scale

Clathrin-coated depressions, for example, occupy about 2% of the surface of the plasma membrane, having an approximate life of up to two minutes.

In this sense, absorptive pinocytosis causes the entire cell membrane to be internalized within the cell through the formation of coated vesicles in a period of one to two hours, which averages between 3 and 5% of the membrane plasma for every minute.

A macrophage, for example, is capable of integrating about 35% of the volume of the cytoplasm in approximately one hour. The amount of dissolved substances and molecules does not affect at any point the speed of vesicle formation and internalization of these.

Difference with phagocytosis

Phagocytosis and pinocytosis are similar processes in which the cell internalizes extracellular material to be processed; both are processes that need energy, so they are considered active transport mechanisms. In contrast to pinocytosis, phagocytosis is literally the way the cell "eats."

Phagocytosis is characterized by the "ingestion" of large particles, including bacteria, various cellular debris, and even intact cells. The particle to be phagocytosed binds to receptors located on the surface of the cell membrane (which recognize residues of mannose, N-aceltiglucosamide, among others) that trigger the extension of pseudopods that surround the particle.

Once the membrane fuses around it, a large vesicle forms (in contrast to those generated in the process of pinocytosis) called a phagosome that is released into the cytoplasm. This is when the phagosome binds to a lysosome to form a phagolysosome.

Within the phagolysosome, digestion of the material occurs thanks to the enzymatic activity of lysosomal acid hydrolases. In this process, receptors and part of the internalized membranes are also recycled, which return in the form of recycling vesicles to the cell surface.

Where does phagocytosis occur?

It is a very common process by which organisms such as protozoa and lower metazoans feed. Furthermore, in multicellular organisms, phagocytosis provides a first line of defense against foreign agents.

The way that specialized cells, including various types of leukocytes (macrophages and neutrophils), destroy external microorganisms and ingest cellular debris, is essential for maintaining the body's system.

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