Nutrients enter the internal environment of cells due to active transport, in which special enzymes take part. In this case, two processes occur - pinocytosis and phagocytosis.
General characteristics of the process
Pinocytosis is a universal way of feeding, which is characteristic of plant and animal cells. Its essence lies in the entry of nutrients into the cell in dissolved form. Phagocytosis is a similar process, but it consumes solid particles.
It is known that pinocytosis is an important stimulus for the formation of lysosomes, and phagocytosis is important when cells are infected with viruses. These two processes have much in common, so they are often combined under the general name - cytosis, or endocytosis, although pinocytosis is more common. If substances, on the contrary, are removed from the cell, then they speak of exocytosis.
To summarize, we can say that pinocytosis is the process of absorption of liquid droplets by the cell.
Process Features
It must be said right away that cytosis depends on temperature and cannot take place at 2 ° C, as well as under the action of metabolic inhibitors, for example, sodium fluoride.
In pinocytosis, outgrowths of the cytoplasm are formed- pseudopodia, which merge with each other and envelop liquid droplets. In this case, vesicles are formed that separate from the cell membrane and begin to migrate through the cytoplasm, turning into vacuoles called pinosomes.
It should be noted that pinocytosis is also the result of cell contact with a suspension of viruses. In this case, the formed vesicles contain vibrios. It is here that they sometimes undergo the "undressing" stage. When large molecules of individual drugs are captured, invagination and the formation of a bubble - vacuole also take place, however, this mechanism of drug transport is not of decisive importance. A greater influence on the absorption of pharmacological agents is their form, the degree of grinding, as well as the presence of concomitant diseases of the digestive system - gastritis, colitis or, for example, peptic ulcer.
Protein reabsorption in the renal tubules
Pinocytosis is an active protein reabsorption mechanism in the proximal renal nephrons. During it, the protein is attached to the brush border. At this point, the membrane is invaginated, and a vesicle containing a protein molecule is formed. When the protein is inside such a vesicle, it begins to decompose into amino acids, which subsequently enter the intercellular fluid through the basolateral membrane. Since such transport requires energy, it is called active.
It is worth noting that there is a concept of maximum transport for substances that are actively reabsorbed. This processassociated with the maximum load of transport systems. It occurs when the amount of compounds that have entered the lumen of the renal tubules exceeds the capabilities of the enzymes and transport proteins involved in the transfer.
As an example, one can also cite a violation of glucose reabsorption, which is observed in the proximal convoluted tubule. If the content of this substance exceeds the functionality of the kidneys, then it begins to be excreted in the urine (normally, glucose is not detected).
Meaning of pinocytosis
This process takes place in the renal tubules and intestinal epithelium. It is responsible for the absorption and reabsorption of many compounds (including proteins and fats) that are necessary for the normal functioning of the body.
In addition, pinocytosis occurs during metabolism through the capillary wall. So, large molecules that are not able to penetrate through the pores of small blood vessels are transferred by pinocytosis. In this case, the membrane of the capillary cell is invaginated, as a result of which a vacuole is formed that surrounds the molecule. On the opposite side of the cell, the opposite process begins to occur - emiocytosis.
It should also be mentioned that pinocytosis is an important component of active transport and ionic deposition. It is he who is the main mechanism for the penetration of macromolecular substances into the internal environment of cells. It is also the main way animal or plant viruses enter host cells.