The intracellular fluid, As its name implies, it is the liquid found inside cells. In humans, for example, the water contained in the internal compartment of all cells represents more than 40% of the body weight of an adult man.
The body of humans and other animals is made up of more or less 60% of water, which is divided between what could be considered two compartments: the intracellular space (within cells) and the extracellular space (around and out of cells).
The fluid contained in the extracellular space is known as extracellular fluid and this can be divided, in turn, into interstitial fluid and blood plasma; representing 20% of the body weight of a human being.
However, the intracellular fluid (that which is contained in the intracellular space) is only one: that which is enclosed by the plasma membrane of any cell. In an adult human being, the sum of the intracellular fluid of all his cells exceeds 40% of his body weight.
Approximately 70% of the fresh weight of a cell, be it animal or plant, is water, which is why we say that this is one of the most abundant elements of the intracellular fluid..
With this statement, we also understand that the interaction of water with other cellular components is of utmost importance for cellular chemistry..
Therefore, the intracellular fluid is the fluid in which most of the catalytic reactions that are necessary for life occur both from the metabolic point (for obtaining and using energy) and homeostatic (for the maintenance of internal conditions ), as for the replication, transcription and translation of cellular DNA.
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Many textbooks refer to the intracellular fluid as the cytosol, which is separated from the extracellular space (and the fluids contained in it) thanks to the presence of the plasma membrane..
The plasma membrane is nothing more than a semi-permeable barrier that allows the passage of some substances and prevents the passage of others. This structure, fundamental for the existence of a cell, is composed of a double layer of lipids and many associated proteins (peripheral and transmembrane)..
Let us remember that in the cytosol of a eukaryotic cell, in addition to the intracellular fluid and everything that is contained there, there are a series of membranous organelles that perform different cellular functions, so the intracellular fluid is only represented by the "fluid" part.
The intracellular fluid, then, is contained in a fairly small space (the size of a cell) so it is, when considered for each individual cell, a very small volume, equivalent to 1 or 2 picoliters (1 picoliter is the millionth part of 1 microliter which, in turn, is one millionth of a liter).
In addition to being composed of water, in the intracellular fluid there is an enormous amount of dissolved substances: ions, proteins and other molecules. However, the viscosity of this liquid is very similar to that of water..
The pH of the intracellular fluid in animal cells remains constant between 6 and 7.5, but this can vary both with respect to the type of cell and with respect to the intracellular region considered (lysosomes, for example, are organelles with a more acidic pH).
Although intracellular and extracellular fluids are not in direct contact with each other, they exchange water and other substances constantly, either by active transport mechanisms (which require energy intake) or passive transport mechanisms (which occur in favor of a concentration gradient)..
These types of transport are what establish the differences in solute concentration between intracellular and extracellular fluid; differences that are very important for various cellular functions.
The movement of solutes through the plasma membrane, either towards the intracellular space or towards the extracellular space, almost always depends on some transport proteins immersed in the membrane, which function as a kind of "channels".
The movement of water from one compartment to the other is also known as osmosis and is a very important factor that determines the redistribution of water (as a solvent) between the inside and outside of the cell, especially when the concentrations of dissolved solutes change. in her.
The existence of a transport medium that communicates the intracellular fluid with the extracellular one makes it possible for, for example, cells to discard some substances or "waste" that they no longer need into the environment that surrounds them and, at the same time, to take from their environment essential nutrients and solutes.
Intracellular fluid is mainly made up of water, after all, we know it for a reason as the universal solvent.
Different molecules and ions are dissolved in the water of the intracellular fluid. There is a large concentration of proteins (including hundreds of enzymes), which represent between 20 and 30% of the total weight of a cell.
In this liquid there are also a large number of ions, whose concentrations in the cells of an average human being are as follows:
- Sodium (Na +): 10-30 mmol / L
- Potassium (K +): 130-150 mmol / L
- Magnesium (Mg2 +): 10-20 mmol / L
- Calcium (Ca2 +): less than 1 mmol / L
- Chlorine (Cl-): 10-20 mmol / L
- Phosphate ions (PO4-): 100-130 mmol / L
- Bicarbonate ion (HCO3-): less than 10 mmol / L
- Sulfate Ions (SO4-): about 20 mmol / L
The main function of the intracellular fluid (especially the water that composes it) is to provide a fluid support medium for the fundamental enzymatic reactions of a cell to occur..
Thus, this liquid, together with its dissolved solutes and the proteins that are distributed and mobilized in it, is by far one of the most important “parts” of a cell..
Although many metabolic reactions occur within the organelles of eukaryotic cells, the cellular fluid harbors pathways such as glycolysis, gluconeogenesis, the pentose phosphate pathway, the translation and synthesis of many soluble proteins, among others..
It is important to comment that the reactions that occur in this liquid depend, to a great extent, on the type of cell that we consider, since there are differences between plants, animals, fungi and protozoa, for example.
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