Cytosol is the fluid portion of the contents of a human cell and thus part of the cytoplasm. The cytosol is composed of about 80% water, with the remaining portion distributed among proteins, lipids, nucleotides, sugars, and ions. They serve important metabolic processes that take place in the aqueous to viscous cytosol.
What is cytosol?
Cytosol is the liquid to gel-like component of all eukaryotic cells and is thus part of the cytoplasm, the total content of cells. Cytosol has approximately 80 percent water content with a variety of different solutes such as proteins, minerals, cations, anions, sugars, enzymes, vitamins, hormones, and many other molecules and compounds required for intermediate metabolic processes. Other substances that are also needed for intermediate metabolism but are not water soluble are found in organelles or special vesicles, tiny vesicles encased in a membrane. Similar to vesicles but much larger compartments are vacuoles. They play an important role in phagocytosis, the entrapment of foreign substances or organisms, and the temporary confinement of secretions. The cytosol is crisscrossed by a dense and constantly changing wickerwork, the cytoskeleton. It consists of actin filaments, intermediate filaments, and microtubules. The cytoskeleton serves to mechanically stabilize the cell internally and externally, but also interacts with the cytosol. Many metabolic processes in the cytosol, such as synthesis and degradation of amino acids, formation of polypeptides as a precursor to the generation of proteins, glycolytic processes, and more, function only in the cooperation of certain components of the cytoskeleton with the cytosol and in exchange with the enclosed organelles and vesicles.
Function, action, and roles
In the cytosol, a large number of enzymatically controlled metabolic processes occur in parallel with each other, some of which are incompatible. The evolution of multicellular organisms (eukaryotes) has therefore made it possible to demarcate tiny areas within the cytosol by membranes, so-called cell compartments. The formation of separated organelles, vesicles, vacuoles, and other cell compartments allows degrading and constructing enzymes to be engaged in opposite metabolic processes in parallel in the same cell. One of the main functions of the cytosol is to exchange substances in cooperation with parts of the cytoskeleton and the compartments, i.e., to release needed substances and to take up other substances that are not or no longer needed in order to supply them to other utilization or to forward them for disposal. Another important task of the cytosol is to take over and organize transports within the cell in cooperation with the cytoskeleton, especially with the microtubules. In order to cope with the many transport tasks, the cytosol can change very rapidly in viscosity from aqueous to gel-like and vice versa. The multitude of biochemical conversions catalytically controlled by enzymes, vitamins and hormones also includes oxidation and reduction processes, so-called redox reactions, which take place not only in the mitochondria. Mitochondria are cell organelles with their own RNA, which are controlled by the so-called respiratory chain, in which, among other things, redox reactions between adenosine triphosphate (ATP) and adenosine diphosphate (ADP) play a major role. Cells, which have a great hunger for energy due to their tasks, can contain several thousand mitochondria. The cytosol not only holds necessary molecules and compounds to enable corresponding synthesis or degradation processes, but also a part of the genetic translation processes takes place within the cytosol. So-called messenger RNA, copies of the complementary nucleic acid sequences of RNA are translated in the cytosol into the synthesis of precursors of proteins (peptides and polypeptides), i.e. into a corresponding sequence of amino acids.
Formation, occurrence, properties, and optimal levels
Cytosol, the liquid portion of the cytoplasm is already formed during cell division. Its composition is controlled hormonally and enzymatically via intercellular and extracellular mass transfer. The cytosol has a different composition depending on the cell type and on the situation and, as already mentioned, can vary in viscosity in rapid successions from liquid to gel-like and vice versa.Hydrophobic compounds required by the cell, which cannot be dissolved in the aqueous cytosol, are stocked in mobile vesicles or vacuoles and shuttled to the place where it is needed. There is also mass transfer with the nucleus, which is separated from the cytosol by a double membrane, usually through nuclear pores in the cell membrane. Optimal values or parameters of cytosol cannot be given because of the variation in composition depending on cell type and situation.
Diseases and disorders
The plethora of tasks and functions performed by components of the cytoplasm, including the cytosol, suggest that exposure to toxins or disease can disrupt or completely eliminate metabolic processes with mild to severe consequences for the organism as a whole. In particular, the exchange of substances between the mitochondria and the cytosol may be disturbed. Many different causes of mitochondriopathies are known, a few of which may also be genetic. In most cases, the energy supply to the cells is insufficient, leading to symptoms such as muscle weakness, general fatigue. If deficiency symptoms or deficiency syndromes are present, the cause of the problems is usually not a disturbed metabolism in the cytosol, but an inadequate supply. A well-known, albeit rare, genetic disorder is Brody myopathy. The genetic defect leads to reduced activity of a Ca2+-ATPase in skeletal muscle, resulting in an accumulation of Ca2+ ions in the cytosol. This results in a delay in the ability of skeletal muscle to relax after contraction.
