Thank you Hello students, in this lecture we will learn about cytosol. The cytosol is the semi fluid or liquid medium component of the cytoplasm located outside of the nucleus and within the cell membrane in eukaryotic cells. The cytosol is also known as intracellular fluid or cytoplasmic matrix.
The cytosol thus forms the liquid matrix around the organelles and generally excludes the part of the cytoplasm contained within the organelles. The term cytosol was first introduced by H. A. Hardy in 1965 and was initially used for the liquid obtained after breaking cells and pelleting all the insoluble components by ultracentrifugation.
Cytosol differs from cytoplasm in that while cytoplasm forms all the part of the cell contained within the cell membrane, cytosol constitute only the part of the cell membrane. part of the cytoplasm that is not occupied by any of the organelles. Let us now see the difference between cytoplasm and cytosol.
Cytoplasm is a gelatin-like and semi-transparent liquid comprising of the cytosol, the cell organelles and the inclusions. Cytoplasm is made up of water which forms about 80% and other components include nucleic acids, enzymes, lipids, carbohydrates, amino acids, inorganic ions and other low molecular weight compounds. The cytoplasm also contains other dissolved salts and nutrients which facilitated the absorption of the water components by the cell.
The central granular part of the cytoplasm, including the cell organelles in eukaryotic cells, is called the endoplasm, while the surrounding layer is known as the cell cortex or the ectoplasm. In contrast to the cytoplasm, the cytosol comprises mostly of water, dissolved ions, proteins and other small or large water-soluble molecules. The cytosol also has different concentrations of ions such as sodium and potassium compared to the extracellular fluid and these differences play an important role in cellular activities such as osmoregulation, cell signaling and in generation of action potentials in endocrine, nerve and muscle cells.
In eukaryotes although most of the metabolic pathways occur within the organelles, many pathways also take place in the cytosol. However, in case of prokaryotes which lack organelles, almost all the metabolic pathways take place in the cytosol. Let us now see the composition of cytosol.
Cytosol forms most of the bulk of many cell types including that of bacteria. However, in case of plant cells, it occupies less volume as the bulk of the cell is occupied by a large central vacuole. Generally, the main composition of cytosol is formed by water and proteins and other components include dissolved ions and other small molecules. Most of the non-protein molecules present in the cytosol have a molecular mass of less than 300 Dalton.
The mixture of these small molecules is complex and number vary from about under 1000 in E.coli and Baker's yeast to about 2 lakhs different small molecules in plants depending upon the metabolites involved in metabolism. The major ions present in the cytosol are potassium, sodium, bicarbonate, chloride, calcium, magnesium, and amino acids. These ions concentrations usually establish gradients between the cytosol and the extracellular fluid or cytosolic organelles and such gradients are utilized in cell-to-cell communication. Example at the synopsis of nerve cells. In addition, the cytosol also contains other substructures like cytoplasmic inclusions such as glycogen, pigments and crystalline inclusions.
Cytoplasmic bodies which are not bound by a membrane such as P-bodies and other non-membrane bound structures can also be found such as egrisomes which are aggregation of misfolded proteins and rods and rings, which are large linear or circular sap structures typically described as polymers of inosine monophosphate dehydrogenase. They generally range from 7 to 7.5 pH. Next, cytosolic water. Water makes up about 70% of the total volume of cytosol in a typical cell. The pH of cytosol or intracellular fluid is 7.4, which is usually higher if the cell is growing.
The viscosity of cytosol is similar to that of pure water, but the diffusion of small molecules in cytosol is about 4-fold slower than in pure water. pure water as the diffusion is affected by the collision with the large number of macromolecules in the cytosol. Out of the total amount of water present in the cytosol, about 5% of it is strongly bound to solute or macromolecules as water of solvation and this water of solvation does not take part in the activation of the cytosol.
actively in osmosis and may also have different solvent properties. The rest of the cytosol water has the same structure as pure water and acts like that of pure water too. Next let us see the ions in cytosol. The concentration of different ions in cytosol is usually different from those in extracellular fluid. The cytosol also contains other charged macromolecules such as nucleic acids and proteins which are not present in the extracellular fluid.
Generally, the cytosol has higher concentration of potassium ions and a lower concentration of sodium ions compared to the extracellular fluid and such gradient of ion concentration help in osmoregulation. Normally water would enter the cell constantly by osmosis due to the higher concentration of macromolecules in the cytosol than in the extracellular fluid. However, sodium ion potassium ATPase further pumps in potassium ions and expels sodium ions.
The potassium ions then flow down their concentration gradient from inside the cell or cytosol to the outside of the cell through potassium ion channels creating a negative membrane potential due to loss of positive ions. This potential difference created across the membrane is compensated by further pumping of negative chloride ions. outside the cell through selective chloride channels.
This pumping out of sodium and chloride ions through the ion channels balance the osmotic effect created by the accumulation of higher concentration of organic molecules inside the cell. Similarly, cytosol also contains lower concentration of calcium ions which allows it to function as a second messenger in calcium signaling. During such signaling, a hormone signal or an action potential leads to opening of calcium ion channels that pumps calcium ions from the extracellular fluid into the cytosol and this sudden increase in cytosolic calcium ion concentration activates other signaling molecules like calmodulin and protein kinase C. Cells also accumulate other osmoprotectants such as betanes or trihalose in their cytosol to regulate larger osmotic changes.
In fact, cells can survive extreme desiccation by undergoing Cryptobiosis or a state of suspended activities where the cytosol and osmoprotactins become glass-like solid that stabilizes proteins and cell membranes during the desiccation. Next, let us see the macromolecules in cytosol. Protein molecules form about 20 to 30 percent of the total cytosol volume and mostly include the dissolved proteins in the cytosol that do not bind to the cell membranes or the cytoskeleton.
Many enzymes present in the cytosol are also believed to be present bound to the cytoskeleton. Unlike in eukaryotic cells in which the genome is contained within the nucleus, the genome in case of prokaryotes occurs as an irregular mass of DNA and associated proteins in a structure in the cytosol known as a nucleoid. Such high concentration of macromolecules in the cytosol causes an effect called macromolecular crowding in which the macromolecules have lesser volume to move in the cytosol. Macromolecular crowding effect results in changes in both the rates and the position of chemical equilibrium of reactions, as well as alter dissociation constants by favoring the association of macromolecules, such as those of multiple proteins to form protein complexes or the binding of DNA binding proteins to the.
targets in the genome present in the cytosol. Next, let us see the protein complexes and compartments in the cytosol. Cytosolic proteins can associate to form protein complexes in the cytosol forming compartments or channels. Such complexes formed by enzymes that catalyzed different steps in the same metabolic pathway are generalized.
Proteins may also form large compartments in the cytosol that are isolated from the rest of the cytosol by the surrounding proteins. For example, proteasomes are hollow barrel-shaped proteolytic cavity formed by a set of subunits and kept by a set of regulatory proteins that recognize proteins with a ubiquitin tag for degradation. Proteasomes contain proteases that degrade the proteins in the isolated chamber. Similarly, in bacterial cells, protein cell and close micro-compartments containing enzymes are common in the cytosol. For example, carboxysomes.
micro compartments containing enzymes involved in carbon fixation such as rubisco. Next let us take up the functions of cytosol. The cytosol serves as the site for many cellular metabolic processes and other cellular activities in eukaryotes while in prokaryotes Most of the metabolic pathways operate in the cytosol. For example, the major cellular processes that occur in the cytosol include protein synthesis or translation, glycolysis of cellular respiration, pentose phosphate pathway, gluconeogenesis, and cytokinesis during mitotic and meiotic cell division occurs in the cytosol. Another major function of cytosol is the intracellular transport of molecules across the cell or between cellular organelles.
During intracellular transport in the cytosol, water-soluble molecules such as amino acids diffuse rapidly through the cytosol while hydrophobic molecules such as fatty acids or sterols are transported by specific binding proteins. The molecules that are taken into the cell by endocytosis are also transported through the cytosol inside vesicles before they are secreted outside. The cytosol also serves as the site for signal transduction from the cell membrane to the nucleus or organelles inside the cell. In addition, the cytosol also plays major role in maintaining water balance within the cell and the exon potential of the cell by accumulating protein and iron gradients. Now coming to the conclusion.
The cytosol serves as the site for all the metabolic reactions and prokaryotes. While in eukaryotic cells containing nucleus and organelles which are sites for many metabolic activities. The cytosol still serves as a site for many cellular activities such as glycolysis, cell division and other metabolic pathways.
Thank you.