- 1 Definition
- 2 Details
- 3 Supplementary
- 4 Further reading
- 5 Reference
A cell is the structural, functional and biological unit of all organisms. It is a membrane-bound structure containing the protoplasm. The protoplasm is the fluid living content of the cell. In certain references, protoplasm is a synonymous of the term cytoplasm. In other sources, the protoplasm includes the nucleoplasm. Thus, in stricter sense, the protoplasm is basically made up of the cytoplasm and the nucleoplasm. The cytoplasm, in turn, is the protoplasmic contents between the cell membrane and the nuclear envelope. The fluid portion of the cytoplasm is called the cytosol. It is the liquid matrix that surrounds the organelles inside the cell.
The cytosol is the aqueous portion of the cytoplasm in an intact cell. It consists of water, organic molecules and dissolved ions. The highest percentage of cytosol component is water, i.e. about 70%. The typical ions in the mammalian cytosol are potassium, sodium, chloride, bicarbonate, amino acids in proteins, magnesium, and calcium. The concentration of potassium ions is greater in cytosol than that in the extracellular fluid. Macromolecules dissolved in the cytosol include protein molecules not bound to the cell membrane or the cytoskeleton.
The intracellular fluid pertains to the internal fluid located within the cell(s). In humans, the total body water composition is made up mainly of intracellular fluid (67%) and extracellular fluid (26%). The intracellular fluid, in turn, is composed of water dissolved ions, and other molecules. The intracellular fluid pertains to the cytosol(s) of the cell. As opposed to the extracellular fluid, which is the bodily fluid outside the cells, the intracellular fluid contains more charged macromolecules.
Common biological reactions
The cytosol contains a number of metabolites involved in various metabolic processes. In eukaryotes, many of the metabolic activities occur inside the organelles although some of the steps occur in the cytosol. For instance, glycolysis, the first step of cellular respiration, occurs in the cytosol. Succeeding steps, such as redox reactions, occur inside the mitochondrion. In prokaryotes, most of the metabolic activities occur in the cytosol since they lack organelles.
The cytosol is involved in cell signaling, e.g. calcium signaling. For instance, a hormone activates a cell to open its calcium channels to allow the entry of calcium ions. The influx of calcium ions leads to a cascade of reactions, such as the activation of other signaling molecules, calmodulin and protein kinase C.
Osmoregulation is the process of regulating water potential in order to keep fluid and electrolyte balance within a cell or organism relative to the surrounding. The ions in the cytosol are regulated to keep up the suitable amounts of ions. For instance, there are greater potassium ions and fewer sodium ions in the cytosol than in blood. Osmoregulation is important to organisms to keep a constant, optimal osmotic pressure within the body or cell. It is the way by which an organism maintains suitable concentration of solutes and amount of water in the body fluids.
The cytosol has osmoprotectants, e.g. trehalose, to make the cell survive in case the cell completely dries out. The osmoprotectants and the cytosol turn into a glass-like solid to stabilize the cell membrane and proteins at desiccation.
The cytosol serves as the site where many chemical reactions take place. In prokaryotes, it is where most metabolic reactions take place (others occur in the cell membrane). In eukaryotes, it is where the organelles and other cytoplasmic structures are suspended. Since the cytosol contains dissolved ions, it plays a role in osmoregulation and cell signaling. It is also involved in generating action potentials such as in endocrine, nerve and muscle cells.
- cyto- (cell) + sol(ution)
- cytoplasmic matrix
- intracellular fluid (ICF)
- Kikkawa, U., Kishimoto, A., & Nishizuka, Y (1989). "The protein kinase C family: heterogeneity and its implications". Annu. Rev. Biochem. 58: 31–44.
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