The cytosol or intracellular fluid (or cytoplasmic matrix) is the liquid found inside cells. It is the liquid of a cell, that is parted from other parts of the cell by cell walls, such as the mitochondrial matrix inside the mitochondrion. The entire contents of a eukaryotic cell within cell membrane, minus the contents of the cell nucleus, are referred to as the cytoplasm. In prokaryotes, most of the chemical reactions of metabolism take place in the cytosol, while a few take place in membranes or in the periplasmic space. In eukaryotes, while many metabolic pathways still occur in the cytosol, others are contained within organelles.
The cytosol is a complex mixture of substances dissolved in water. Although water forms the large majority of the cytosol, its structure and properties within cells is not well understood. The concentrations of ions such as sodium and potassium are different in the cytosol than in the extracellular fluid; these differences in ion levels are important in processes such as osmoregulation and cell signaling. The cytosol also contains large amounts of macromolecules, which can alter how molecules behave, through macromolecular crowding.
Although once thought to be a simple solution of molecules, multiple levels of organization exist in the cytosol. These include concentration gradients of small molecules such as calcium, large complexes of enzymes that act together to carry out metabolic pathways, and protein complexes such as proteasomes and carboxysomes that enclose and separate parts of the cytosol.
In the early 20th century the sum total of the contents of the cell (the protoplasm) were commonly classified into the contents of the nucleus (the nucleoplasm) and the remainder of the cell contents, the cytoplasm. The cytoplasm was further classified into solid structures, such as organelles and the cytoskeleton, and a liquid component that was variously called the cell sap, enchylema, hyaloplasm, paramitone, interfilar substance, or ground substance.
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