Cytosol

The cytosol, sometimes called the cytoplasmic matrix or groundplasm, is the liquid portion of the intracellular fluid. Membranes divide it into compartments; for instance, the mitochondrial matrix is one such compartment. In eukaryotic cells, the cytosol sits inside the cell membrane and forms part of the cytoplasm—which also includes organelles like mitochondria and plastids, though not their internal fluids—while the nucleus is separate. The cytosol acts as a liquid matrix around these organelles. In prokaryotes, nearly all metabolic reactions happen in the cytosol, with a few occurring in membranes or the periplasmic space. In eukaryotes, many metabolic pathways still take place in the cytosol, but others are carried out within organelles.

The cytosol is a complex mixture of substances dissolved in water. Water makes up the vast majority of it, but its structure and properties inside cells remain poorly understood. The concentrations of ions like sodium and potassium differ between the cytosol and the extracellular fluid; these differences are crucial for osmoregulation, cell signaling, and generating action potentials in excitable cells such as endocrine, nerve, and muscle cells. The cytosol also contains many large macromolecules, which can alter how other molecules behave through a phenomenon called macromolecular crowding.

Though once considered a simple solution, the cytosol has multiple levels of organization. These include concentration gradients of small molecules like calcium, large enzyme complexes that work together in metabolic pathways, and protein complexes such as proteasomes and carboxysomes (in bacteria) that enclose and separate parts of the cytosol.

The term "cytosol" was first introduced in 1965 by H. A. Lardy. It originally referred to the liquid produced by breaking cells apart and pelleting insoluble components via ultracentrifugation. That soluble cell extract, known as a cytoplasmic fraction, is not identical to the soluble part of the cell cytoplasm. Today, "cytosol" means the liquid phase of the cytoplasm in an intact cell, excluding any cytoplasm inside organelles. To avoid confusion between these uses, the phrase "aqueous cytoplasm" has been used for the liquid contents of the cytoplasm in living cells. Earlier terms, such as hyaloplasm, were also used for cell fluid, though not always consistently, as its nature was not well understood.

The proportion of cell volume that is cytosol varies. In bacteria, it forms the bulk of cell structure, while in plant cells, the large central vacuole is the main compartment. The cytosol is mostly water, along with dissolved ions, small molecules, and large water-soluble molecules like proteins. Most non-protein molecules in it have a molecular mass under 300 Da. This mixture is extraordinarily complex, given the immense variety of metabolites involved in metabolism. For example, plants may produce up to 200,000 different small molecules, though not all in the same species or cell. Estimates for single cells like *E. coli* and baker's yeast suggest fewer than 1,000 metabolites are present.

Water makes up about 70% of a typical cell's total volume. The pH of intracellular fluid is 7.4, while mouse cell cytosolic pH ranges from 7.0 to 7.4, usually higher in growing cells. The viscosity of cytoplasm is roughly that of pure water, but diffusion of small molecules is about four times slower, mostly due to collisions with the many macromolecules. Studies in brine shrimp show that a 20% reduction in cell water inhibits metabolism, which progressively slows as the cell dries, halting entirely when water levels drop 70% below normal. Although water is vital, its structure in the cytosol is not well understood, partly because methods like nuclear magnetic resonance spectroscopy only measure average water structure, not local microscopic variations. Even pure water's structure is poorly understood due to hydrogen-bonded clusters. The classic view holds that about 5% of cell water is strongly bound as water of solvation to solutes or macromolecules, while the rest behaves like pure water. This bound water is not active in osmosis and may have different solvent properties, excluding some molecules and concentrating others. However, some argue that high macromolecule concentrations affect water throughout the cytosol, making it behave very differently from dilute solutions. These ideas include zones of low- and high-density water that could influence cell structures and functions. Yet advanced nuclear magnetic resonance methods measuring water mobility in living cells contradict this, suggesting 85% of cell water acts like pure water, with the remainder less mobile and likely bound to macromolecules.