<P> The proportion of cell volume that is cytosol varies: for example while this compartment forms the bulk of cell structure in bacteria, in plant cells the main compartment is the large central vacuole . The cytosol consists mostly of water, dissolved ions, small molecules, and large water - soluble molecules (such as proteins). The majority of these non-protein molecules have a molecular mass of less than 300 Da . This mixture of small molecules is extraordinarily complex, as the variety of molecules that are involved in metabolism (the metabolites) is immense . For example, up to 200,000 different small molecules might be made in plants, although not all these will be present in the same species, or in a single cell . Estimates of the number of metabolites in single cells such as E. coli and baker's yeast predict that under 1,000 are made . </P> <P> Most of the cytosol is water, which makes up about 70% of the total volume of a typical cell . The pH of the intracellular fluid is 7.4. while human cytosolic pH ranges between 7.0 - 7.4, and is usually higher if a cell is growing . The viscosity of cytoplasm is roughly the same as pure water, although diffusion of small molecules through this liquid is about fourfold slower than in pure water, due mostly to collisions with the large numbers of macromolecules in the cytosol . Studies in the brine shrimp have examined how water affects cell functions; these saw that a 20% reduction in the amount of water in a cell inhibits metabolism, with metabolism decreasing progressively as the cell dries out and all metabolic activity halting when the water level reaches 70% below normal . </P> <P> Although water is vital for life, the structure of this water in the cytosol is not well understood, mostly because methods such as nuclear magnetic resonance spectroscopy only give information on the average structure of water, and cannot measure local variations at the microscopic scale . Even the structure of pure water is poorly understood, due to the ability of water to form structures such as water clusters through hydrogen bonds . </P> <P> The classic view of water in cells is that about 5% of this water is strongly bound in by solutes or macromolecules as water of solvation, while the majority has the same structure as pure water . This water of solvation is not active in osmosis and may have different solvent properties, so that some dissolved molecules are excluded, while others become concentrated . However, others argue that the effects of the high concentrations of macromolecules in cells extend throughout the cytosol and that water in cells behaves very differently from the water in dilute solutions . These ideas include the proposal that cells contain zones of low and high - density water, which could have widespread effects on the structures and functions of the other parts of the cell . However, the use of advanced nuclear magnetic resonance methods to directly measure the mobility of water in living cells contradicts this idea, as it suggests that 85% of cell water acts like that pure water, while the remainder is less mobile and probably bound to macromolecules . </P>

Where is the cytosol located in a cell