Osmolytes are compounds affecting osmosis. They are soluble in the solution within a cell, or in the surrounding fluid, e.g. as plasma osmolytes. They play a role in maintaining cell volume and fluid balance. For example, when a cell swells due to external osmotic pressure, membrane channels open and allow efflux of osmolytes which carry water with them, restoring normal cell volume. Osmolytes also contributes to protein folding. Natural osmolytes that can act as osmoprotectants include trimethylamine N-oxide (TMAO), dimethylsulfoniopropionate, trimethylglycine, sarcosine, betaine, glycerophosphorylcholine, myo-inositol, taurine, glycine, and others. Bacteria accumulate osmolytes for protection against a high osmotic environment. The osmolytes will be neutral non-electrolytes, except in bacteria that can tolerate salts. In humans, osmolytes are of particular importance in the renal medulla. Current understanding of osmolytes have been used to calculate the maximum depth where a fish can survive at 26,900 feet (8,200 meters).