Osmolytes are compounds affecting osmosis

Osmosis

Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...

. 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

Fluid balance

Fluid balance is the concept of human homeostasis that the amount of fluid lost from the body is equal to the amount of fluid taken in. Euvolemia is the state of normal body fluid volume. Water is necessary for all life on Earth...

. For example, when a cell swells due to external osmotic pressure

Osmotic pressure

Osmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane....

, membrane channels open and allow efflux of osmolytes which carry water with them, restoring normal cell volume.

Natural osmolytes that can act as osmoprotectant

Osmoprotectant

Osmoprotectants or compatible solutes are small molecules that act as osmolytes and help organisms survive extreme osmotic stress. Examples include betaines, amino acids, and the sugar trehalose. These molecules accumulate in cells and balance the osmotic difference between the cell's surroundings...

s include trimethylamine N-oxide

Trimethylamine N-oxide

Trimethylamine N-oxide, also known by several other names and acronyms, is the organic compound with the formula 3NO. This colorless solid is usually encountered as the dihydrate. It is an oxidation product of trimethylamine and a common metabolite in animals. It is an osmolyte found in saltwater...

 (TMAO), dimethylsulfoniopropionate

Dimethylsulfoniopropionate

Dimethylsulfoniopropionate , is an organosulfur compound with the formula 2S+CH2CH2COO−. This zwitterionic metabolite found in marine phytoplankton, seaweeds, and some species of terrestrial and aquatic vascular plants...

, trimethylglycine

Trimethylglycine

Trimethylglycine is an organic compound that occurs in plants as either glycine betaine or a choline-containing glycine betaine compound. Trimethylglycine was the first betaine discovered by science; originally it was simply called betaine because, in the 19th century, it was discovered in sugar...

, sarcosine

Sarcosine

Sarcosine, also known as N-methylglycine, is an intermediate and byproduct in glycine synthesis and degradation. Sarcosine is metabolized to glycine by the enzyme sarcosine dehydrogenase, while glycine-N-methyl transferase generates sarcosine from glycine. Sarcosine is a natural amino acid found in...

, betaine, glycerophosphorylcholine

Alpha-GPC

L-Alpha Glycerylphosphorylcholine is a natural choline compound found in the brain and in milk. It is also a parasympathomimetic acetylcholine precursor which may have potential for the treatment of Alzheimer's disease and is used as a nootropic dietary supplement to enhance memory and...

, myo-inositol, taurine and others. In humans, osmolytes are of particular importance in the renal medulla

Renal medulla

The renal medulla is the innermost part of the kidney. The renal medulla is split up into a number of sections, known as the renal pyramids. Blood enters into the kidney via the renal artery, which then splits up to form the arcuate arterioles. The arcuate arterioles each in turn branch into...

.