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 and the cytosol. In extreme cases, such as in bdelloid rotifers, tardigrades, brine shrimp and nematodes, these molecules can allow cells to survive being completely dried out and let them enter a state of suspended animation called cryptobiosis. In this state the cytosol and osmoprotectants become a glass-like solid that helps stabilize proteins and cell membranes from the damaging effects of desiccation.
Compatible solutes have also been shown to play a protective role by maintaining enzyme activity through freeze-thaw cycles and at higher temperatures. Their specific action is unknown but is thought that they are preferentially excluded from the proteins interface due to their propensity to form water structures.
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