Practice question

Cryopreservation of mammalian cells routinely employs dimethyl sulfoxide as penetrating cryoprotectant at 5-10% concentration. DMSO is small amphipathic molecule freely crossing lipid bilayer, replacing intracellular water and disrupting hydrogen bonding network required for ice lattice formation. During controlled slow cooling at -1°C per minute achieved using isopropanol containers like Mr. Frosty or programmable freezers, extracellular ice forms first creating osmotic gradient that draws water out of cells dehydrating them. DMSO lowers freezing point, promotes vitrification where residual intracellular water transitions to amorphous glass rather than sharp damaging crystals that would shear membranes, organelles and chromosomes. DMSO also stabilizes proteins through preferential exclusion. At ambient temperature DMSO exhibits cytotoxicity and can trigger differentiation of hematopoietic lines, so after thawing rapid dilution in large volume medium reduces concentration below 1%. Glycerol penetrates slower favoring insect and embryonic stem cells, while ethanol and formaldehyde are lethal and unsuitable. DMSO thus became standard in ATCC and cell bank protocols.