Protection of Biomembranes and Freezing Tolerance
Soluble material accumulates in the residual liquid of the cell to an enormous extent during extracellular freezing. In addition to the danger of membrane disintegration as a consequence of water withdrawal (see above), severe stress is incurred by the concentration of ions at membrane surfaces. High ionic charge at the membrane surface changes the membrane potential, and can thus also lead to membrane disintegration or at least to the dissociation of peripheral proteins.
The Degree of Frost Hardiness Depends on the Extent and Quality of Membrane Protection
Relatively high concentrations of soluble, low molecular weight, so-called cryoprotective substances can be found in frost-tolerant plants [polyols, sucrose and its galactosides raffinose and stachyose, poorly degradable carbohydrates such as hammamelitol (Fig. 1.3.26), amino acids, polyamines and many more]. These compounds "dilute" the ionic charge at the membrane surface and, at the same time, stabilise the bilayer struc-
CH2OH HOH2C— C—OH H— C—OH H— C—OH CH2OH
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