Pushy Ice Crystals

A glass bottle will break if water freezes inside it, because the water expands as ice crystals form.  In the same way, cells will break open and die if the fluid inside turns to ice.  The equilibrium freezing point of all water solutions, including body fluids, is -0.5oC.  If a plant or animal cannot find a warm winter home (above -0.5oC), it must find a way keep its cells from freezing.  Some plants and animals tolerate extracellular freezing , that is, ice formation in the fluid outside of the cell.  The cell is protected by its membrane, which forces water out for the winter and keeps ice from getting in.

1back.gif (1823 bytes) Extracellular Freezing
Freezing outside cells. This type of freezing is very common in plant tissue that survives winter, including tree branches, and leaves that stay green under snow. Water moves into spaces between the cells, where it can freeze without damaging the plant.