What occurs to bacterial cells when they freeze, leading to outer membrane rupture?

When bacterial cells freeze, the cytoplasm volume expands due to the formation of ice crystals. As water inside the cells freezes, it expands, which can lead to an increase in internal pressure. This expansion can cause significant mechanical stress on the outer membrane of the cell, ultimately resulting in its rupture. This process is particularly critical because the integrity of the cell membrane is vital for the survival of the cell and for maintaining its internal environment.

The other options do not accurately reflect the effects of freezing on bacterial cells. The cytoplasm does not shrink during freezing; rather, it is the expansion that leads to structural damage. Similarly, the temperature of the cell does not increase during freezing; it decreases as the cell is subjected to lower temperatures. Lastly, osmotic pressure is not decreased as a direct result of freezing; the changes in cellular water content due to ice formation and potential rupture lead to different osmotic dynamics. Thus, the correct understanding of what happens during freezing highlights the crucial role of cytoplasmic expansion in cell membrane integrity.