Final answer:
Cell membranes require fluidity for their function and selective permeability. 'Cracks' in the membrane would disrupt these functions, leading to cell damage or death. The bilayer structure with its mosaic nature can usually self-repair minor punctures.
Step-by-step explanation:
Cell membranes must be fluid in nature because this fluidity allows for the transit of materials through the membrane and contributes to its selective permeability. If there were cracks in cell membranes, the integrity and selective permeability would be compromised, resulting in the inability to protect the cell's internal environment and potential cell destruction. Since the membrane is composed of a bilayer of phospholipids with embedded proteins and cholesterol, it has a particular mosaic nature that allows it to be penetrable by a fine needle without bursting and self-seal afterward. However, a disruption such as a 'crack' would suggest a failure in the membrane to repair itself, which contradicts the observed ability of cell membranes to mend minor disruptions.
The fluidity of the membrane is also dependent on the presence of certain lipids with unsaturated hydrocarbon tails (with C-C double bonds) and cholesterol, as well as the temperature of the environment. Any significant 'cracks' in the cell membrane would hinder its essential functions, which include maintaining homeostasis by allowing certain substances to enter and leave the cell, while keeping out harmful materials and preventing the loss of essential internal components.