Question

what would happen if plasma membrane had not been selectively permeable

Answer 1

Then any sustance can easily travel from one cell to another.

Answer 2

If the membrane were fully permeable to all substances, then anything could enter the cell. This would upset the balance between the cell's contents and the outside environment. There is only so much ability to store substances and utilize substances, therefore, the cell would not be able to maintain homeostasis.

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