What would have happened ? If.............

Plasma membrane had not been selectively permeable.

Selectively permeable means, that the membrane allows the entry and exit of certain substances/molecules only if there is a concentration gradient whiles others are restricted. For example- water, oxygen, carbon-di-oxide etc. can move easily through the Plasma membrane. If the Plasma membrane would have not been selectively permeable than any molecule/compound could enter the cell which would disturb the balance of the Cell i. e. it will disturb the "homeostasis ". If the membrane was fully permeable, substances would enter constantly and the cell couldn't handle everything coming in or moving out which will lead to the death of the Cell.