What would happen if the cell membrane were fully permeable to all substances in the cell’s environment?

Interesting question. 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. For example, the cell membrane is selectively permeable and allows
substances to cross the membrane if there is a concentration gradient, and if those
substances are small enough to cross the membrane. Sometimes, the proteins in the cell
membrane help materials enter the cell. However, this is all regulated at a particular
rate. If the membrane was fully permeable, substances would enter constantly and the
cell couldn't use or handle everything coming in, resulting in the death of the
cell.