In cells that are constantly involved in secretion, an equivalent amount of membrane must be returned to the interior of the cell for each vesicle that fuses with the plasma membrane; if this does not occur, then what would happen?

       The cell membrane would keep expanding in a way that could lead into a process called apoptosis.

Explanation:

       As said on the question, in secretion, the same amount of membrane must return to the cell, because, when a cell wants to send something to outside of it, it needs to have a membrane involving it for protection, and the organelle that does that is the Golgi Apparatus.

      Once it involves this thing that needs to go outside the cell, this Golgi will be less larger than before, and the plasmatic membrane will be bigger once the exocytosis happen, and if it keeps expanding and not going back, the golgi apparatus would stop working and this cell would program its own death by the apoptosis process.

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