While pondering how exocytosis works, it occurred to me that "Intersections of magnetic parabolas" potentially explains this phenomenon.
If the overall macro-molecular construct of the cytoplasm/protoplasm, referring to the internal "charge" of the cell within its membranes reaches a point where the synthesis or breakdown (anabolic/catabolic) reactions becomes non-homeostatic, the cell's pressure might push a vacuole to the edge allowing material to escape the pressure.
While that can occur in isolation (within a single cell's walls), there would also be a macro-effect of multiple cells' "charge" that affects a single cell.
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