Answer:
This is because fluids have a polar nature, so they couldn´t cross the lipidic bilayer that constitutes the plasma membrane
Explanation:
Plasma membranes are contituted by lipid bilayers. These kind of compounds are hidrophobic (they reject water) and don´t permit the freely passage of hidriphilic molecules that can disolve in water, constituing fluids. Because of that cells developed transport mechanism like endocytosis.
