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.
 
        
                    
             
        
        
        
Check points throughout the cell cycle that determine when the cell should begin the next phase
        
             
        
        
        
Answer:
Hope the above picture might help you :)