Answer: The answer is that have to bind to receptors to cross the membrane.
Explanation:
A hydrophilic substance can easily dissolve or absorbed in a water containing vessel, not a fat or lipid vessel like biological membranes.
Now, protein-based hormones like dopamine or serotonin ARE NOT LIPID-SOLUBLE; so, have to bind to receptors positioned in the membrane and MAY also require energy in form of ATP for across the membrane before they would exert their function on the target cell or organelles.
