Secretion A process in which molecules are actively transported out of an organ. Solubility Ability of a substance to be dissolved in a solvent.  The solubility is expressed according to the solvent, e.g., water solubility, solubility in acetone, etc. Substance Physical material of which something is made.  It may be element, compound, or a mixture of materials. Substrate A substance acted upon.  It often refers to the chemical that undergoes reaction with an enzyme. Systemic toxin A toxin that affects the entire body or many organs.