Which urinary structure serves as the transporter for urine toward the bladder?

The answer is ureter. They concentrate the urea from nephrons and collect in the bladder.  They are larger in diameter (25–30 cm) compared to the proximal and distal tubules, loop of henle and collecting tubules. There is one ureter for each kidney connecting to the bladder. The ureter has smooth muscles that cotract and direct the urine to the bladder

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products