The function of which cell part is most similar to that of the human excretory system?

I believe it is the cell membrane since it is in charge of regulating what comes in and out of a cell. The human excretory systems functions to eliminate the waste products of metabolism, which includes removal of nitrogenous waste in the form of urine and solid wastes mostly from the breakdown of hemoglobin.

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