Nhumans, zygote is the correct term for a developing organism during which period of time?

No, the correct term is embryo. Embryo is in humans, the name for the developing organism during the first eight weeks after conception. While Zygote is for fertilised egg. And Fetus is the term for the developing organism between the embryonic stage and birth.

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