Some reflexes, like the rooting, sucking and grasping reflex, are theorized to exist because

Some reflexes are theorized to exist because they have the survival value. Sigmund Freud First to theorize that we pass through different stages, behaviour is governed by the reflexes that we are born with, start to develop schemata, but need to develop permanence.

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