Final answer:
Repressible systems like the trp operon are usually 'on' by default, constantly producing necessary compounds unless the end product is prevalent enough to repress its production. Inducible systems, like the lac operon, are generally 'off' by default and are activated only in the presence of a specific substrate required for metabolism.
Step-by-step explanation:
Repressible systems and inducible systems refer to how certain genes are regulated in prokaryotic cells, such as those found in bacteria. Repressible operons, like the tryptophan (trp) operon, are generally on because they typically contain genes necessary for a biosynthetic pathway that the cell continually needs to maintain.
They remain on by default to produce essential compounds such as amino acids, unless the end product is sufficiently present and it then acts to suppress its own production.
In contrast, inducible operons such as the lactose (lac) operon, are commonly off by default. They are only turned on in the presence of a substrate that needs to be metabolized, such as lactose. Once lactose is available, the operon is induced, leading to the production of enzymes that will digest the lactose.