Inducible enzymes are those that are only synthesised when their substrate is present.

For example, β-galactosidase is an inducible enzyme as it is only synthesised when lactose is present.

A repressible enzyme is an enzyme whose production is stopped when it is not required.

Lactose permease allows lactose to enter the bacterial cell.

The lac operon only produces enzymes for lactose metabolism when lactose is available, saving energy.

β-galactosidase catalyses the hydrolysis of lactose into glucose and galactose.

The lac operon is a group of genes in bacteria that control the metabolism of lactose.

The genes are controlled by the same promoter region and are transcribed together.

1. lacZ - codes for β-galactosidase
2. lacY - codes for lactose permease
3. lacA - codes for transacetylase

The repressor protein binds to the operator region. 

This prevents RNA polymerase from binding to the promoter region, which prevents (downregulates) the transcription of the structural genes.

Lactose binds to the repressor protein, changing its shape and preventing it from binding to the operator region.

This allows the structural genes to be transcribed.

1. Regulatory gene (lacI) - codes for a repressor protein that can inhibit the lac operon
2. Promoter region - where RNA polymerase binds to start transcription
3. Operator region - where the repressor protein can bind to control expression of structural genes
4. Structural genes - genes coding for a proteins that have a function within a cell, in this case, the enzymes that help metabolise lactose