What is the biochemical explanation for why adding glucose to a bacterial culture containing lactose significantly decreases production of beta-galactosidase, whereas adding lactose to a bacterial culture containing glucose leads to only a minimal increase in the production of beta-galactosidase?

If we add glucose to the culture with lactose, the lac operon would be turned off and the production of beta-galactosidase, other enzymes and molecules that the operon transcribes, would decrease.

On the contrary, if we add lactose to a bacterial culture containing glucose, the lac operon would be on, transcribing the genes, hence, producing beta-galactosidase but only at a small rate. Because the bacteria already got the energy from the glucose.

Explanation:

An operon is a group of structural genes whose expression is regulated by a promoter, operator and regulator genes. In this particular case, they are talking about the lactose operon, lac operon. This operon is required, in bacteria, to obtain energy from lactose when there is no glucose present.

The lac operon contains a series of genes, lacZ, lacY, and lacA. The gene LacZ codifies an enzyme required to break the lactose, beta-galactosidase. LacY codifies for another enzyme required in the transport of lactose inward the cell. And finally, LacA produces a molecule that is involved in detoxification.

The lac operon is regulated, among others, by the presence of glucose and lactose. The glucose is the preferable source of energy for bacteria so if there are lactose and glucose the glucose would be used for energy at a bigger extent. In the absence of glucose, the lac operon plays a vital role and helps the bacteria to obtain energy from lactose.

When there is glucose but no lactose available, there is a repressor than binds to the promoter so the lac operon is OFF cannot express the genes.

On the other hand,when there is lactose and no glucose, lactose enters the cell and some of this molecules turn to alolactose. Alolactose molecules bind to the repressor so the operon is not repressed anymore and the lac operon would be ON producing enzimes and molecules.

If the lac operon is ON ( glucose absent ) the production of beta-galactosidase would be high, on the other hand, if the operon is OFF (glucose present) the production of beta-galactosidase decreases.

In the presence of both lactose and glucose, the genes for the lactose metabolisms are transcribed at low rates in the lac operon.

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