Describe the state of the lac operon system if the level of lactose is low

The lac operon is something that I know (at least at my university) you can take an entire college course on, so I'm not going to go into supreme detail. Its up to you to do your own homework in that respect (looking up enzymes and proteins and their function, etc) but I will get you started.

The lac operon is a set of three genes that are responsible for producing three proteins: B-galactosidase, which cleaves lactose, B-galactoside permease, which helps to transport lactose into the cell, and B-galactoside transacetylase-which attaches an acetyl group to B-galactosides, but why this function is important is still unknown.

So, the cell wants these proteins only when lactose is present. It will not waste resources producing these proteins when there is no lactose to be digested. (Although, no operons are ever truly "off." They are "leaky"- always producing a little bit). If the level of lactose it low, the lac operon is off. If lactose is present, the lac operon is on. This is achieved by a regulatory protein that binds to the operator. When lactose is present, lactose will bind to the regulatory protein, the protein will fall off, and the genes can be transcribed. If lactose is not present, the regulatory protein remains bound and the genes are not transcribed.

Something important to keep in mind is that cells will always utilize glucose as an energy source first. If you culture bacteria with both lactose and glucose, you wont see appreciable lac operon activity. This is because lactose requires processing (in other words, cleavage by B-galactosidase to be turned into glucose) before it can undergo cellular respiration to produce energy. Glucose is already ready to undergo cellular respiration. Only after there is no glucose present will the lactose begin to be used by the bacteria as an energy source.

basically the lac operon, or all operons for that matter, consist of three parts (as mentioned above): the promoter, operator, and related genes. in a resting state - that is, when lactose is not available, a binding protein is binded at the site of the operator so that the operon may not function. however, when lactose is indeed available, a lactose molecule binds to the regulatory protein and thus changes its binding properties so that it releases it "grip" from the operator DNA and allows RNA polymerase to transcribe the mRNA for lactase-enzymes that would break down the available lactose for energy