Genetic Regulation and the Lactose Operon

ID #1405

I have a question regarding the graph showing glucose and beta-galactosidase concentration vs. cell growth. Why is it that there is no increase of glucose shown in the cell as lactose is broken down into glucose and galactose?

Great question; I love seeing questions that show that you're thinking beyond just what is presented! There are two main reasons for why you don't see the level of glucose in the graph rise when you start metabolizing lactose. First is that the timeline would need to be extended further to see this effect if it were likely to happen. The second is that it's actually not likely to happen, because just as soon as you're "producing" glucose by cleaving lactose, you're using it up in glycolysis. The level of glucose was so high at the beginning of the experiment because we gave those cultures of E. coli cells more glucose than they could initially use, but eventually they did use it all up. Even if you had an enormous amount of lactose available in addition to all that glucose, you might predict that over time, all that lactose would be converted to glucose. But think about what is really happening. Once all the gluose is exhausted, just as soon as some of the lactose is brought in, you start using it, which means now you have some glucose. This is going to quickly lead to a drop in cAMP levels, which in turn means less lac operon expression, and a shift back to glucose metabolism. You'll reach a balance of using just enough lactose to turn into the glucose you really wanted, and the level of glucose won't really "peak" again.

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