A student asks: the Bt toxin kills insects, so why does it not kill the bacterium that makes it? …
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A student asks: the Bt toxin kills insects, so why does it not kill the bacterium that makes it?
- In what form is the toxin stored in the crystals?
- What changes it to the active form, and where?
- Suppose the gut of a certain insect were acidic instead of alkaline. What would you expect, and why?
- Name the group of insects that includes the tobacco budworm.
Need a reminder? The key ideaShow
The soil bacterium Bacillus thuringiensis makes crystals of an inactive protoxin. In the alkaline gut of an insect, the crystals dissolve and the protoxin becomes the active toxin. It binds to the midgut cells, makes pores, and the cells swell and burst. The toxin gene is called cry. The genes cryIAc and cryIIAb control the cotton bollworms, and cryIAb controls the corn borer. Most Bt toxins act on one group of insects only.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) As an inactive protoxin.
Step 2. (b) The alkaline pH of the gut of the insect dissolves the crystals, and the protoxin is changed to the active toxin.
Step 3. (c) The crystals would not dissolve well, so little active toxin would form, and the insect would be harmed less. This is a prediction from the mechanism, and not a fact that NCERT states.
Step 4. (d) The lepidopterans, the moths and butterflies.
Answer: (a) inactive protoxin; (b) the alkaline gut; (c) little toxin forms, so little harm; (d) lepidopterans.
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