Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): The crystals of Bt toxin are solubilised in the gut of an insect that ingests them.
Reason (R): The activated Bt toxin binds to the surface of midgut epithelial cells and creates pores that cause cell swelling and lysis.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).
Assertion (A): The crystals of Bt toxin are solubilised in the gut of an insect that ingests them. — True. A is about what dissolves the crystals. The Bt toxin is present in the crystals as an inactive protoxin. The alkaline pH of the insect gut solubilises the crystals and the protoxin becomes the active toxin.
Reason (R): The activated Bt toxin binds to the surface of midgut epithelial cells and creates pores that cause cell swelling and lysis. — True. R is about how the active toxin kills the insect. It binds to the midgut epithelial cells and makes pores in them; the cells swell and burst, and the insect dies.
Why R does not explain A? The crystals dissolve because of the alkaline pH of the gut. R tells what happens after the toxin has become active, so it is a later step and not the cause of solubilisation. Both are true, but R is not the correct explanation of A.
NCERT Extract:
Bt Cotton – “… B. thuringiensis forms protein crystals during a particular phase of their growth. These crystals contain a toxic insecticidal protein. Why does this toxin not kill the Bacillus? Actually, the Bt toxin protein exist as inactive protoxins but once an insect ingest the inactive toxin, it is converted into an active form of toxin due to the alkaline pH of the gut which solubilise the crystals. The activated toxin binds to the surface of midgut epithelial cells and create pores that cause cell swelling and lysis and eventually cause death of the insect.”