Question

The inactive protoxin is activated in the gut of the insect by
A. Acidic $pH$
B. Alkaline $pH$
C. Low temperature
D. High temperature

Answer 1

Protoxin refers to an inactive stage of toxin that gets activated by providing stimuli. Bt toxins are an example of protoxins. These are secreted by a bacterium that acts as biopesticides for many insects. Unlike other pesticides, it acts on the alkaline guts of insects.

Complete answer: Protoxin is the inactive stage of toxins. These can be stimulated by temperature or $pH$change to become active. A naturally occurring gram-positive bacteria called Bacillus thuringiensis produces a wide range of toxins effective against many insects and pests species. These bacteria reproduce by spore formation. Various toxins are known to be formed during the sporulation process by this bacterium. During the sporulation process, the bacterium synthesizes a cytoplasmic inclusion. This inclusion contains one or more proteins that act as a toxin for a large number of insect larvae. These toxins are named BT toxins. The various known Bt toxins are endotoxin, cytolytic proteins, insecticidal proteins, etc. When the bacterium passes the sporulation stage it lyses and releases thousands of spores along with the inclusion bodies made in the cytoplasm. The inclusion body encloses inactive toxins called protoxins. When insects or pests try to ingest the inclusion bodies, the alkaline gut of the insects stimulates the toxin to get activated. Thus, the alkaline $pH$ leads to the activation of protoxin inside the insect’s gut. The toxin is usually in crystal form. These crystals are solubilized in the insect gut and produce pores in the gut lining. This leads to the leakage of essential fluids from the insect’s body leading to sudden death. Hence, this bacterium is used as an important biopesticide.
Therefore, the right answer is option B.

Note:Bacillus thuringiensis is also used in recombinant DNA technology. Crops such as cotton and corn have been genetically modified by inserting toxin-producing genes of this bacterium. These are called GM or genetically modified crops. Whenever insects ingest these crops they die.