Question

Bt toxins are
A. Intracellular lipids
B. Intracellular Crystalline proteins
C. Extracellular crystalline proteins
D. Intracellular polysaccharides

Answer 1

Bacillus thuringiensis (Bt) is a soil-dwelling bacteria that produces a toxin that kills several herbivorous insects. Since then, $1920s$ the toxin it produces has been employed as an insecticide spray, and it is widely employed in organic farming. Bt is also the source of the genes that have been used to genetically edit a variety of food crops so that they may manufacture the toxin on their own to deter certain insect pests. Although a number of Bt strains are available to make its application more target-specific, the toxin is fatal to multiple insect orders, including Lepidoptera (butterflies), Diptera (flies), and Coleoptera (beetles).

Complete explanation:
Option A: Bacillus thuringiensis $\left( {Bt} \right)$ is a bacterium that produces poisonous proteins for insects. They release toxins, which is a crystalline protein that can obstruct the gut of insects when consumed. As a result, it's commonly used as a biological pesticide.
So, option A is incorrect.
Option B: Some protein crystals are formed by Bacillus thuringiensis $\left( {Bt} \right)$. A hazardous insecticidal protein is present in these crystals.
So, option B is incorrect.
Option C: Bacillus thuringiensis $\left( {Bt} \right)$ is a bacterium that produces poisonous proteins for insects. They release toxins, which is a crystalline protein that can obstruct the gut of insects when consumed.
So, option C is correct.
Option D:
Bt is an insecticide that produces a crystal-shaped protein (Cry toxin) that kills only particular insects.
So, option D is incorrect.

Hence, Option C is the correct answer.

Note:
The Gram-positive bacterium Bacillus thuringiensis $\left( {Bt} \right)$ generates crystals during sporulation. In the Bacillus, these crystals are made up of one or a limited number of $130{\text{ }}kDa$ protoxins known as crystal proteins, which exist as inactive protoxin. These proteins are very selective and powerful insecticides. Because of the alkaline pH of the alimentary canal, which solubilizes the crystals, Bacillus thuringiensis $\left( {Bt} \right)$ toxin protein exists as an inactive toxin that is transformed into an active form. The active toxin binds to the surface of midgut epithelial cells, creating holes that cause cell swelling and lysis, eventually resulting in an insect's death.