Q.Why does the Bt toxin not kill the bacterium that produces it but kills the insect that ingest it?
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Bt Toxin Inactivity
Let’s start with something you already know. Imagine you eat a piece of stale, spoiled food. Your stomach immediately feels uneasy — maybe you get cramps or nausea. Your body is reacting to something harmful that entered it. Now, what if that harmful thing was a tiny crystal that only dissolves and becomes active in your stomach’s specific acidic environment? That’s the basic idea behind Bt toxin inactivity.
The everyday intuition
Think of a medicine capsule that has a special coating. That coating is designed to survive the acid in your stomach and only dissolve in your intestines, where the medicine is needed. If the coating dissolved too early, the medicine would be destroyed before it could work. If it never dissolved, the medicine would pass right through you, useless.
Bt toxin works on a similar principle — but in reverse. The toxin is produced by a bacterium called Bacillus thuringiensis (Bt). This bacterium makes a protein crystal that is harmless to most organisms, including humans, because it is produced in an inactive form. It only becomes dangerous when it reaches the very specific environment of an insect’s midgut.
The precise meaning
Bt toxin inactivity means that the toxin protein is initially non-toxic. It is a protoxin — a precursor that needs to be activated inside the body of a target insect. The activation happens only when the protoxin is eaten by a susceptible insect larva. Inside the insect’s midgut, which has a high pH (alkaline conditions) and specific digestive enzymes, the inactive crystal dissolves and gets chopped into a smaller, active toxin molecule.
That active toxin then binds to the insect’s gut wall, creating pores that cause the gut to leak. The insect stops feeding and eventually dies. But for any other animal — including humans, cattle, birds, or fish — the toxin remains inactive because their digestive systems lack the right alkaline pH and the specific enzymes needed to activate it.
This is why Bt toxin is considered safe for humans and non-target animals. The toxin is not poisonous by itself; it is the insect’s own digestive chemistry that turns it into a poison. If you ate the Bt crystal, it would pass through your body unchanged, like an undigested seed.
Why it matters
This concept is the foundation of Bt crops — genetically modified plants like Bt cotton, Bt corn, and Bt brinjal. Scientists take the gene that codes for this inactive toxin from the bacterium and insert it into the plant’s DNA. The plant then produces the inactive toxin in its tissues. When a pest insect chews on the plant, it ingests the inactive toxin, which becomes active only inside that insect.
The key advantage is target specificity. The toxin does not harm beneficial insects like bees, ladybugs, or earthworms, because their gut conditions are different. It also does not harm humans or livestock. This reduces the need for chemical insecticides, which are broad-spectrum poisons that kill many organisms, including helpful ones.
What the NCERT textbook says
The NCERT Class 12 Biology textbook (Chapter 12: Biotechnology and its Applications) explains this clearly: …
Inside the bacterium, Bt toxin exists as a harmless, inactive protoxin crystal; it becomes an active, cell-lysing toxin only after being solubilised and cleaved by the alkaline pH and proteases found in an insect's gut. …
Bt toxin is produced by the bacterium as an inactive protoxin; it only becomes an active, gut-cell-lysing toxin after being solubilised and processed by the alkaline pH and proteases of an insect's gut — an environment the bacterium's own cell doesn't experience internally.
Bacillus thuringiensis (Bt) produces crystal (Cry) proteins that act as insecticidal toxins. Inside the bacterium, this protein exists in an inactive protoxin form as crystals, which are harmless to the bacterium's own cells.
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Showing the 12 most recent of 35 on this concept.
- CBSE 2026Set V11 markMCQQ.The corn borers are controlled by the protein encoded by the gene(a) Cry I Ab(b) Cry II Ab(c) Cry IV Ab(d) Cry I Ac
›Reveal solutionSolution
The cryIAb protein controls the corn borer.
Different cry genes encode Bt toxins active against different insects. The cryIAb protein controls the corn borer, whereas cryIAc and cryIIAb control the cotton bollwor …
- CBSE 2026Set A1 markMCQQ.Why does Bt toxin protein not kill the Bacillus?(a) Bt toxin protein exists as inactive protoxins(b) Bt toxin protein is converted into harmless form by Bacillus(c) One enzyme of Bacillus inactivates Bt toxin protein(d) All of these
›Reveal solutionSolution
Bt toxin is stored as an inactive protoxin, so it does not kill Bacillus; the correct option is (a).
Bacillus thuringiensis produces the Bt toxin as inactive crystalline protoxin (a pro-toxin). Because it is inactive in this form, it does no harm to the bacterium itself. When an insect larva ingests the crystals, the alkaline pH of the insect's gut solubilises them, and gut proteases convert the inactive protoxin into the active toxin. The active toxin binds to the midgut epit …
- CBSE 2026Set BOTANY1 markMCQQ.Bacillus thuringiensis has _____ property.(a) fungicidal(b) bactericidal(c) insecticidal(d) herbicidal
›Reveal solutionSolution
Bacillus thuringiensis is a soil bacterium whose Cry (crystal) protein toxins kill specific insect pests, so its property is insecticidal.
Bacillus thuringiensis (Bt) forms protein crystals during sporulation that contain a toxin called delta endotoxin, encoded by cry genes. These crystals are inactive as produced, but once an insect larva ingests them, the alkaline pH of its gut solubilises the crystal, releasing the active toxin. The activated toxin binds to specific receptors on the midgut epithelial cells, creating pores that cause the cells to swell and lyse, killing the larva. Because each Cry protein binds a narrow range of insect receptors, a given Bt toxin (e.g. Cry I Ac, Cry II Ab) is specific to a particular group of insects (bollworms, corn borer, etc.) and harmless to non-target organisms, including humans. This natural insecticidal property is exploited biotec …
- CBSE 2026Set ANNUAL1 markQ.The protein that controls cotton bollworm is encoded by which gene?
›Reveal solutionSolution
The Bt cotton (bollworm-resistant) toxin protein is encoded by cry genes borrowed from the soil bacterium Bacillus thuringiensis.
Bacillus thuringiensis produces crystal (Cry) proteins that are toxic to specific insect larvae, including cotton bollworm. Genes encoding these toxins (e.g., cryIAc and cryIIAb, effective against cotton bollworms) were isolated from the bacterium and introduced into the cotton genome, producing Bt cotton. The Cry protein remains inactive in the bacterium/plant but is activated in the insect' …
- CBSE 2026Set ANNUAL1 markMCQQ.The Bt gene is isolated from which organism?(a) E.coli(b) Lactobacillus(c) Streptococcus cerevisae(d) Bacillus thuringiensis
›Reveal solutionSolution
The 'Bt' in Bt cotton/Bt crops refers to the bacterium Bacillus thuringiensis, whose cry genes encode insecticidal toxin proteins.
Bacillus thuringiensis is a Gram-positive soil bacterium that, during a particular phase of its growth, produces crystal (cry) proteins toxic to specific insect larvae (such as the bollworm affecting cotton). Different strains produce different cry proteins toxic to particular insect groups — e.g., cryIAc and cryIIAb control cotton bollworms, while cryIAb controls corn borer.
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- CBSE 2026Set ANNUAL1 markQ.CRY gene protects which type of crop by bollworms?
›Reveal solutionSolution
The cry gene (from Bacillus thuringiensis) codes for the Bt toxin protein that protects cotton crop from bollworm attack.
The soil bacterium Bacillus thuringiensis (Bt) produces crystal proteins, coded by cry genes (e.g. cryIAc, cryIIAb), that are toxic to specific insect larvae (in their inactive form, they are non-toxic pro-toxins). When a susceptible insect ingests the toxin, it is activated by the alkaline pH of the insect gut, binds to the gut epithelium, creates pores, causing cell swelling/lysis and death of the larva. Genetically engineering th …
- CBSE 2026Set ANNUAL1 markMCQQ.Assertion (A): Plants having genes from Bacillus thuringiensis are resistant to insects. Reason (R): The transgenic plants have receptors which convert protoxin into toxin.(a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.(b) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.(c) Assertion is true but Reason is false.(d) Assertion is false but Reason is true.
›Reveal solutionSolution
Bt cotton plants ARE insect-resistant (Assertion true), but the protoxin-to-toxin conversion happens inside the INSECT'S GUT (due to its alkaline pH), not via any 'receptor' in the plant — so the given Reason is false.
Assertion: Bt cotton is genetically engineered to carry a cry gene from the soil bacterium Bacillus thuringiensis, which codes for a Bt toxin (Cry) protein. This makes the plant resistant to bollworms and other lepidopteran/coleopteran/dipteran pests — true.
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- CBSE 2026Set ANNUAL1 markMCQQ.What causes the inactive form of Bt toxin (Protoxin) to get converted into it's active form in the body of an insect?(a) Temperature of the gut(b) Enzymes present in saliva(c) Alkaline pH of the gut(d) There is no specific reason
›Reveal solutionSolution
The inactive Bt protoxin is solubilised and activated by the alkaline pH of the insect's gut, so the toxin only becomes active after the insect eats it.
Bacillus thuringiensis makes the insecticidal Cry protein as an inactive protoxin (a crystal, Cry protein). When a susceptible insect (e.g. a bollworm larva) ingests it:
- The alkaline pH of the insect midgut dissolves the crystal, releasing the protoxin.
- Gut proteases then cleave it into the active toxin.
- The active toxin binds the midgut epithelium and creates pores, so the cells swell, lyse and the insect dies. …
- CBSE 2025Set F1 markMCQQ.What does Cry-toxin do?(a) Swelling of cells(b) Binding of epithelium of midgut(c) Lysis of intestinal cells(d) More than one of these
›Reveal solutionSolution
The Cry (Bt) toxin binds the midgut epithelium and causes cell swelling and lysis.
The Cry proteins of Bacillus thuringiensis are produced as inactive protoxin crystals. When an insect ingests them, the alkaline pH of its gut solubilises and activates the toxin. The activated toxin binds to the surface of the midgut epithelial cells and creates pores in the membrane, which makes the cells swell and lys …
- CBSE 2025Set ANNUAL1 markMCQQ.Bt cotton plants protected from which organism?(a) Nematode(b) Bacteria(c) Fungus(d) Insect
›Reveal solutionSolution
Bt cotton carries a gene from Bacillus thuringiensis that produces a crystal (Cry) protein toxic specifically to certain insects.
Bacillus thuringiensis produces Cry proteins that exist as inactive protoxins, but once ingested by a susceptible insect (e.g., cotton bollworms, corn borers), the alkaline gut environment converts the protoxin into its active, toxic form, which binds to the gut epithelium, causing cell lysis and killing the insect larva. Genes encoding these Cry tox …
- CBSE 2024Set ANNUAL1 markQ.What is Bt?
›Reveal solutionSolution
Bt refers to Bacillus thuringiensis, a soil bacterium whose toxin genes are used to make insect-resistant transgenic crops.
Bt (Bacillus thuringiensis) is a Gram-positive soil bacterium that produces crystal (cry) proteins toxic to specific insect larvae (e.g. Bt cotton uses cry genes toxic to bollworms). These cry genes, isolated from Bt, are genetically engineered into crop plants to conf …
- CBSE 2024Set E1 markMCQQ.At what pH insecticidal protein synthesized by Bacillus thuringiensis becomes active?(a) Acidic pH(b) Alkaline pH(c) Neutral pH(d) First acidic then alkaline pH
›Reveal solutionSolution
The inactive Bt crystal protoxin becomes an active toxin in the alkaline (basic) pH of the insect midgut.
Bacillus thuringiensis makes the insecticidal Cry protein as an inactive protoxin stored in crystals. When an insect eats it, the alkaline pH of the insect's gut dissolves the crystal and activates the protoxin into the active toxin. The activated toxin then binds to the midgut epithelium, creates pores, causes cell swell …
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