Question
Explain the mechanism by which Bt toxin (Cry protein) kills insect larvae in Bt cotton plants. Why is this toxin harmless to humans?
(CBSE 2024 Board Exam — 3 marks)
Solution — Step by Step
Bacillus thuringiensis (Bt) is a soil bacterium that naturally produces insecticidal proteins called crystal proteins or Cry proteins (encoded by cry genes). Scientists isolated the cry gene and introduced it into cotton plants — that’s your Bt cotton.
Inside the bacterial cell, Cry proteins exist as protoxins — inactive crystalline inclusions. This is the key WHY behind the whole mechanism: the toxin must be activated before it can kill. This protects the bacterium’s own cell machinery.
When a bollworm larva chews on Bt cotton, it ingests these protoxins. The insect midgut is highly alkaline (pH ~10). This alkaline environment, combined with gut proteases, cleaves the protoxin into its active toxic form.
The activated Cry toxin binds to specific receptor proteins on the epithelial cells of the midgut wall. It then inserts itself into the cell membrane, creating pores (holes). The midgut cells swell due to osmotic imbalance and lyse — the gut is perforated.
With a perforated gut, the larva stops feeding within hours. The intestinal contents leak into the body cavity, causing septicaemia. The larva dies. The cotton plant is protected.
Why This Works
The entire mechanism depends on two conditions existing simultaneously: alkaline pH and specific receptor proteins. The insect gut happens to provide both. This is why Bt toxin is a remarkably targeted bioinsecticide.
Human digestive systems are acidic (pH ~2 in the stomach). The protoxin cannot be activated in our gut. Additionally, human intestinal cells do not carry the specific receptor proteins that Cry toxin binds to. Both conditions for activation and binding are absent — hence, complete safety for humans and other mammals.
This specificity is also why different Cry proteins target different insects. CryIAc and CryIIAb target Helicoverpa (bollworm) specifically. Cry proteins targeting beetles won’t affect lepidopteran larvae — the receptor proteins are different.
Alternative Method (Memory Framework for NEET)
For MCQs, NEET often tests the sequence of events. Memorise it as:
IABP — Ingestion → Alkaline activation → Binding to receptors → Pore formation
NEET 2023 asked which condition activates Bt toxin — the answer is alkaline pH of insect midgut. Don’t write “stomach acid” — that’s the trap option for mammals. The insect midgut is alkaline, not acidic.
Common Mistake
Many students write: “Bt toxin kills insects by acting as a poison that insects eat.” This gets zero marks in CBSE because it skips the mechanism entirely.
The examiner wants: protoxin → alkaline gut activation → receptor binding → pore formation → cell lysis → death. Each link in that chain is a potential 1-mark point. Write the mechanism, not the outcome.
Also — never say the toxin is “harmful to insects in general.” It is specific to certain insect orders. Bt cotton is designed against lepidopteran larvae (caterpillars). If the exam asks why it’s safe for bees and butterflies in the plant’s vicinity, the answer is receptor specificity.
Quick Revision Table
| Step | What Happens | Key Term |
|---|---|---|
| 1 | Bt gene expressed in plant cells | cry gene, transgenic plant |
| 2 | Protoxin ingested by larva | Inactive crystal protein |
| 3 | Alkaline gut activates it | pH ~10, midgut proteases |
| 4 | Binds epithelial receptors | Receptor-specific binding |
| 5 | Pores form, cells lyse | Gut perforation |
| 6 | Larva dies, plant survives | Bioinsecticide mechanism |
Final Answer Summary: Bt toxin is a protoxin activated by the alkaline pH of the insect midgut. The activated Cry protein binds to specific receptors on gut epithelial cells, creates pores, causes cell lysis and gut perforation, and kills the larva. It is harmless to humans because our acidic stomach cannot activate the protoxin, and our gut cells lack the required receptor proteins.