Why are membrane receptors required for insulin signalling?

Question

BlackTom AI · Accepted Answer

Insulin is a peptide hormone composed of amino acids and is therefore polar and relatively large, so it cannot diffuse through the hydrophobic lipid bilayer of the plasma membrane; this is why cell-surface (membrane) receptors are required.

1. Relevant knowledge points and concepts
- Cell membranes have a hydrophobic lipid bilayer core that restricts passage of large or polar molecules.
- Small nonpolar (lipophilic) molecules (e.g., steroid hormones) can cross membranes and act on intracellular receptors.
- Peptide hormones (like insulin) are hydrophilic and bind to transmembrane receptors that initiate intracellular signal transduction cascades (e.g., insulin receptor tyrosine kinase → IRS → PI3K → Akt → GLUT4 translocation).

2. Step-by-step reasoning
- Insulin’s structure: it is a peptide of two chains linked by disulfide bonds (about 51 amino acids, several kDa). Its peptide nature makes it polar and not membrane-permeable.
- A polar molecule cannot passively diffuse through the hydrophobic interior of the lipid bilayer.
- Therefore, target cells detect insulin using membrane-bound receptors. Binding to the extracellular domain of the insulin receptor triggers receptor autophosphorylation and downstream signaling inside the cell.
- This signaling leads to effects such as increased glucose uptake (via GLUT4 translocation), glycogen synthesis, and changes in gene expression without insulin entering the cell.

3. Intermediate “steps” (conceptual)
- Insulin (extracellular) → binds insulin receptor (transmembrane) → receptor kinase activity → intracellular signaling cascade → physiological response.

4. Why the correct answer is correct
- “Insulin is a large, polar molecule” directly explains why it cannot cross the lipid bilayer and therefore requires membrane receptors to transmit its signal into the cell.

5. Why other options are incorrect
- “Insulin is hydrophobic.” Wrong: insulin is hydrophilic/polar, not hydrophobic; hydrophobic hormones do not need membrane receptors to enter cells.
- “Insulin enters mitochondria.” Wrong: insulin acts at the plasma membrane by receptor binding; it does not need to enter mitochondria for its primary signaling.
- “Insulin binds DNA.” Wrong: insulin does not directly bind DNA; it influences gene expression indirectly via signaling cascades and transcription factors.

6. Conclusion
The correct answer is “Insulin is a large, polar molecule,” which matches the provided answer.

Why are membrane receptors required for insulin signalling?Single choice

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