Question
Explain how hormones exert their effects on target cells using the lock and key mechanism. Distinguish between how steroid hormones and peptide hormones interact with their receptors.
Solution — Step by Step
Hormones are chemical messengers secreted by endocrine glands into the blood. However, not every cell in the body responds to every hormone. Only target cells — cells that possess the specific receptor for that hormone — are affected.
This selectivity is the lock and key mechanism: the hormone (key) fits only into its specific receptor (lock). A slightly different shape doesn’t fit. This explains why adrenaline affects the heart and liver but not the lens of the eye, even though adrenaline reaches every cell via the bloodstream.
Receptors are large protein molecules with a highly specific three-dimensional binding site. The hormone binds non-covalently (like enzyme-substrate interaction) with precise complementarity.
Hormones differ in their chemical nature. This determines where the receptor is located:
Peptide hormones and amines (e.g., insulin, glucagon, adrenaline, FSH, LH, TSH): Water-soluble. Cannot cross the hydrophobic lipid bilayer of the cell membrane. Their receptors are on the cell surface (plasma membrane).
Steroid hormones and thyroid hormones (e.g., cortisol, testosterone, oestrogen, aldosterone, T₃/T₄): Lipid-soluble. Easily diffuse through the cell membrane. Their receptors are inside the cell — either in the cytoplasm or the nucleus.
This difference in receptor location leads to fundamentally different mechanisms of action.
When a peptide hormone (e.g., adrenaline) binds to its surface receptor:
- The receptor (usually a G protein-coupled receptor, GPCR) undergoes a conformational change
- This activates an associated G protein, which activates adenylyl cyclase
- Adenylyl cyclase converts ATP → cAMP (cyclic AMP — the “second messenger”)
- cAMP activates protein kinase A
- Protein kinase A phosphorylates (adds phosphate to) specific enzymes, activating or inhibiting them
- This triggers the cellular response (e.g., in the liver, breakdown of glycogen → glucose released into blood)
The hormone itself (first messenger) never enters the cell. The cAMP is the second messenger that relays the signal inside the cell.
Amplification: One hormone molecule can activate many G proteins → many cAMP molecules → many enzyme activations → large cellular response. This is why nanomolar concentrations of hormones produce powerful effects.
When a steroid hormone (e.g., testosterone) enters the cell:
- It diffuses through the plasma membrane
- It binds to its intracellular receptor (in cytoplasm or nucleus)
- The hormone-receptor complex undergoes a conformational change
- The complex moves to the nucleus (or is already there)
- It binds to specific DNA sequences called Hormone Response Elements (HREs) — specific promoter regions of target genes
- This activates (or represses) transcription of those genes
- New mRNA is made → translated into new proteins → cellular response
Key difference from peptide hormones: Steroid hormones alter gene expression directly, producing long-lasting changes in protein synthesis. Peptide hormones work faster (seconds to minutes) but effects are shorter-lasting; steroid hormone effects develop over hours but are more prolonged.
The lock-and-key specificity of hormone-receptor interactions has several important consequences:
-
Hormone specificity: Two hormones can have opposite effects because they bind different receptors (e.g., insulin promotes glucose uptake; glucagon promotes glucose release — both “about glucose” but different responses).
-
Hormone analogues as drugs: Drugs can mimic or block hormones by fitting their receptors. Beta-blockers block adrenaline receptors (treating hypertension). Anabolic steroids mimic testosterone. Birth control pills mimic progesterone/oestrogen to prevent ovulation.
-
Receptor downregulation: Prolonged high hormone levels can cause cells to reduce the number of surface receptors — a form of desensitisation. This is why type 2 diabetes involves insulin resistance: cells downregulate insulin receptors in response to chronically high insulin levels.
Why This Works
The lock-and-key model works because receptor proteins have specific 3D shapes that create binding pockets complementary to the hormone’s shape. This is identical in principle to enzyme-substrate specificity.
The difference between surface receptors and intracellular receptors is a direct consequence of the hormone’s chemical nature (water-soluble vs lipid-soluble). Evolution matched receptor location to hormone chemistry: a water-soluble hormone that can’t enter the cell still communicates its message via a second messenger relay; a lipid-soluble hormone that can enter goes directly to the nucleus to change gene expression.
Alternative Method
For CBSE answers, a simplified description is:
“Hormones act on specific target organs/cells because those cells contain specific protein receptors that fit the hormone like a lock and key. On binding, the receptor changes shape and triggers a response inside the cell — either through a second messenger (for protein hormones) or by directly affecting gene expression (for steroid hormones).”
Common Mistake
Students often write “all hormones enter the cell to exert their effect.” This is only true for lipid-soluble hormones (steroids, thyroid hormones). Peptide and protein hormones (insulin, FSH, TSH, adrenaline) cannot cross the lipid bilayer and act through surface receptors + second messengers. Mixing up these two mechanisms is a common error in CBSE Class 11/12 and NEET.
NEET frequently asks about the second messenger. Common options in MCQs: cAMP, cGMP, IP₃ (inositol trisphosphate), DAG (diacylglycerol), Ca²⁺. The most common second messenger is cAMP (used by adrenaline, glucagon, TSH, FSH). Insulin is unusual — it uses a tyrosine kinase receptor, not a GPCR, and does not use cAMP.