Question
Compare the mechanism of action of steroid hormones and peptide hormones. Why can steroid hormones act on genes directly while peptide hormones cannot?
Solution — Step by Step
Steroid hormones (cortisol, estrogen, testosterone, aldosterone) are lipid-soluble — they are derived from cholesterol. Because the cell membrane is a lipid bilayer, these hormones pass right through it.
Peptide hormones (insulin, glucagon, GH, ADH) are water-soluble proteins or polypeptides. They cannot cross the lipid bilayer. They need a different strategy.
This single difference — lipid-soluble vs water-soluble — determines the entire mechanism downstream.
Since steroid hormones cross the membrane, they bind to intracellular receptors (usually in the cytoplasm or nucleus).
The hormone-receptor complex acts as a transcription factor — it binds to specific DNA sequences called Hormone Response Elements (HREs) and directly activates or represses gene transcription.
The result: new mRNA is made, new proteins are synthesized. This is a slow but long-lasting response (hours to days).
Peptide hormones bind to cell-surface receptors (membrane receptors). The receptor activates an enzyme (often adenylyl cyclase) that converts ATP to cyclic AMP (cAMP) — the second messenger.
cAMP activates protein kinase A (PKA), which phosphorylates target proteins and triggers the cellular response.
This is a fast but short-lived response (seconds to minutes). No new gene expression needed — existing proteins are activated by phosphorylation.
graph TD
A[Hormone arrives] --> B{Lipid-soluble?}
B -->|Yes: Steroid| C[Crosses membrane]
C --> D[Binds intracellular receptor]
D --> E[Hormone-receptor complex]
E --> F[Binds DNA at HRE]
F --> G[Gene transcription]
G --> H[Slow, long-lasting response]
B -->|No: Peptide| I[Binds surface receptor]
I --> J[Activates adenylyl cyclase]
J --> K[ATP to cAMP]
K --> L[Activates protein kinase A]
L --> M[Phosphorylates proteins]
M --> N[Fast, short-lived response]Why This Works
The fundamental principle here is membrane permeability. The lipid bilayer is selectively permeable — lipid-soluble molecules pass freely, water-soluble ones do not. This forces peptide hormones to use a relay system (second messengers) to communicate their signal across the membrane.
Think of it this way: a steroid hormone is like someone who has a key to the office and walks straight to the boss’s desk (DNA). A peptide hormone is like a delivery person who rings the doorbell (surface receptor) and the message gets relayed inside through a chain of people (second messengers).
A quick comparison table for revision:
| Feature | Steroid Hormones | Peptide Hormones |
|---|---|---|
| Solubility | Lipid-soluble | Water-soluble |
| Receptor location | Intracellular | Cell surface |
| Mechanism | Direct gene regulation | Second messenger (cAMP) |
| Speed of action | Slow (hours-days) | Fast (seconds-minutes) |
| Duration | Long-lasting | Short-lived |
| Examples | Cortisol, estrogen, testosterone | Insulin, glucagon, ADH, GH |
Alternative Method
Some textbooks categorise hormone mechanisms into three groups, not two:
- Intracellular receptor mechanism — steroids, thyroid hormones (T3/T4), vitamin D
- Second messenger (cAMP) mechanism — most peptide hormones, catecholamines acting on beta-receptors
- Second messenger (IP3/DAG) mechanism — some hormones use phospholipase C instead of adenylyl cyclase, producing IP3 and DAG as second messengers (e.g., oxytocin, GnRH)
For NEET, the two-pathway model (steroid vs peptide) is sufficient. For JEE and higher-level biology, knowing about IP3/DAG adds an edge.
Common Mistake
Many students assume thyroid hormones (T3, T4) follow the peptide hormone pathway because the thyroid gland also produces peptide-like hormones. But T3 and T4 are amino acid derivatives that are lipid-soluble — they cross the membrane and bind intracellular (nuclear) receptors, just like steroid hormones. NEET has tested this distinction directly.