Mechanism of Hormone Action – Easy NEET Notes
Introduction
Hormones regulate different physiological activities of the body.
Based on their chemical nature, hormones are divided into four groups.
Types of Hormones
1. Peptide / Polypeptide / Protein Hormones
Examples
- Insulin
- Glucagon
- Pituitary hormones
- Hypothalamic hormones
Important Features
- Made up of amino acids.
- Water soluble.
- Cannot pass through cell membrane.
- Act through membrane-bound receptors.
NEET Point
These hormones use second messengers like:
- cAMP
- IP₃
- Ca²⁺
2. Steroid Hormones
Examples
- Cortisol
- Testosterone
- Estradiol
- Progesterone
Important Features
- Lipid soluble.
- Can easily pass through plasma membrane.
- Receptors are inside the cell.
NEET Point
They directly influence:
- Gene expression
- Protein synthesis
3. Iodothyronines (Thyroid Hormones)
Examples
- T₃ (Triiodothyronine)
- T₄ (Thyroxine)
Important Features
- Derived from amino acid tyrosine.
- Regulate metabolism and growth.
4. Amino Acid Derivative Hormones
Example
- Epinephrine (Adrenaline)
Important Features
- Derived from amino acids.
- Usually act quickly.
Mechanism of Hormone Action
Hormones act in two major ways:
- Through membrane receptors
- Through intracellular receptors
A. Mechanism of Protein Hormone Action
(Example: FSH)
Step-by-Step Explanation
Step 1: Hormone reaches target cell
- Protein hormone travels through blood.
- Reaches target cell membrane.
Step 2: Hormone binds receptor
- Hormone cannot enter the cell.
- It binds to receptor present on plasma membrane.
Step 3: Receptor gets activated
- Hormone-receptor binding activates receptor.
Step 4: Formation of second messenger
Second messengers formed are:
- cAMP
- Ca²⁺
Step 5: Biochemical responses occur
Inside the cell:
- Enzymes become active.
- Cellular metabolism changes.
Step 6: Physiological response produced
Final body response occurs.
Example
- Ovarian growth due to FSH.
Important NEET Concept → Second Messenger
What is a Second Messenger?
A molecule formed inside the cell after hormone binds receptor.
Function
- Transfers signal inside the cell.
- Produces rapid response.
Examples
- cAMP
- Ca²⁺
- IP₃
Key Features of Protein Hormones
| Feature | Protein Hormones |
|---|---|
| Solubility | Water soluble |
| Receptor location | Cell membrane |
| Cell entry | Cannot enter |
| Action speed | Fast |
| Mechanism | Second messenger system |
B. Mechanism of Steroid Hormone Action
(Example: Estrogen)
Step-by-Step Explanation
Step 1: Hormone enters target cell
- Steroid hormones are lipid soluble.
- Easily diffuse through plasma membrane.
Step 2: Hormone binds intracellular receptor
- Receptor is present in:
- Cytoplasm or
- Nucleus
Step 3: Hormone-receptor complex formed
- Hormone combines with receptor.
Step 4: Complex enters nucleus
- Hormone-receptor complex binds DNA/genome.
Step 5: Gene activation occurs
- Specific genes become active.
Step 6: mRNA synthesis
- Transcription occurs.
- mRNA is produced.
Step 7: Protein synthesis
- Proteins are synthesized.
Step 8: Physiological response occurs
Results include:
- Tissue growth
- Differentiation
Key Features of Steroid Hormones
| Feature | Steroid Hormones |
|---|---|
| Solubility | Lipid soluble |
| Receptor location | Inside cell |
| Cell entry | Easily enters |
| Action speed | Slow but long-lasting |
| Mechanism | Gene activation |
Difference Between Protein & Steroid Hormones
| Character | Protein Hormone | Steroid Hormone |
|---|---|---|
| Solubility | Water soluble | Lipid soluble |
| Receptor | Membrane receptor | Intracellular receptor |
| Cell entry | Cannot enter | Can enter |
| Second messenger | Present | Absent |
| Main effect | Enzyme activation | Gene activation |
| Speed | Fast | Slow |
| Duration | Short | Long-lasting |
Flowchart for Revision
Protein Hormone
Hormone → Membrane receptor → Second messenger → Enzyme activation → Physiological response
Steroid Hormone
Hormone → Intracellular receptor → Gene activation → mRNA → Protein synthesis → Physiological response
NCERT Keywords for NEET
- Membrane-bound receptor
- Intracellular receptor
- Second messenger
- cAMP
- Gene expression
- Hormone-receptor complex
- Physiological response
- Biochemical response
NEET Important One-Liners
- Protein hormones act through second messengers.
- Steroid hormones regulate gene expression.
- cAMP is a common second messenger.
- Steroid hormones are lipid soluble.
- Protein hormones cannot cross plasma membrane.
- Steroid hormone receptors are intracellular.
- Protein hormones produce rapid responses.
- Steroid hormones cause long-lasting effects.
 |
| Diagram showing the mechanism of action of protein hormones and steroid hormones in target cells. |
CBSE Class 11 Biology Questions
Chapter: Mechanism of Hormone Action
1. MCQs (Multiple Choice Questions)
1. Hormones that cannot enter target cells are:
A. Steroid hormones
B. Protein hormones
C. Thyroid hormones
D. Estrogen
Answer:
B. Protein hormones
2. Which of the following acts as a second messenger?
A. DNA
B. RNA
C. cAMP
D. Protein
Answer:
C. cAMP
3. Steroid hormones bind to receptors present in:
A. Cell wall
B. Cell membrane
C. Cytoplasm/Nucleus
D. Ribosome
Answer:
C. Cytoplasm/Nucleus
4. Which hormone is lipid soluble?
A. Insulin
B. Glucagon
C. Estrogen
D. FSH
Answer:
C. Estrogen
5. Protein hormones act through:
A. Genome activation
B. Second messenger system
C. Protein synthesis only
D. DNA replication
Answer:
B. Second messenger system
6. Which of the following is a steroid hormone?
A. Insulin
B. Glucagon
C. Testosterone
D. Epinephrine
Answer:
C. Testosterone
7. Hormone-receptor complex in steroid hormones mainly affects:
A. Cell wall
B. Gene expression
C. Lysosome
D. Centriole
Answer:
B. Gene expression
8. Example of amino acid derivative hormone:
A. Cortisol
B. Insulin
C. Epinephrine
D. Progesterone
Answer:
C. Epinephrine
9. Protein hormones are generally:
A. Fat soluble
B. Water soluble
C. Insoluble
D. Gaseous
Answer:
B. Water soluble
10. Steroid hormones produce:
A. Immediate short effects
B. Long-lasting effects
C. No effect
D. Mechanical response
Answer:
B. Long-lasting effects
2. Very Short Answer Questions (1 Mark)
Q1. What are second messengers?
Answer:
Molecules formed inside the cell that transmit hormone signals are called second messengers.
Q2. Name one steroid hormone.
Answer:
Testosterone
Q3. Where are receptors for protein hormones located?
Answer:
On the cell membrane.
Q4. Give one example of a second messenger.
Answer:
cAMP
Q5. Which hormones regulate gene expression?
Answer:
Steroid hormones
3. Short Answer Questions (2–3 Marks)
Q1. Why can steroid hormones easily enter target cells?
Answer:
Steroid hormones are lipid soluble. The plasma membrane is made of lipids, so steroid hormones can easily diffuse through it and enter the target cell.
Q2. Explain the role of second messengers.
Answer:
Second messengers transfer signals from the hormone-receptor complex into the cell. They activate enzymes and produce biochemical and physiological responses.
Q3. Differentiate between protein and steroid hormones.
| Protein Hormones | Steroid Hormones |
|---|---|
| Water soluble | Lipid soluble |
| Cannot enter cell | Can enter cell |
| Receptor on membrane | Receptor inside cell |
| Use second messenger | Direct gene activation |
Q4. What happens after hormone-receptor complex formation in steroid hormones?
Answer:
The hormone-receptor complex enters the nucleus and binds with DNA. This activates genes and leads to protein synthesis.
4. Long Answer Questions (5 Marks)
Q1. Explain the mechanism of action of protein hormones.
Answer:
- Protein hormones travel through blood to target cells.
- They cannot enter the cell because they are water soluble.
- They bind to receptors present on the cell membrane.
- This activates second messengers like cAMP or Ca²⁺.
- Second messengers activate enzymes inside the cell.
- Biochemical reactions occur.
- Finally physiological responses are produced.
Example:
FSH stimulates ovarian growth.
Q2. Describe the mechanism of action of steroid hormones.
Answer:
- Steroid hormones are lipid soluble and diffuse through the plasma membrane.
- They bind with intracellular receptors in cytoplasm or nucleus.
- Hormone-receptor complex is formed.
- This complex binds to DNA in the nucleus.
- Genes become activated.
- mRNA is synthesized.
- Proteins are formed.
- Physiological effects like growth and differentiation occur.
5. Assertion and Reason Questions
Q1.
Assertion (A):
Protein hormones cannot enter target cells.
Reason (R):
Protein hormones are water soluble.
Answer:
Both A and R are true and R is the correct explanation of A.
Q2.
Assertion (A):
Steroid hormones act faster than protein hormones.
Reason (R):
Steroid hormones activate gene expression.
Answer:
Assertion is false but Reason is true.
Q3.
Assertion (A):
cAMP acts as a second messenger.
Reason (R):
It transfers signals inside the cell.
Answer:
Both A and R are true and R is the correct explanation of A.
6. Fill in the Blanks
- Protein hormones are ______ soluble.
Answer: water
- Steroid hormones bind to ______ receptors.
Answer: intracellular
- cAMP acts as a ______ messenger.
Answer: second
- Estrogen is a ______ hormone.
Answer: steroid
- Protein hormones act through ______ receptors.
Answer: membrane-bound
7. Match the Column
| Column A | Column B |
|---|---|
| 1. Insulin | a. Steroid hormone |
| 2. Estrogen | b. Second messenger |
| 3. cAMP | c. Protein hormone |
| 4. Testosterone | d. Lipid soluble hormone |
Answers:
1 → c
2 → a
3 → b
4 → d
8. Statement-Based Questions
Q1. State whether True or False.
i. Steroid hormones use second messenger system.
Answer: False
ii. Protein hormones bind membrane receptors.
Answer: True
iii. Estrogen can cross plasma membrane.
Answer: True
iv. cAMP is produced inside the target cell.
Answer: True
9. Case Study Questions
Case Study
A hormone reaches its target cell and binds to receptors present on the cell membrane. After binding, cAMP is produced inside the cell, leading to enzyme activation and physiological response.
Q1. Identify the type of hormone.
Answer:
Protein hormone
Q2. Name the second messenger involved.
Answer:
cAMP
Q3. Why cannot this hormone enter the cell?
Answer:
Because it is water soluble.
Q4. Where are its receptors located?
Answer:
On the plasma membrane.
10. Important NEET/CBSE One-Liners
- Protein hormones act through second messengers.
- Steroid hormones regulate gene expression.
- cAMP is a common second messenger.
- Steroid hormones are lipid soluble.
- Membrane receptors are used by protein hormones.
- Intracellular receptors are used by steroid hormones.
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