Answer: A hormone is a chemical messenger secreted by an endocrine gland directly into the blood, acting on specific target cells.
(a) Hormone: a chemical messenger secreted in very small amounts by ductless (endocrine) glands or cells directly into the blood, which carries it to distant target organs where it regulates specific physiological activities.
(b) Properties (any two):
- Secreted directly into blood (no ducts) and act at a distance from the site of secretion.
- Effective in very low concentration.
- Act only on specific target cells having specific receptors.
- Are not stored in large amounts; they are destroyed after action, so they are continuously secreted.
- Chemically they are proteins/peptides, amino acid derivatives or steroids.
(c) Mode of action through membrane-bound receptors (for protein/peptide and amine hormones that cannot enter the cell, e.g. adrenaline, glucagon, FSH):
[Flow diagram]
Hormone (first messenger) in blood
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Binds to specific receptor on the plasma membrane of the target cell → hormone–receptor complex
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Activates G-protein, which activates the enzyme adenylate cyclase on the inner side of the membrane
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ATP → cyclic AMP (cAMP), the second messenger
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cAMP activates protein kinases
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Protein kinases phosphorylate enzymes/proteins, altering their activity
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Physiological response (e.g. glycogen breakdown, secretion, change in metabolism)
