Signaling Molecules | Zoology Optional Notes for UPSC PDF Download

Types of Signals

Intercellular Signaling

• Localized signals between nearby cells.
• Quick responses with short durations.
• Ligands diffuse through the extracellular matrix.
• Examples include synaptic signals in nerve cells.

• Signals from distant cells.
• Involves hormones that travel through the bloodstream.
• Slower response but longer-lasting effects.
• Produced by endocrine cells in glands like the thyroid and pituitary.

• Cells produce signals that bind to their own receptors.
• Ensures proper tissue development and regulates processes like inflammation.

• In animals (gap junctions) and plants (plasmodesmata).
• Small signaling molecules diffuse between neighboring cells.
• Enables coordinated responses among a group of cells.

Signaling Molecules

Small Hydrophobic Ligands

• Steroid hormones (e.g., estradiol, testosterone, cholesterol).
• Diffuse through the plasma membrane and interact with internal receptors.
• Soluble in blood when bound to carrier proteins.

Water-Soluble Ligands

• Polar molecules that bind to extracellular domains of cell-surface receptors.
• Diverse, including small molecules, peptides, and proteins.

Other Ligands

• Nitric oxide (NO) is a gas that can diffuse directly across the membrane.
• Functions over short distances and regulates processes like smooth muscle relaxation.

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Signaling Receptors

Internal Receptors

• Found in the cytoplasm, respond to hydrophobic ligands.
• Act as regulators of mRNA synthesis and gene expression.
• Example: Steroid hormone receptors.

Cell-Surface Receptors (Transmembrane Receptors)

Ion Channel-Linked Receptors

• Ligand binding opens a channel for specific ion passage.
• Have extensive membrane-spanning regions.

G-Protein-Linked Receptors

• Ligand binding activates a G-protein.
• G-protein then interacts with ion channels or enzymes in the membrane.
• Involved in cyclic series of events.

Enzyme-Linked Receptors

• Associated with an enzyme in the intracellular domain.
• Ligand binding activates the enzyme, triggering intracellular signaling.
• Example: Tyrosine kinase receptors.

Virus Recognition of Host

• Viruses often bind to cell-surface receptors on host cells.
• Chemical differences in receptors among hosts determine viral host specificity.
• Viral reproduction can lead to changes in binding properties, allowing the virus to adapt to new hosts.

Role of Cell-Surface Receptors in Diseases

• Errors in cell-surface receptor proteins linked to diseases like hypertension, asthma, heart disease, and cancer.
• Bacterial toxins can disrupt G-protein-linked receptor function, causing diseases like pertussis, botulism, and cholera.

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Example: Cholera and G-Protein-Linked Receptors

• Vibrio cholerae produces choleragen, which modifies a G-protein, leading to continuous activation of chloride channels.
• Results in fluid loss, dehydration, and potentially fatal consequences.

Enzyme-Linked Receptor Example: Tyrosine Kinase Receptor

• Ligand binding leads to dimerization of receptors.
• Phosphorylation of tyrosine residues on the intracellular domain.
• Transmission of signals within the cytoplasm for cellular responses.

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Conclusion

Understanding the diverse types of signals, signaling molecules, and receptors is crucial for comprehending the intricate communication systems within multicellular organisms and the implications for health and disease.