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.

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.

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.

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.