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Signal Transduction in Cells

Definition

Signal transduction is the process by which a cell converts an external signal (chemical or physical) into a specific cellular response through a series of molecular events.

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Components of Signal Transduction
1. Signal (Ligand)
• Hormones, neurotransmitters, growth factors, cytokines
• Can be hydrophilic or hydrophobic
2. Receptor
• Protein that specifically binds the signal
• Located on the cell membrane or inside the cell
3. Transduction Pathway
• Series of biochemical reactions involving enzymes and second messengers
4. Cellular Response
• Changes in gene expression, enzyme activity, metabolism, or cell behavior

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Types of Cell Signaling
1. Autocrine – Cell signals itself
2. Paracrine – Signal acts on nearby cells
3. Endocrine – Hormones travel through blood to distant cells
4. Juxtacrine – Requires direct cell-to-cell contact

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Types of Receptors

1. Cell Surface Receptors
Used by hydrophilic (water-soluble) ligands

a) G-Protein Coupled Receptors (GPCRs)
• Activate G proteins (Gs, Gi, Gq)
• Example: Adrenaline, glucagon

b) Receptor Tyrosine Kinases (RTKs)
• Dimerize and autophosphorylate
• Example: Insulin receptor

c) Ion Channel Receptors
• Open/close ion channels
• Example: Neurotransmitter receptors

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2. Intracellular Receptors
Used by hydrophobic (lipid-soluble) ligands
• Located in cytoplasm or nucleus
• Ligand-receptor complex acts as a transcription factor
• Example: Steroid hormones, thyroid hormone

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Second Messengers

Small molecules that amplify the signal inside the cell.

Second Messenger Function
cAMP Activates protein kinase A
Ca²⁺ Regulates muscle contraction, secretion
IP₃ Releases Ca²⁺ from ER
DAG Activates protein kinase C
cGMP Vision and smooth muscle relaxation


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Signal Amplification
• One ligand → multiple activated proteins
• Ensures strong response even at low signal concentration

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Termination of Signal
• Ligand degradation
• Receptor desensitization
• GTP hydrolysis
• Removal of second messengers

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Importance of Signal Transduction
• Controls growth and development
• Regulates metabolism
• Maintains homeostasis
• Coordinates immune responses

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Disorders Related to Signal Transduction
• Cancer (mutated RTKs, Ras proteins)
• Diabetes (insulin signaling defects)
• Hormonal disorders

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Flow Chart (Summary)

Signal → Receptor → Second messengers → Protein kinases → Cellular response