IV. Cell-to-Cell Communication and Signal Transduction

Cells communicate via signaling molecules that bind to receptor molecules on target cells.

1. Signaling Molecules

• Delivery Methods: Include endocrine (via bloodstream), paracrine (to nearby cells), and autocrine (to the releasing cell itself).
• Types:
  • Lipid-soluble molecules: Can penetrate the plasma membrane to bind with receptors in the cytoplasm or nucleus.
  • Hydrophilic molecules: Bind to cell-surface receptors.

1. Membrane Receptors

These are typically integral glycoproteins with extracellular, transmembrane, and intracellular domains that transduce signals.

• Functions:
  • Control membrane permeability by regulating ion channels.
  • Regulate the entry of molecules (e.g., cholesterol via LDL receptors).
  • Bind the extracellular matrix to the cytoskeleton via integrins.
  • Act as transducers through second messenger systems.
• Types of Membrane Receptors:
  • Channel-linked Receptors: Binding of a ligand opens or closes an ion channel gate (e.g., nicotinic acetylcholine receptors).
  • Catalytic Receptors: Single-pass proteins where the extracellular domain is a receptor and the cytoplasmic domain is a protein kinase (e.g., insulin and growth factor receptors).
  • G Protein-linked Receptors: Transmembrane proteins that interact with a heterotrimeric G protein (composed of α, β, and γ subunits) upon ligand binding. This activates intracellular second messengers like cyclic AMP (cAMP), Ca²⁺, and the inositol phospholipid pathway.