Understanding Cellular Communication Mechanisms

Sep 12, 2024

Lecture on Cellular Communication and Signal Transduction

Overview of Cells and Homeostasis

  • Human body comprises trillions of cells.
  • Cells form tissues and organ systems.
  • Tissues and organ systems maintain homeostasis.
  • Cells must communicate to maintain homeostasis.

Signal Transduction

  • Process by which cells respond to chemical messengers.
  • "Transduction" means converting energy from one form to another.
  • Target cells respond to messengers if appropriate receptor is present.
    • Example: 3 cells with different receptors, but only one binds to the messenger.

Chemical Messengers

  • Agonists: Activate receptors and bring about a response.
  • Antagonists: Bind but do not activate receptors, blocking them.

Categories of Messengers

  • Lipophilic Messengers
    • Poor plasma solubility; require carrier molecules.
    • Can diffuse directly across cell membranes.
    • Act as transcription factors, impacting DNA directly.
    • Effects take longer to manifest compared to hydrophilic messengers.
  • Hydrophilic Messengers
    • Cannot cross cell membranes by themselves.
    • Must bind to membrane-bound receptors.

Types of Receptors

1. Enzyme-Linked Receptors

  • Receptor linked to an enzyme, usually a kinase.
  • Messenger binding activates enzyme, phosphorylating substrate.

2. Channel-Linked Receptors

  • Transmembrane proteins with a binding site.
  • Binding causes conformational change, opening the channel.
  • Allows ion movement, resulting in a response.

3. G Protein-Linked Receptors

  • Involves a receptor linked to a G protein.
  • Binding causes alpha subunit dissociation, interacting with other structures.

G Protein-Linked Receptor Mechanisms

Fast vs. Slow Channels

  • Fast Channels (Ionotropic): Quick response (e.g., channel-linked receptor).
  • Slow Channels (Metabotropic): Longer to respond (e.g., G protein-linked receptor).

G Protein Activation

  • Transmembrane Protein Activation: Opens ion channels.
  • Activator Enzyme Activation: Produces secondary messengers.

Second Messengers

  • Generated by amplifier enzyme after first messenger interaction.
  • Examples:
    • Cyclic AMP (cAMP)
    • Cyclic GMP (cGMP)
    • Diacylglycerol (DAG)
    • Inositol Triphosphate (IP3)

Functions of Second Messengers

  • cAMP, cGMP, DAG: Activate protein kinases.
  • IP3: Opens channels in smooth ER for calcium release.
    • Calcium interacts with calmodulin to activate protein kinase.

Calcium as a Second Messenger

  • Some cells have calcium channels that respond to first messengers.
  • Calcium influx binds calmodulin, activating protein kinase and eliciting response.

These notes provide an overview of how cells communicate using chemical messengers and the diverse signal transduction mechanisms involved, detailing the roles of different types of messengers, receptors, and secondary messengers.

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