Lecture on Chemical Signaling and Receptors

Overview of Chemical Signaling

• Higher level topic: C2.1 Chemical Signaling
• Theme C: Interaction and Interdependence
• Cells communicate through signaling molecules and receptors

Signaling Molecules and Receptors

• Signaling Molecules: Also known as ligands
  • Act as signaling molecules
  • Must bind with protein receptors
  • Protein receptors are embedded in cell membranes
  • Binding causes a change in receptor shape, leading to cellular responses
• Differences from Enzymes:
  • Ligands remain unchanged
  • Can stay bound to receptors for a longer duration compared to enzyme-substrate interactions

Quorum Sensing

• Describes a change in behavior of a colony when population density reaches a certain threshold
• Example: Vibrio fischeri bacteria and bioluminescence in bobtail squid

Types of Chemical Signaling Molecules

1. Hormones
   • Come from endocrine glands, secreted directly into bloodstream
   • Only affect target cells with specific receptors
   • Long effect time
   • Examples: Insulin, Glucagon, Sex hormones
2. Neurotransmitters
   • Travel between two neurons in a synapse
   • Short effect time
   • Examples: Dopamine, Serotonin, Acetylcholine
3. Cytokines
   • Passed between nearby cells
   • Example: Interferons, Erythropoietin
4. Calcium Ions
   • Not biological molecules but can cause effects with specific receptors
   • Role in muscle contraction and neurotransmitter release

Hormones vs. Neurotransmitters

• Evolved independently, have different forms and functions
• Both small, soluble in water, and compatible with receptors
• Hormones: Act over large distances
• Neurotransmitters: Localized effect between neurons

Categorizing Signaling Molecules

• Intracellular Receptors:
  • For signaling molecules that can enter the cell
  • Receptors are hydrophilic
  • Example: Steroid hormones
• Transmembrane Receptors:
  • For signaling molecules that cannot enter the cell
  • Receptors have hydrophilic and hydrophobic regions
  • Example: Non-steroid hormones (e.g., Insulin)

Conclusion

• Transmembrane receptors are for molecules that cannot enter the cell
• Intracellular receptors are for molecules that can enter the cell
• Importance of receptor location and amino acid composition in function of signaling pathways