Cell Signalling

Cell Signalling

1. Signal binds to receptor on target cell and induces an intracellular signal, which affects its behaviour

   • Signal transduction = transmission of molecular signals from a cell's exterior to its interior
   • Signal = usually a ligand

2. Types

   • Endocrine
     • Hormones produced by endocrine glands
       • Released into the blood
     • Long distance
   • Paracrine
     • Paracrine signals released locally
       • Affect neighbouring cells
     • Simple diffusion
   • Neuronal
     • Signals transmitted along axons
       • To neighbouring and remote target cells
     • Communication mediated by electrical signals

3. Receptors

   • Cell surface
     • Bind large, hydrophilic molecules
       • Unable to cross membrane
   • Intracellular
     • Bind small, hydrophobic molecules
       • Diffuse across membrane
     • E.g. Steroid hormones
       • When cortisol binds, the receptor changes shape and goes into the nucleus where it acts as a transcription factor
         • Activates transcription

4. Molecular switches

   • Intracellular signalling proteins act as switches
     • Turn pathways on and off
   • Phosphorylation
     • Molecules activated by addition of phosphate group
       • By a kinase
     • Molecules inactivated by removal of phosphate group
       • By a phosphatase
   • GTP-binding
     • Molecules activated when they bind GTP
     • Molecules inactivated when they bind GDP