Signal Transduction and Second Messengers

Module 4: Signal Transduction and Second Messengers

By the end of this module, you should be able to:

  1. Describe the different ways cells can communicate with each other.

  • Gap Junctions: connexin proteins make little ion-carrying tunnels between cells called connexons, and control the rapid and direct ion passage 

  • Cell-to-cell binding: surface molecules bind together 

  • Extracellular messengers:

  1. Hormones: Extracellular messengers carried by blood

  2. Neurotransmitters: released from a neuron into the blood where it then acts similar to hormones

  3. Local mediators: Released by cells and act locally on other cells 

    1. Autocrine = shuts off signal

    2. Paracrine = turns on signal

3 types: 

  1. Cytokine = cell growth

  2. Nitric oxide = causes relaxation of blood vessel (dilation)

  3. Eicosanoids = released after stimuli

    1. Prostaglines

    2. Leukotrienes

    3. Thromboxane



  1. Compare and contrast the signal transduction mechanisms for lipophilic and lipophobic messengers.

Lipophilic messengers

Lipophobic messengers

  • Likes water 🙂 = can diffuse

  • Released upon synthesis

  • Regulated by regulating rate of synthesis

  • Doesnt like water 🙁 = uses exocytosis

  • Stored in vesicles

  • Regulated by regulating rate of exocytosis


  1. Describe how extracellular messengers reach their target cells.

Depends on type: 

  • Hormones: through the blood 

  • Neurotransmitters: through synapse 


  1. Discuss the properties of messenger-receptor binding.

    • Receptor: protein on the cell, accepts or denies 

    • There can be one receptor for many messengers 

    • There can be many receptors that accept the same messenger 

      1. Which receptor uptakes it depends on the affinity

*ones with higher affinity to the messenger will take priority 

  1. Explain how receptors are regulated.

Upregulations: messenger deficiency = the cells may increase the number of receptors. 

Downregulation: excess messengers = cell receptor number decreases 

  1. Identify the components of a signal transduction pathway and explain how the pathways are amplified.

  • The sequence of events between receptor activation & the cell’s response

  1. Extracellular (first) messenger binds to receptor 

    1. Open or close channel

    2. Activate intracellular (second) messenger system  

      1. G protein activated 

      2. Adenyl cyclase activated 

      3. Generates hundreds of cAMP molecules 

      4. Activates protein kinase A

      5. Phosphorylated hundreds of proteins 

  • (each step multiplies the responders) 

  • Terminated by protein degradation, diffusion of messenger, removal of calcium from cytosol and engulfing the complex 


  1. Describe the following functional classes of receptors:

    • Channel-linked receptors: Simplest receptors. Messenger binds → channel opens (ex: Ca++ channels)

  • Enzyme-linked receptors: Messenger binds → receptor activates attached enzyme → catalyzes chemical reaction (tyrosine kinase… think growth factors)

  • G protein-linked receptors: Largest category. G protein binds to a GDP or GTP. “switch”(receptor communicating to ion channels or enzymes).

    1. Independent of a second messenger. 

    2. Direct communication between G proteins to open OR close ion channels. (slow gating) 

    3. Indirect communication - G protein cAMP → phosphorylate plasma membrane channel = open/close