1. Chapter 5.6+ Cellular Biology
Cell Communication
Chapter 5.6+
Cellular Biology

2. What you need to know! The 3 stages of cell communication.
How G-protein-coupled receptors receive cell signals and start transduction
How a cell signal is amplified by a phosphorylation cascade.
An example of a second messenger and its role in a signal transduction pathway.
How a cell response in the nucleus turns on genes, whereas in the cytoplasm it activates enzymes.
What apoptosis means and why it is important to normal functioning of multicellular organisms.

3. Local Signals
Adjacent cells have a variety of junctions: tight, desmosomes, gap
These adjacent cells communicate via gap junctions (plasmodesmata in plants), or cell-cell recognition (antigen and receptor protein)
Local regulators work to influence several localized cells
Neurotransmitters are released into the synapse and stimulate the target cell

4. Long-distance Signaling
Hormones released by the endocrine system travel through the blood and activate cells elsewhere in the body
Neurons can send electrical impulses from the brain to any part of the body

5. External Signals  Responses
Animal cells communicate by contact, secreting regulators (hormones), or neurotransmitters
There are three stages of cell signaling:
Reception: target cell’s detection of signal molecule
Transduction: converting the signal into a cellular response
Response: The specific cellular response to the signal

6. Signaling Overview

7. Reception
The ligand or signaling molecule can be sent throughout the entire body
Only cells with the appropriate receptor protein “hear” the signal
Most receptors are membrane proteins
Water soluble ligands that cannot cross the plasma membrane
Others receptors are intracellular
Lipid soluble ligands can cross membranes
Examples:
G Protein-Coupled Receptors
Receptor Tyrosine Kinases
Ligand-gated ion channels

8. G Protein-Coupled Receptors
Membrane imbedded protein receptor works with a G protein:
Ligand binds to a protein receptor causing a conformational change which binds the receptor protein to an inactive G protein
A GTP molecule now replaces a GDP molecule in the G protein which activates the G protein
The G protein leaves the receptor protein and binds to an enzyme at the allosteric site, this activates the enzyme (which in turn activates transduction)
The G protein hydrolyzes the GTP to GDP, this inactivates the G protein, which in turn detaches it from and deactivates the enzyme

9. G Protein-Coupled Receptors

10. Ligand Gated Ion Channels
Ion channels are simple gates that open when ligands bind to them
The ions trigger the cellular response upon entry
These are most common in the nervous system where ligands are neurotransmitters and the ions change the polarity of the cell

11. Intracellular Receptors
Intracellular receptors require ligands that can cross the plasma membrane
Steroids and small gas molecules qualify
The hormone-receptor complex move into the nuclease and act as transcrption factors
These transcription factors turn on certain genes by stimulating RNA polymerase to transcribe the gene(s)
The gene activation triggers the response
The receptor carries out the entire transduction

12. Transduction: Phosphorylation Cascade
The enzyme activated at the end of reception is a relay protein that phosphorylates (activates) protein kinases
The activated protein kinases in-turn activate other protein kinases (potentially amplifying the signal)
The cascade ends with the phosphorylation of a protein that triggers the cell’s response
Protein phosphatases (PP) remove the phosphate from the kinases and the final protein so they can all be reused

13. Transduction

14. Transduction: Second Messengers
Not all transduction molecules are proteins
cAMP is a converted ATP (NA) that cause a phosphorylation cascade

15. Response
Change in cellular function based on a signal transduction induced by a ligand
The final product of many transduction are transcription factors that activate genes
Other pathways simply activate proteins (i.e. enzymes) already in the cytoplasm
Signal transductions can lead to multiple responses from a single ligand

16. Signal Transduction Review

17. Apoptosis Advanced cell signaling which ends in scheduled cell suicide
Systematic dismantling and digestion of the cell
This prevents the digestive and metabolic enzymes from spilling out and damaging adjacent cells
Crucial in: brain development, the immune system, and morphogenesis of the fingers and toes