You Must Know  3 stages of cell communication Reception, transduction, & response  How G-protein-coupled receptors receive cell signals & start transduction  How receptor tyrosine kinase receive cell signals & start transduction  How a cell response in the nucleus turns on genes while in the cytoplasm it activates enzymes  What apoptosis means & why it is important to normal functioning of multicellular organisms

11.1  External signals are converted into responses within the cell  Animal cells communicate: by direct contact by secreting local regulators (growth factors or neurotransmitters)

 3 stages of cell signaling: 1) Reception: ○ The target cell’s detection of a signal molecule coming from outside the cell 2) Transduction: ○ Conversion of the signal to a form that can bring about a specific cellular response 3) Response: ○ Specific cellular response to the signal molecule

11.2  Reception: a signal molecule binds to a receptor protein, causing it to change shape  The binding between a signal molecule (LIGAND) & a RECEPTOR is highly specific  A change in the shape is the initial transduction of the signal

 Receptors are found in 2 places: 1) Intracellular ○ Inside membrane in the cytoplasm or nucleus ○ Signal molecule MUST cross the membrane (hydrophobic) : examples – steroids & NO 2) Plasma membrane receptors ○ Bind to water-soluble ligands

 3 types of plasma membrane receptors: 1) G-Protein-Coupled receptor 2) Receptor tyrosine kinase 3) Ligand-gated ion channels

 1) G-Protein-Coupled receptor Step 1 ○ The ligand (signaling molecule) has bound to the G- protein-coupled receptor ○ Causes a conformational change in the receptor so it can bind to an inactive G-protein ○ This causes a GTP to displace the GDP ○ This activates the G-protein Step 2 ○ The G protein binds to a specific enzyme & activates it ○ When activated, it triggers the next step in a pathway leading to cellular response ○ All shape changes are temporary ○ To continue, new molecules are required

 2) Receptor tyrosine kinase Step 1 ○ Shows binding of signal molecules to the receptors & formation of a dimer ○ Each tyrosine kinase adds a phosphate from an ATP Step 2 ○ Fully activated receptor protein that initiates a unique response ○ The ability of a single ligand to activate multiple responses is the difference between #2 & #1

 3) Ligand-gated ion channels Caused by specific signal molecules Opens & closes to regulate the flow of specific ions - Na + or Ca 2+

11.3  Transduction – Multiple molecular interactions relay signals from receptors to target molecules  Involve a phosphorylation cascade Usually a multistep pathway Leads to greater amplifying of the signal Enzymes called protein kinases phosphorylate & activate proteins Allows for a greater cellular response

 Not all components are proteins May include small, nonprotein water-soluble molecules or ions called SECOND MESSENGERS Calcium ions & cyclic AMP are examples Once activated, they can initiate a phosphorylation cascade

11.4  Response – Cell signaling leads to regulation of transcription or cytoplasmic activities  Many pathways regulate protein synthesis Done by turning specific genes on or off in the nucleus The final activated molecule in a signaling pathway functions as a transcription factor

 In the cytoplasm Signaling pathways often regulate the activity of proteins rather than their synthesis Example: final step in signaling pathway may affect the activity of enzymes or cause cytoskeleton rearrangement

11.5  Apoptosis – integrates multiple cell signaling pathways Controlled cell suicide The cell is systematically dismantled & digested Protects neighboring cells from damage that would occur if a dying cell leaked out its enzymes

 Apoptosis Triggered by signals that activate a cascade of suicide proteins in the cells In vertebrates, a normal part of development: ○ Normal nervous system ○ Operation of immune system ○ Normal morphogenesis of hands & feet in humans