Cell Communication

Signal Transduction Pathways Signal recognized by receptor molecule, then converted to response ligand: the signaling molecule receptor protein: the molecule to which the receptor binds may be on the plasma membrane or within the cell

Signaling Junctions directly connect cytoplasms (including plasmodesmata) Direct cell-cell contact/recognition Cells can identify each other by cell surface markers. major histocompatibility complex (MHC) proteins are used by cells to distinguish “self” from “non-self”

Local Signaling Messenger molecules secreted Growth factors example of paracrine signaling Synaptic signaling uses neurotransmitters

Long-distance Signaling Hormones released into blood Can move through cells/air, too Electrical signals travel through nerve cells

1. Reception Chemical signal binds to receptor Receptors can turn on/off genes Ex: ion-channel receptors

Receptor Types 3 subclasses of membrane receptors: 1. Channel linked receptors – ion channel that opens in response to a ligand 2. G protein-coupled receptor – a G-protein (bound to GTP) assists in transmitting the signal 3. Enzymatic receptors (receptor tyrosine kinases) – receptor is an enzyme that is activated by the ligand

2. Transduction Receptor changes shape Kinases phosphorylate proteins “phosphorylation cascade” Phosphatases dephosphorylate

Example: Cyclic AMP “second messenger” Enzyme converts ATP to cAMP in response to signal cAMP activates kinase, which causes cellular responses

3. Response Regulate activity/synthesis of enzymes Transcription factors Signal “amplified” after each step

Cell Suicide Apoptosis- programmed cell death Discards of old and/or damaged cells Uses multiple signaling pathways