Cell Communication Topics 4.1 through 4.2
Topic 4.1 Cell Communication
Importance of Cell Communication • Cell signaling allows cells to communicate with one another and coordinate the functions/activities of the organism • Communication pathways usually involve the plasma membrane
Cell Communication • Cells communicate with one another through cell-to-cell contact or from a distance via chemical signaling • Correct and appropriate signal transduction pathways are generally under strong selective pressure
Single-Celled Organisms (How Bacteria “Talk”, 18:14) • In single-celled organisms, cell communication pathways influence how the cell responds to its environment • Example: quorum sensing in bacteria
Multicellular Organisms • In multicellular organisms, cell communication pathways coordinate the activities within individual cells that support the function of the organism as a whole • Example: Epinephrine stimulation of glycogen breakdown in mammals
Cell Communication Cell-to-Cell Chemical Contact Signaling Local Plasmodesmata Regulators Immune Hormones System Cells
Examples of Cell-to-Cell Contact
Plasmodesmata • Microscopic channels which traverse the cell walls of plant cells and some algal cells, enabling transport and communication between them • Allow material to be transported from cell to cell (proteins, ions, etc.)
Immune System Cells • Some immune cells interact by cell-to-cell contact • Examples include antigen-presenting cells (APCs), helper T-cells , and killer T-cells
What are antigens and antibodies? • Antigen: any substance that causes the immune system to produce antibodies (“antibody - generating”) • Antibodies: Y-shaped proteins that are produced by B cells , identify and neutralize pathogens (antigens)
How Antibodies Work
Immune Cell: Antigen-Presenting Cell (Antigen Processing and Presentation Animation 2:30) • Antigen-presenting cells process and present antigens for recognition by certain lymphocytes such as T cells • Examples of antigen-presenting cells include macrophages , dendritic cells , and B cells
MHC Molecules
Immune Cell: Helper T • Helper T cells are the most important cells in adaptive immunity • Three ways in which helper T cells “help” the immune response: – stimulate B cells to produce more antibodies to the specific antigen – activate and increase the number of macrophages – activate cytotoxic (killer) T cells to kill infected cells
Helper T Cells
Immune Cell: Cytotoxic (Killer) T • Cytotoxic T cells target and destroy cells that are infected with pathogens
Examples of Chemical Signaling
Local Regulators • Local regulators are signaling molecules that only target cells in the vicinity of the signal emitting cell (short distance) • Examples of local regulators include neurotransmitters , morphogens , and quorum sensing
Local Regulator: Neurotransmitters
Local Regulator: Morphogens • Morphogens are a group of chemicals that are involved in the patterning of cells during embryonic development
Local Regulator: Quorum Sensing • Bacteria produce and release chemical signal molecules called autoinducers that increase in concentration as a function of cell density
Hormones • Hormones produced by endocrine cells travel long distances, through the blood, to reach their target cells • Examples of hormones include insulin and human growth hormone
Insulin Signaling Pathway (Insulin Signaling Animation 4:42) • Insulin is produced by the pancreas and targets cells of the liver, skeletal muscle and fat
Plant Hormones • Ethylene – gaseous hormone that ripens fruit • Auxin – chemical messenger that influences fruit development and cell growth
Topic 4.2 Introduction to Signal Transduction
Signal Transduction Pathway • The general pathway of signal transduction for cell-to-cell contact or chemical signaling is the same
Signal Transduction Pathway Professor Dave Explains Signal Transduction (6:32)
Signal Transduction • Signal transduction pathways link signal reception with a cellular response • Many signal transduction pathways include protein modification (shape change) and phosphorylation cascades
Stage 1: Signal Reception • Signaling begins with the recognition of a chemical messenger – a ligand - by a receptor protein in a target cell • The ligand has a complementary shape to the receptor
Ligand Types • The chemical messengers, or ligands, can be a peptide, a small chemical, or protein, in a specific one-to-one relationship with the receptor
Types of Receptor Proteins • Receptor proteins may be embedded within the plasma membrane or located within the cytoplasm or nucleus
Transmembrane Receptors • Embedded within the membrane, also known as extracellular receptors • Examples include GPCRs and ligand-gated ion channels
G protein-coupled receptors (GPCRs) • An example of a receptor protein found in eukaryotes, thousands of different types • The epinephrine signaling pathway involves a GPCR
Epinephrine Signaling Pathway Epinephrine Signaling Animation 3:10
Ligand-Gated Ion Channels
Intracellular Receptors • Located within the cytoplasm or nucleus • Targeted by lipid soluble ligands that are able to pass through the plasma membrane
Stage 2: Signal Transduction • Signal transduction is the process by which a signal is converted to a cellular response • After the ligand binds, the intracellular domain of a receptor protein changes shape, initiating transduction of the signal • Occurs in one step or a series of many steps
Signal Transduction • Signaling cascades relay signals from receptors to cell targets, often amplifying the incoming signals • Some signaling cascades utilize second messengers (such as cyclic AMP, or cAMP) during the signal transduction pathway to relay and amplify the intracellular signal
Enzymes Involved in Signal Transduction • Protein kinases – turn “on” or activate proteins by adding phosphates to the proteins ( phosphorylation cascade ) • Protein phosphatases – turn “off” or deactivate proteins and kinases by removing phosphates from the proteins ( dephosphorylation )
Second Messengers
Stage 3: Cellular Response • The signal transduction pathway initiates a change in cellular activity • Response occurs in the cytoplasm or nucleus
Cellular Response • Cellular responses include cell growth, secretion of molecules, or gene expression, among others • Most signaling pathways activate transcription factors that control the cellular response – Examples: Epidermal growth factor and testosterone
Epidermal Growth Factor • Signaling pathway that promotes cell growth and division
Testosterone Cellular response increases gene activity for proteins involved in: • Muscle mass • Bone growth • Body hair • Reproductive tissue
Transcription Factors • Transcription factors regulate cellular responses by: - Turning “on” genes Protein OR - Turning “off” genes No protein OR - Regulating the activity of a particular protein
Signal Transduction Review
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