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Chapter 8


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York University
BIOL 1000
Julie Clark

Tanya Sivamanoharan Fri, Dec, 2/11 Textbook Notes- Chapter# 8 CHAPTER# 8- CELL COMMUNICATION 8.1- Cell Communication: An Overview -communication critical for function+ survival of cell -commun. is responsible for controlled growth + develop. of animal -cell communicate with each other in 3 ways: -adjacent cells use direct channels of communication: -small molecules and ions are exchanged directly between cytoplasm of 2 cells -gap junctions direct channel of communication in adjacent animal cells, between the cytoplasm of both cells -gap junctions synchronize metabolic activities or electronic signals b/w cells in a tissue -plasmodesmata direct channel of commun. in adjacent plant cells -plasmodemata move small molecules, like plant hormones that regulate growth -cells communicate through specific contacts between cells: -some cells have molec. on surface that allow direct interaction with another cell -some cells use surface molecules to recognize only specific molecules, preventing pathogens/ viruses - cell adhesion molecules integral membrane proteins that allow cells to bind to other cells/ extracellular matrix -cells communicate through intracellular (between cell) chemical messengers: -most common cell communication -one cell (controlling cell), synthesizes specific signalling molecule to affect activity of another cell (target cell) -the target cell is not on cell synthesizing signalling molecules, but it is nearby/distant from the controlling cell -a controlling cell synthesizes/releases a signal molecule that causes a response of target cell -the target cell processes the signal from control cell in 3 diff. steps: -reception binding of a signal molec. with a specific receptor on target cell -signal molec. are usually peptides/steroids/amines -each molec. will have cellular receptors that bonds to specific molecules st -epinephrine was 1 amine discovered signalling messenger -some receptors are found within target cells, where the signal molec. freely passes through the cell and binds with receptor -membrane receptors found on cell surface, and short-lived -internal- receptor act directly on genome (nuclear receptors), activating specific genes, and long term -transduction changing signal into form necessary to cause cellular response -transduction normally occurs over several reactions (signalling cascade) or single step -response transduced signal from step 2 causes specific cellular response -response depends on the signal and receptors on the target cell -signal transduction all 3 steps of intracellular cell communication combined -signal transduction occurs by diff. mechanisms, depending on receptor type 8.2- Characteristics of Cell Communication Systems with Surface Receptors - cell communication systems based on surface receptors have 3 components: -the extracellular signal molecules released by controlling cells -surface receptors on target cells that receive the signals -internal response pathways triggered when receptors bind a signal 8.2a- Hormones and Neurotransmitters Are Extracellular Signal Molecules Recognized by Surface Receptors in Animals -surface receptors in mammals/vertebrates recognize + bind 2 main types of extracellular signals (hormones + neurotransmitters) -signal molec. released by control cells and enter fluids surrounding the cell (including blood) -hormones molecules (peptides/steroids), released by special gland cells (adrenal glands) -special class of peptides, the growth factors affects cell growth, divisions, + distribution -neurotransmitters molecules released by neurons that trigger other neurons or cells in the body -some neurotransmitters affect 1 or few cells immediately around neuron, releasing signal molecule, affecting many types of tissues -signal molecules released into the body’s circulation, they remain for certain time -signalling molecules are secreted or broken down by enzymes in target cell -removal process ensures that signalling molecules are active only until controlling cells secrete them 8.2b- Surface Receptors Are Integral Membrane Glycoproteins -surface receptors that recognize+ bind signal molecules are all glycoproteins -glycoproteins proteins with attached carbohydrate chains, integral membrane proteins extend throughout plasma membrane -signal-binding site of receptor extends from outer membrane surface, folded closely to fit signal molecule -the fit of signal molecule and signal-binding site of receptor is specific, so a particular receptor binds only one type or closely related signal molecules -signal molecules causes specific changes to receptor’s binding site and/or target cell -the change in binding site helps transmit signal through plasma membrane and activates cytoplasmic side of receptor -activate receptor than triggers 1 step of signal transduction pathway -animals cells have 100s to 1000s of surface receptors -membrane receptors for 1 particular hormone may exists in 100s-1000s 8.2c- The Signalling Molecule Bound by a Surface Receptor Triggers Response Pathways within the Cell -signal transcription pathways triggered by surface receptors common to all animal cell -parts of pathways found in protists, fungi, and plants -binding of signal molec. triggers cellular response without entering cell -experiments have shown: -a signal molec. produces no response if it is injected directly into cytoplasm -unrelated molec. that mimic structure of normal extracellular signal molec. can trigger full cellular response, when binded to recognition site of target cells receptor -the signal from signal transduction is relayed inside the cell by protein kinase -protein kinase-> enzymes that transfer a phosphate group from ATP to 1 or more sites on proteins -target proteins the phosphorylated proteins, b/c they are modified by signalling pathway -added phosphate stimulate or inhibit activity of target proteins, directly or indirectly affects cellular response -protein kinase cascade protein kinases act in chains to pass along a signal -each kinase is phosphorylated by previously phosphorylated kinase in protein kinase cascade -proteins bringing out cellular response may take part in reaction pathways, as enzymes of other cellular reactions, or be end targets of signal transduction pathways (transport proteins), or be proteins regulating gene transcription] -protein phosphatases a group of enzymes balance or reverse effects of protein kinases in signal transduction pathways -protein phosphatases removes phosphate groups from target proteins, which are active only when a surface receptor binds a signal molecule -most protein phosphatases are continuously active by removing phosphate groups from target proteins -protein phosphatases shuts off signal transduction pathway if signal molec. is no longer bonded to cell surface -signal transduction pathways involving surface receptors is amplification -amplification an increase in magnitude of each step in signal transduction pathway -amplification occurs b/c many proteins that carry out indiv. steps in pathway -the more enzyme-catalyzed steps in a responsive pathway, the greater amplification -amplification also occur in pathways involving internal receptors -receptors and bound signal molecules are removed from cell surface by endocytosis -receptors and bound signal molecules are degraded by lysosomes after entering cell OR receptors may separate from signal molecules and recycled to the cell surface and only the signal molecules are degraded -2 large families of surface receptors: -receptor tyrosine kinases -G protein- coupled receptors 8.3- Surface Receptors with Built-In Protein Kinase Activity: Receptor Tyrosine Kinases -in simplest signal transduction, the receptor itself has a protein kinase site at cytoplasmic end -this type of receptor is activated when a 2 receptor molec. each bind a signal molec. in the reception step, move together in the membrane and assemble into a pair called dimer -dimer assembly activates the receptor’s protein kinase -autophosphorylation adds phosphate groups to sites on receptor itself -target proteins recognize and bind to the phosphorylated sites on receptor and are then activated by phosphorylating themselves -the phosphorylations are to initiate the signal transduction pathway controlled by receptor -in autophosphorylation, phosphate groups are added to tyrosine amino acids on the receptor -the protein kinase activity of the activated receptors also adds phosphate groups to tyrosine in amino acid chains of target proteins -receptor tyrosine kinases specified phosphorylation (above) -cellular responses triggered by receptor tyrosine kinases are import. processes of animals -other receptor tyrosine kinases bind growth factors (epidermal, platelet- derived, nerve), all import. peptide hormones that regulate cell growth and division in animals -heredity defects in insulin receptors are responsible for certain diabetes, -the defects impair ability of receptor to bind insulin or block its ability to trigger cellular response -ultimately by defects, the cell is unresponsive to insulin and does not add sufficient glucose transporters to take up glucose 8.4- G Protein- Coupled Receptors -G-protein- coupled receptors 2 ndlarge family of surface receptors -G-protein-coupled receptors respond to a signal by activating an inner membrane protein called G protein -G-protein is closely associated with cytoplasmic end of the receptor -there are many G-proteins usually involved in recognizing and binding odour molecules as sense/smell -most receptors of G-proteins are large glycoproteins built from single polypeptide chain anchored in plasma membrane by 7 segments of amino acid chain across membrane (back/forth 7 times) -unlike receptor tyrosine kinase, these rece
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