ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Review Questions and Answers		AP BIOLOGY: Chapter Seven Outline CELLS OF MULTICELLULAR ORGANISMS TOUCH AND COMMUNICATE WITH EACH OTHER Send and Receive Chemical Signals Coordinate Activities to Behave as Group, Not Individuals RECEPTOR PROTEINS AND SIGNALING BETWEEN CELLS Use a Variety of Molecules Attached to cell surface Released from cell Cells Choose to What Signal to Respond Accomplished by receptor proteins Have three dimensional shape fig 7.1 Signal molecule binds to receptor if correct shape Induces shape change in receptor protein Results in response by cell Characterizing small number of receptor proteins difficult Monoclonal antibodies used to bind to particular receptors Genetic engineering identifies and sequences receptor genes TYPES OF CELL SIGNALING fig 7.2 Direct Contact Molecules of plasma membrane bind in specific ways Example: cell interaction in early development fig 7.2a Paracrine Signaling Molecules released by cells and taken up by neighboring cells Paracrine signals are short-lived with local effects fig 7.2b Plays important role in early development Endocrine Signaling Released signal molecule collected and distributed via blood stream Molecules called hormones, signaling is endocrine fig 7.2a Used by plants and animals Synaptic Signaling Nervous systems neurons produce neurotransmitters Released from neurons close to the target cells, persist briefly fig 7.2d Site of release called chemical synapse MECHANISMS OF CELL SIGNALING: INTRACELLULAR RECEPTORS Intracellular Receptors Pass Through Target Cell Plasma Membrane fig 7.3 Function of Intracellular Receptors Act as enzymes Example: Nitrous oxide (NO) gas Binds to guanylyl cyclase in neighboring cells Activated enzyme catalyzes synthesis of cyclic GMP NO initiated response relaxes smooth muscle surrounding blood vessels Blood vessels expand, increasing blood flow Regulate gene transcription Include similarly structured steroid hormone receptors fig 7.3 Genes may be evolved from single ancestral gene Grouped in intracellular receptor superfamily Each receptor has DNA binding site occupied by inhibitory protein Signal molecule binding to another site on receptor releases inhibitor Receptor binds then to DNA to activate or suppress gene MECHANISMS OF CELL SIGNALING: CELL SURFACE RECEPTORS Cell Surface Receptors Cannot Diffuse Through Cell Membranes fig 7.4 Signals bind to receptor proteins on cell surface Convert extracellular signal to intracellular signal Produces change in cell's cytoplasm Include three superfamilies Chemically Gated Ion Channels Receptor is multi-pass transmembrane protein fig 7.4a Winds across plasma membrane several times Center of protein forms a pore through which ions can pass Ion channel opens or closes when neurotransmitter binds to protein Called chemical gating Type of ion determined by three dimensional shape of ion channel Enzymatic Receptors Acts as or are directly linked to enzymes fig 7.4b Binding between signal molecule and receptor activates the enzyme Most are protein kinases, add phosphate groups to proteins Single pass transmembrane protein Signal molecule binds outside cell Portion initiating enzyme activity is in cell's cytoplasm G Protein-Linked Receptors GTP binding G protein assists membrane-bound enzymes or ion channels fig 7.4c Largest superfamily composed of seven-pass transmembrane protein fig 7.4d Signal binding causes G protein to bind GTP and become activated Activated protein diffuses away from receptor to begin actions G proteins involved in mechanism of half of medicines currently in use INITIATING THE INTRACELLULAR SIGNAL Second Messengers Relay Message Also called intracellular mediators Small molecules or ions that change shape and behavior of receptor proteins cAMP Used as second messenger by all known animal cells fig 7.5 Example: adrenaline binding to beta-adrenergic receptor (G protein-linked) fig 7.6 Binding adrenaline activates G protein Enzyme adenylyl cyclase produces large amounts of cAMP in target cell cAMP binds to A-kinase Activates it to phosphorylate cell proteins fig 7.7a Action dependent on cell type, in muscle stimulates glycogen to glucose Calcium Chemically-gated calcium channels in endoplasmic reticulum membrane Influx of Ca++ from inside ER to cytoplasm triggers many activities Skeletal muscles contract, some endocrine cells release hormones Receptor activates G protein which activates phospholipase C enzyme Phospholipase C catalyzes production of inositol triphosphate (IP3) IP3 binds to Ca++ channels opening them Also initiates response by binding to calmodulin fig 7.8 AMPLIFYING THE SIGNAL: PROTEIN KINASE CASCADES Receptors at Surface Receive Signal, But Response Is Elsewhere Second messengers relay signal to enzymes or genes Most receptors use other protein messengers to amplify signal to nucleus Mechanism of the Amplification Process Receptor phosphorylates stage-one protein These in turn activate stage-two, then stage-three proteins fig 7.9 Example: vision Single light-activated rhodopsin activates many transducin molecules Each transducin causes modification of cyclic GMP One rhodopsin ultimately causes split of 105 cyclic GMP's fig 7.10 Example: cell division Receptor phosphorylates ras protein Ras in turn activates multiple phosphorylation cascades Hyperactive ras (as in cancer) results in uncontrolled cell division CELL-CELL INTERACTIONS AND THE EXPRESSION OF CELL IDENTITY Tissues Are a Fundamental Property of Multicellular Organisms All cells within a tissue are identified as members of that tissue Identification results from the presence of unique cell surface markers Cell Surface Markers Some are glycolipids, lipids with carbohydrate tails Differentiate organs and tissues within the vertebrate body Markers on surface of red blood cells identify A, B, O blood types Cell populations of glycolipids change as cells differentiate Some are proteins anchored in the plasma membrane Immune system "self" marker proteins Major histocompatibility complex (MHC) proteins INTERCELLULAR ADHESION Cell Junctions Are Long-Lasting Physical Connections Between Cells fig 7.12 Nature of the connection determines what tissue is like Tissue function dependent on how individual cells arranged within it Tight Junctions Connect adjacent cells to prevent small molecules from leaking fig 7.13 Cells act as wall within an organ Molecules sequestered within a region Example: cells lining digestive tract Partition plasma membranes of lining cells together Nutrient transport proteins must stay in proper orientation to function Anchoring Junctions Common in sheets of tissues exposed to stress Cadherin protein junctions Attach cell cytoskeleton to other cells or extracellular matrix Desmosomes: connect cytoskeletons of adjacent cells fig 7.14 Hemidesmosomes: anchor epilthelial cells to basement membrane fig 7.12 Single-pass transmembrane glycoproteins fig 7.15 Cytoplasmic end linked to intermediate filaments Other end projects through membrane links to cadherin of next cell More secure than connection to free-floating membrane proteins Cadherins also connect to cell's actin framework, less stable connection fig 7.16 Adherens junctions Connect actin filaments of neighboring cells or extracellular matrix fig 7.12 Linking proteins belong to superfamily of receptors called integrins Integrin is transmembrane protein made of two glycoprotein subunits Communicating Junctions Pass ions or small molecules from one cell to another Example: chemical synapses passing neurotransmitters Example: gap junctions fig 7.17 Composed of connexons Six identical transmembrane proteins arranged in a circle Connexons of two adjacent cells must be perfectly aligned Small molecules like sugars and amino acids can pass Are dynamic structures that can open and close Respond to factors like Ca++ and H+ ions If cell damaged ions flow in, close gap junctions, seal off cell In plants, plasmodesmata provide cytoplasmic connections between cells fig 7.18 Occur only at gaps in cell walls Function like animal cell gap junctions Are lined with plasma membrane Contain central tubule connecting ER of both cells SUMMARY OF CELL COMMUNICATION MECHANISMS tbl 7.1