notes 11

Molecular Biology Chapter 11 Cell- Cell Interactions If you are multicellular, your cells need to coordinate with each other Physical connections Communication Synchronize functions Extracellular Layer Cell wall in plants, fungi, bacteria, algae Extracellular matrix in animals Fiber composites Cross- linked network of longer fibers embedded in rigid surrounding material Tensile strength vs compression Cell walls in plants Wood, paper, cotton threads, etc…. Cell wall is dynamic Repaired if damaged Softened in ripe fruit Primary cell wall Fiber= cellulose, cross-linked into microfibrils, laid out in crisscross network Surrounding material= pectins Pectin= gelatinous polysaccharide Primary Cell Wall Plant cells fill with water Plasma membrane presses against the cell water, creating turgor pressure Secondary Cell Wall in Plants Cell wall produced in mature plant cells Composition depends on the cell’s function Stem cells Lots of lignin Wood Leaf Cells Waxy compounds Extracellular matrix in animals Extracellular matrix= ECM Same basic idea- fiber composites Fibers are mainly collagen Surrounding material is gel-forming polysaccharides ECM is more flexible than cellulose or lignin Animals much more flexible!!! Exact nature of ECM is related to the cell type Where would you expect to find….. Cells with very little ECM between them? ECM with lots of elastin (elastin= stretchy ECM) Cells with lots of ECM between them Hint: It’s a structure where the ECM is doing the “work” Hint 2: It’s a structure where the ECM has lots of Ca salts in it Cells are connected to the ECM via transmembrane proteins Keeps cells in their place Helps neighboring cells stick to each other If it doesn’t= metastasis SIDE NOTE Case study: 12 patients Bleeding gums, capillaries break (small bruises), breakdown of cartiage (joint pain), loss of dentin (tooth pain) How can the breakdown of ECM be reversed? Captain James Lind, HMS Salisbury in 1747 Experimental design: 6 groups with 2 people All got the same diet with varying supplements Apple cider Sulfuric acid Vinegar Herbal mixture Quart of seawater 2 oranges and a lemon Why did the oranges and lemon work? Enzyme: Prolyl hydroxylase Catalyzes procollagen collagen Ascorbic acid is a cofactor for prolyl hydroxylase Intercellular connection and communication In multicellular organisms, cells have to stick together Cells with similar functions that are stuck together= movement 1 muscle cell working alone= lame 1000s of muscles working together= movement Need to be physically connected Need to synchronize activity/ communicate Attachment in plants Middle lamella between neighboring cell walls Gelatinous pectins Works like glue to hold cells together During the fall, middle lamella is broken down (the leaves fall off the trees) No more “glue” between leaf cells and stem cells Attachments in animals Gelatinous polysaccharides In animals, integrins also help hold cells together Can also have “cables” that link cells together Tight junctions Proteins that “stitch” cells together Can make a watertight seal Found in cells that form barriers Skin, lining of the digestive system Why are tight junctions b=needed in between these cells? Junctions vary between tissues Bladder cells and stomach cells- totally impervious Small intestine cells- small molecules and ions can get through Junctions are dynamic, can change over time Desmosomes---- Rivets!!! Proteins inside the plasma membrane attach to the cytoskeleton Reach out of the cell to connect to same protein in neighbor Cytoskeletons of neighboring cells are physically connected!!! Found between muscle cells, epithelial cells How do certain cells connect with each other? Hypothesis 1: They’re next to each other during development, basically randomly Hypothesis 2: Different cell types recognize each other, selectively link up with similar types Test by breaking down connections, letting them reform If hypothesis 1: cells will rejoin each other randomly If hypothesis 2: cells will rejoin in their original pattern Must be a molecular recognition system Specialized proteins embedded in the membranes Protein in cell A binds to a protein in cell B How can you identify specific membrane proteins that might be involved? Break them 1 by 1 Antibodies=====Bind to specific proteins If a recognition protein has an antibody stuck to it, can’t bind with normal partner Individual antibodies are against specific proteins Don’t know the protein’s function, only that is binds to that antibody Test antibodies 1 by 1 Identify the antibody that stops cell adhesion Characterize the protein Cadherins---- adhesion proteins Specific cadherins ensure that specific cells adhere to each other Liver cells to liver cells Muscle cells to muscle cells Skin cells to skin cells Cell to Cell Communication Cells communicate through gaps In plants= plasmodesmata Gaps in the cell walls, membrane fuse, cytoplasm is shared Smooth ER connects to 2 cells Activity between the cells can be coordinated Passage through the plasmodesmata is probably controlled In animals, occurs through gap junctions Transmembrane proteins from both cells line up and form channels Small molecules can pass through Water, ions, amino acids, sugars, etc.… Ions can coordinate cell function Heart contractions coordinated by the flow of potassium/calcium/sodium ions Communication between distant cells Cells in multicellular organisms need to coordinate efforts Hormones= small signaling molecules Varying small concentration can have huge effects Key variable is lipid solubility Signal Reception Extracellular messages are converted into intracellular response Hormones deliver their message by binding to specific receptors Cells without the proper receptors will not respond to hormone signals A wide range of cell types can respond to specific hormone Cells in your heart, lungs, and liver have receptors that identify adrenaline Number of receptors is dynamic Can decline if there are high levels of hormones for a long time Sensitivity to signal declines Receptors can be blocked Drugs can bind to receptors, prevent hormones from binding, signal is not received Beta-blockers –prevent adrenaline from affec7ng heart cells, lowers blood pressure Signal Processing Lipid soluble hormones Lipid soluble = can pass right through the membrane Receptor is inside the cell Signals causes changes in gene expression, pump activity Signal Transduction Lipid insoluble hormones Transduction = physical form of the signal changes Receptor is transmembrane protein Hormone causes receptor to create intracellular signal Intercellular signal changes gene expression for specific genes More proteins are made Signal Amplification Hormones occur in tiny quantities Signal needs amplification If hormones open ion channels, changes the electrical properties of the membrane Secondary messengers within the cell 1. Triggered by G proteins 2. Triggered by enzyme-linked receptors Signal Transduction and G Proteins Signal molecule binds to the outward facing part of membrane bound receptor G protein is bound to the inward facing part of the receptor G protein has a GDP molecule bound to it When the hormone binds to the receptor, its shape changes The receptor’s shape change switches the G protein to its active state Releases GDP, attaches to GTP Activated G protein activities enzyme which produces second messengers Second messenger= non-protein molecule that causes a response Many second messengers activate protein kinases Protein kinase= enzyme that activates (or deactivates) other proteins Second messengers can have different roles in different cells More than 1 second messenger can be activated by a single stimulus Signal transduction and enzyme- linked receptors Hormone binds to its transmembrane receptor protein In this example, receptor tyrosine kinase (RTK) Hormone binding creates dimer, which gets a phosphate group from ATP Activates the RTK Activated RTK forms a bridge with Ras protein Ras is activated by GDP ? GTP Activated Ras phosphorylates a protein to activate it, which can then phosphorylate other proteins, etc…. Phosphorylation cascade If protein 1 activates 10 protein 2s, and each protein 2 activates 10 protein 3s… Signal Response 2 general categories 1. Change in gene expression 2. Activate/deactivate specific protein Enzyme, membrane channel, etc… Single signals can have huge effects In seeds Hormone GA binds to cells ? increase in cGMP ? produc7on of ?-amylase Hormone GA binds to cells ?ac7vates membrane channels ? increase in intracellular Ca2+ ? vesicles full of ?-amylase are secreted into the starch storage area Signal Deactivation Unless signal continues, intracellular processes stop For G proteins G proteins convert GTP that activated them back into GDP, inactivate themselves Second messengers are quickly broken down For enzyme-linked receptors Inside the cell, phosphatases are always removing phosphate groups from proteins Once RTK is turned off (no signal), cascade slows Entire process can happen very quickly, only need tiny amount of signal Cross- talk Cells receive lots of different types of signals All these different signals causes different responses that need to be coordinated Cross-talk= interactions between responses Hormone sends message to cell telling it to divide Cell has a mutation in Ras such that Ras never deactivates What will happen? Acetylcholine is a neurotransmitter that binds to receptors on skeletal muscle cells. The receptor-signal complex starts a series of reactions that leads to contraction of skeletal muscle. Black widow spider venom causes an explosive release of acetylcholine. What will that do to its victims? Not just multicellular eukaryotes…. Quorum sensing in bacteria When population size for a bacterial species reaches a threshold, they may change their behavior Large groups may produce biofilms Biofilms= hard, polysaccharide-rich coatings Allows attachment to diverse surfaces Protects them