Ap Biology Notes Cellular Communication Essay

Cell-to-cell communication is essential in multicellular organisms. They must communicate to coordinate activities such as growth and development, and reproduction. In addition unicellular organisms communicate with each other. Signals may use light, or touch but we will focus on chemical signals. 1. External signals are converted to responses within the cell a. Evolution of cell signaling i. In yeast a cells and ÃŽ ±cells both secrete chemicals, which can only be received by the alternate type yeast. This signals the two cells to join via fusion 1. The process by which the signal on the surface of the cell is converted to a series of steps by the cell in response is called a signal transduction pathway ii. Signal transduction pathways are very similar in yeast and in complex multicellular organisms 2. This leads scientists to believe that this pathway evolved first in ancient prokaryotes b. Local and long distance signaling iii. Local signaling 3. Adjacent cells of plants and animals may communicate through cell junctions a. Signaling substances dissolved in the cytoplasm travel between cells i. Plants = plasmodesmata ii. Animals = gap junctions 4. Animal cells may use the following b. Cell-to-cell recognition iii. Direct contact between membrane-bound cell-surface molecules iv. Important in embryonic development and immune response c. Paracrine signaling v. Uses local regulators which are released and travel only a short distance to nearby cells vi. Ex. Growth factors target nearby cells to grow and divide d. Synaptic signaling vii. Electrical signal along a nerve cell triggers a chemical release across a synapse to trigger response in target cell viii. Ex. Nerve cells iv. Long- distance signaling 5. Both plants and animals use hormones e. Animals (endocrine signaling) cells release hormones which travel in the circulatory system to target cells f. Plants hormones travel in vessels or by diffusion through the air as gas g. Hormones vary in size and shape 6. Nervous system signals can also be long distance c. The three stages of cell signaling: A preview v. Reception: when the target cell detects a signaling molecule. The signaling molecule binds to a receptor protein on the target cell’s surface vi. Transduction: After binding the receptor protein is changed in some way, this converts the signal to a form that will bring about a specific cellular response 7. May occur in a single step or a series of changes vii. Response: The transduced signal triggers a specific cellular response. 8. Catalysis of an enzyme, rearrangement of the cytoskeleton, activation of a specific gene 2. Reception: A signaling molecule binds to a receptor protein, causing it to change shape d. To ensure signals are sent to the correct cell signaling molecules act as a ligand. viii. Ligand- molecule that specifically binds to another (usually larger) molecule ix. The receptor protein then usually changes shape x. May be located on the membrane or inside the cell e. Receptors in the plasma membrane xi. Water-soluble signaling molecule binds to receptor on the membrane causing it to change shape or aggregate. f. Intracellular Receptors xii. Found in cytoplasm or nucleus of target cells 9. Signaling molecule must be hydrophobic enough or small enough to pass through the plasma membrane h. Steroid hormones, thyroid hormones, nitric oxide | Examples| Pathway| Other| G-Protein Coupled Receptors| Yeast mating factors, epinephrine, hormones, neurotransmitters| 1. signaling molecule binds to the g-protein receptor 2. receptor changes shape and the cytoplasmic side binds to the inactive G protein 3. GTP then displaces to form GDP and activates the protein 4. Activated G protein diffuses along the membrane to an enzyme altering the enzyme to trigger the next step| Bacteria such as whooping cough, botulism and cholera disrupt this pathway| Receptor Tyrosine Kinases| Enzymes that catalyze the transfer of phosphate groups| 1. binding of two signaling molecules to two tyrosine chains causes the two to associate with each other forming a dimer 2. dimerization activates the tyrosine kinase region to add a phosphate from and ATP to each tyrosine in the polypeptide 3. each tail can now bind to and activate a different specific relay protein within the cell| One receptor may activate ten+ pathways. Absence can result in cancer| Ion Channel Receptors| Nervous system| 1. signaling molecule binds to the ion channel in the membrane 2. protein changes shape creating a channel through the membrane 3. specific ions can now flow through the membrane which may cause a change in the cell or trigger another pathway| Some ion gated channels are controlled by change in voltage rather than binding of a ligand| g. Intracellular Receptors xiii. Ex. Testosterone 10. Hormone passes through the plasma membrane 11. Testosterone binds to a receptor protein in the cytoplasm activating it 12. The hormone-receptor complex enters the nucleus and binds to a specific gene 13. The bound protein acts as a transcription factor, stimulating the transcription of the gene into mRNA 14. The mRNA is translated into a specific protein 3. Transduction: cascades of molecular interactions relay signals from receptor to target molecules in the cell h. Protein phosphorylation and dephosphorylation xiv. Proteins can be activated by the addition of a phosphate group (often broken off of ATP or GTP) 15. Phosphates are transferred from ATP to a protein by a general group of enzymes known as protein kinases i. Phosphorylation often causes the protein to change shape j. This happens because the added phosphate group interacts with polar or charged amino acids within the protein xv. Protein phosphatases are enzymes that remove phosphate groups from a protein 16. Mechanism for turning off signal transduction 17. These also allow for turning off and reusing pathways i. Small molecules and ions as second messengers xvi. Molecules other than proteins act as second messengers 18. Small and water soluble such as ions k. This allows them to rapidly spread throughout the cell via diffusion 19. Second messenger refers to anything after the first messenger which is the extracellular signaling molecule that binds to the membrane 20. Most common second messengers are cyclic AMP and Ca+2 xvii. Cyclic AMP as a second messenger in response to the hormone epinephrine 21. Epinephrine binds to receptor molecule protein activates adenylyl cyclase which can catalyze the synthesis of many molecules of cAMP l. Adenylyl cyclase catalyzes the conversion of ATP into cAMP ix. cAMP usually activates a serine/threonine kinase known as protein kinase A which phosphorylates many other proteins m. cAMP is converted back to AMP by phosphodiesterase xviii. Calcium ions and inositol tripohosphate 22. Increasing calcium concentration causes responses such as muscle contraction, secretion of substances, and cell division in animals, and greening in response to light in plants 23. Calcium is usually in high concentrations outside of the cell and in the ER but in low concentrations in the cytosol 24. Pathway n. Signaling molecule binds to receptor   o. Phospholipid pinches off membrane IP3 is released as second messenger p. IP3 binds to receptor on ER causing protein channel to open q. Ca+2 is released into cytosol 4. Response: Cell signaling leads to regulation of transcription or cytoplasmic activities j. Nuclear and cytoplasmic responses xix. Pathways lead to the regulation of one or more cellular activities 25. Regulate protein synthesis r. Turning specific genes on or off (calls for the synthesis of mRNA from DNA) 26. Regulate protein activity s. Cause a shape change to turn a protein on or off 27. Regulate overall shape change of cell 28. Release of mating factors 29. Cell division k. Fine-tuning of the response xx. Signal amplification 30. Enzyme cascades amplify effects by increasing the product at each step t. Enzymes stay active long enough to work on multiple products before becoming inactive xxi. The specificity of cell signaling and coordination of the response 31. Different types of cells are programmed to respond to only certain types of signals u. Some cells will respond to the same signals but in different ways v. This is because different cells have different collections of proteins xxii. Signaling efficiency: scaffolding proteins and signaling complexes 32. Scaffolding proteins increase the efficiency of the response w. A large protein with multiple protein kinases attached x. Decreases the time of the response because diffusion between proteins is not needed 33. Pathways are not linear, in fact the same protein may act in multiple pathways 34. Relay proteins serve as branch points where the signal may go in one of two directions xxiii. Termination of the signal 35. Each step in the pathway lasts only a short time, this makes the proteins ready for a new signal 36. When the signaling molecule leaves leave the receptor it reverts to its inactive form and the relay molecules follow

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