Signal Transduction Pathways

Signal Transduction Pathways What happens inside of a cell when a signal has been received? Why? When cells release signal molecules (ligands) to send a message, and the message is received, a whole host of events take place inside the cell. The ultimate goal is a response—a gene is turned on, a protein is manufactured, an enzyme is activated, the cell divides or dies, etc. There are many responses that could occur, but the pathways to those responses are very similar. 1. According to Model 1, does the signal transduction pathway occur inside or outside of a cell? It occurs inside of the cell. 2. Compare the shape of the ligand in Model 1 to the shape of the receptor protein. The ligand is specific to the receptor site of the protein. In other words, the ligand perfectly fits inside the receptor and the receptor is formed to perfectly fit the ligand. 3. The four steps in the signal transduction pathway are listed below. Label the diagram above with the Roman numerals to indicate where on the diagram each step is taking place. I. Signaling II. Reception III. Transduction IV. Response 4. Based on the diagram in Model 1 and your labels, propose a definition for “transduction” in the context of the signal transduction pathway. Transduction is when a signal is transmitted through the cell to the location where the reaction should happen. 5. Amplification often occurs during the transduction step in the signal transduction pathway. a. Define “amplification” as it is used in everyday language. To make something bigger or more profound. b. Explain how the signal in Model 1 was amplified. One protein was able to activate several other proteins causing a response between them all. 6. List several possible responses that could occur due to a signal being received by a cell. (Hint: There are several listed in the Why? box.) a gene is turned on, a protein is manufactured, an enzyme is activated, the cell divides or dies 7. Once the response is achieved in a cell, what would need to occur to stop the response? The activated proteins would deactivated thoroughly. Read This! Signal transduction pathways are as varied as the species on Earth, but each of them follow the same basic steps. Reception may occur by the ligand attaching to a receptor protein or the ligand may travel through the cell membrane by diffusion. Transduction may occur by one protein or by several proteins activating each other in a cascade. Keep this in mind as you continue working through this activity. PP The ligand attached to an active site in a receptor protein in a cell membrane. PP 8. Refer to Model 2. Describe the event that sets off a phosphorylation cascade inside of a cell. 9. Phosphorylation is a process that adds a phosphate group onto a protein to “activate” it—that is, to change its shape enough that it can function properly. a. Which step(s) in the phosphorylation cascade illustrated in Model 2 include phosphorylation? 4, 5, and 7 b. Where do the phosphate groups come from that are added to the proteins during phosphorylation? ATP to ADP 10. According to Model 2, what class of enzymes performs phosphorylation? Kinases 11. Identify the steps in Model 2 that represent reception, transduction, and response for the phosphorylation pathway. Reception: One Transduction: Two, three, four, five, and Six Response: Seven 12. What is the cell’s response to the signal received in Model 2? Transcription of a part of DNA is started 13. In Model 2, steps 3, 4, and 5 are shown as amplification steps. Describe what that means in terms of this signal transduction pathway example. Every Kinase can activate more than one kinase enzyme in the sequence. 14. What advantage would there be to an organism if the signal transduction pathway had several amplification steps? Amplification would allow the reception of a ligand to cause a response in many different places throughout the cell. 15. Describe what would occur in the cell if the activated protein kinase enzymes continued to be active for a long period of time. The transcription process would continue to transcribe the DNA strand as long as the protein kinases are active. 16. What would need to occur in the cell to deactivate the protein kinase enzymes? The phosphate group would need to be removed. 17. Protein phosphatases are enzymes that remove phosphate groups from proteins. Complete the illustration in Model 2 by adding at least two protein phosphatases (PP) to show how the cell is returned to inactive status. Done 18. Although signal transduction pathways vary among species, there are several common elements. Explain how a biologist might use details about signal transduction pathways used in different species as evidence for evolutionary relatedness. Species that share the same ligands, receptor proteins, and relay proteins are closely related by evolution. Extension Questions 19. Consider the signal transduction pathway in Model 3. What event begins the process of producing a cellular response? In the beginning a ligand has to attach to the active site of a receptor protein molecule. 20. Locate the secondary messenger molecules in Model 3. a. Describe the type of diffusion that is used to get the secondary messengers into the cell. Facilitated diffusion, high to low concentration b. What activates or opens the transport protein channel that allows the secondary messengers to enter the cell? The membrane channel is opened by the activated relay protein, which then attaches to and active site on the transport chain. Read This! Relay molecules and secondary messengers have essentially the same jobs in signal transduction pathways. However, relay molecules are almost always proteins that require activation. They are large and do not diffuse through the cell quickly. Secondary messengers on the other hand are small, water soluble molecules that can diffuse quickly. They may or may not need activation before they are able to move the signal on to the next step in the process. Cyclic AMP (cAMP) and calcium ions (Ca2+) are common secondary messengers in human systems. 21. Within an organism it is critical that signals between cells are very specific. For example, if ligand A is meant to activate immune system cells to reproduce in response to an infection, it should not also cause other cells to grow as if they had received a growth hormone. When a ligand is released, what prevents all of the cells in the body from being affected? A cell needs a specific receptor protein to be affected by a released ligand.