![]() This variation in response is due to differences in protein expression in different cell types. A single pathway can branch off toward different endpoints based on the interplay between two or more signaling pathways, and the same ligands are often used to initiate different signals in different cell types. Signaling pathways can get very complicated very quickly because most cellular proteins can affect different downstream events, depending on the conditions within the cell. Interactions that occur before a certain point are defined as upstream events, and events after that point are called downstream events. The events in the cascade occur in a series, much like a current flows in a river. In a signaling pathway, second messengers, enzymes, and activated proteins interact with specific proteins, which are in turn activated in a chain reaction that eventually leads to a change in the cell’s environment ( Figure 9.10). Binding Initiates a Signaling PathwayĪfter the ligand binds to the cell-surface receptor, the activation of the receptor’s intracellular components sets off a chain of events that is called a signaling pathway or a signaling cascade. The binding of the receptors in this manner enables their intracellular domains to come into close contact and activate each other. A dimer is a chemical compound formed when two molecules (often identical) join together. In some cases, binding of the ligand causes dimerization of the receptor, which means that two receptors bind to each other to form a stable complex called a dimer. Conformational changes of the extracellular domain upon ligand binding can propagate through the membrane region of the receptor and lead to activation of the intracellular domain or its associated proteins. When a ligand binds to its receptor, conformational changes occur that affect the receptor’s intracellular domain. Signal transduction only occurs with cell-surface receptors because internal receptors are able to interact directly with DNA in the nucleus to initiate protein synthesis. Continuation of a signal in this manner is called signal transduction. Once a ligand binds to a receptor, the signal is transmitted through the membrane and into the cytoplasm. These questions address the following standards: The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. An animation of fight-or-flight response can be seen here. The same signal that activates all the systems needed for survival also shuts down the systems which are not essential for the rapid reaction needed to escape danger. The likely response is that nauseous feeling and digestion cut short, courtesy of our sympathetic system. Other organs have their activities dampened: the stomach halts digestion and salivary glands stop production.Īsk students what happens if they get a loud alarm sound while eating. Some organs must be activated for the response: skeletal muscle, heart, and the release of glucose from liver. It should trigger the “fight-or-flight” response. A Learning Objective merges required content with one or more of the seven Science Practices.Īsk students what would happen if suddenly the fire alarm went off. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® Exam questions. Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 3 of the AP ® Biology Curriculum Framework. Second messengers, e.g., cAMP and Ca 2+, are often used to transmit signals within a cell. This change in shape activates or inactivates them. (Substrate–level phosphorylation was studied when you learned about glycolysis.) By adding a phosphate group, phosphorylation changes the shapes of proteins. A significant contributor to cell signaling cascades is the phosphorylation of molecules by enzymes known as kinases. These cascades are complex because of the interplay between proteins. What is the role of second messengers in signal transduction?ĭuring signal transduction, a series of relay proteins inside the cytoplasm of the target cell activate target proteins, resulting in a cellular response.How does the binding of a ligand initiate signal transduction throughout a cell?.In this section, you will explore the following questions: ![]()
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