Chapter 11 Study Guide Notes for BIL 150 (Campbell Biology)
Chapter 11 Study Guide Notes for BIL 150 (Campbell Biology) BIL 150
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This 15 page Study Guide was uploaded by Kaman Vale on Thursday March 24, 2016. The Study Guide belongs to BIL 150 at University of Miami taught by Yunqiu Wang/ Floria Mora-Kepfer Uy in Spring 2016. Since its upload, it has received 21 views. For similar materials see General Biology (Lecture) in Biology at University of Miami.
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Date Created: 03/24/16
Chapter 11 Notes Concept 11.1 Evolution of cell signaling Occurs in a series of steps called a signal transduction pathway. This pathway exists in yeast and animal cells. Local and Long Distance Signaling Cells in multicellular organism communicate via signaling molecules. -eukaryotic cells communicate with direct contact (one type of local signaling) -Both animal, plant have cell junctions that directly connect the cytoplasm of adjacent cells. In this case, signaling substances dissolved in cytosol can pass freely. -animal cells may communicate direct contact between membrane bound cell surface molecules in process called cell-cell recognition. Other cases of local signaling: 1. messenger molecules are secreted by the signaling cell. Some travel short distances. Local regulators influence cells in the area. -local regulators in animals are called growth factors, which are compounds that stimulate nearby target cells to grow and divide. Numerous cells can receive and respond to molecules. This is called paracrine signaling. 2. called synaptic signaling, which occurs in the animal nervous system. Electrical signal along a nerve cell triggers secretion of neurotransmitter molecules. Molecules act as chemical signals, diffusing across the synapse (the narrow space between the nerve cell and its target cell) triggering response in target cell. Local signaling in plants is not as well understood. Because of their cell walls, plants use different mechanisms. Animal and plants both: 1. Use hormones for long distance signaling. - Hormone signaling aka endocrine signaling, specialized cells release hormone molecules which travel via circulatory system to other parts of the body where they reach target cells that respond. 2. Plant hormones aka plant growth regulators travel in vessels but reach their target by moving thru cells or diffusing through the air as gas. 3. Hormones vary in size and type. Three Stages of Cell Signaling 1. Reception: target cell’s detection of a signal molecule coming from outside of the cell. Signal is detected when signaling molecule binds to a receptor protein in the cell’s surface. 2. Transduction: binding of the signaling molecule changes receptor protein in ways, initiating process of transduction. This stage converts the signal to a form that can bring out specific cellular response. Transduction sometimes happens in one step, but more often requires a sequence of changes in a series of diff. molecules—signal transduction pathway. (Molecules in pathway are called relay molecules) 3. Response: transduced signal triggers a specific cellular response. The cell signaling process helps ensure that these things occur in the right cells, right time, and proper coordination within the activities of other cells of the organism. Section 11.2 Reception: A signaling molecule binds to a receptor protein, causing it to change shape Receptor protein allows cell to “hear” the signal and respond to it. The signaling molecule is complementary in shape to a specific site on the receptor and attaches there. Signaling molecule acts as a ligand (term for molecule that specifically binds to another molecule). Ligand binding makes receptor protein change shape. This shape change activates the receptor, allowing it to interact with other cellular molecules. Receptors in the plasma membrane Cell surface receptor proteins play big role in biological system of animals. Most water soluble signaling molecules bind to specific site on transmembrane receptor proteins that transmit information from outside environment to the inside. There are three major types of cell surface transmembrane receptors: G coupled receptors (GPRCs), receptor tyrosine kinases, and ion channel receptors. G Protein-Coupled Receptors: Is a cell-surface transmembrane receptor that works with a G protein (protein that binds GTP). Many signaling molecules like epinephrine, and hormones use GPCRs. These receptors vary in the binding sites for their signaling molecules. GPCR proteins are similar in structure. Malfunctions of the associated g proteins are involved in many diseases. GPCRs are responsible for vision, taste, smell. Receptor Tyrosine Kinases: Belong to a major class of plasma membrane receptors characterized by having enzymatic activity. A kinase is an enzyme that catalyzes transfer of phosphate groups from ATP to amino acid tyrosine on a substrate protein. RTKs are membrane receptors that attach phosphates to tyrosines. One RTK may activate 10 or more different transduction pathways and cellular responses at once, helping cell regulate and make cell growth and cell reproduction. Ability of single ligand binding event to trigger many pathways is what extinguishes this from GPCRs. Ion Channel Receptors: A ligand-gated ion channel is a type of membrane receptor containing a region that can act as a “gate” when receptor changes shape. When signaling molecule binds as a ligand to receptor protein, the gate opens or closes, allowing/blocking the flow of specific ions, such as NA+ or CA2+, through a channel in the receptor. Proteins bind the ligand at specific site on their extracellular sides. Ligand gated ion channel are very important in the nervous system. -Abnormal functioning of RTKs are associated with many types of cancers. Intracellular Receptors Found in cytoplasm or nucleus of target cells. To reach receptor, a signaling molecule passes thru the target cell’s plasma membrane. (possible because they are hydrophobic enough to or small enough to cross the hydrophobic interior of the membrane) Include steroid hormones, and thyroid hormones of animals. Once hormone has entered a cell, it may bind to intracellular receptor in the cytoplasm or the nucleus. This binding changes receptor to hormone-receptor complex that can cause a response. Ex: Behavior of aldosterone (steroid hormone). Hormone secreted by cells of adrenal gland. Aldosterone travels thru the blood and enters the cells all over the body. Response only happen in kidneycells which contain the receptor molecule for this hormone. How does activated hormone receptor complex turn on genes? Special proteins called transcription factors control which genes are turned on… a.k.a, which genes are transcribed into mRNA in a particular cell at a particular time. Concept 11.3 Transduction: Cascades of molecular interactions relay signals from receptors to target molecules in the cell When receptors for signaling molecules are plasma membrane proteins, the transduction stage of cell signaling is a multistep pathway involving many molecules. One benefit of multistep is the possibility of amplifying a signal. (This causes an increase in the # of activated molecules by the end of the pathway). Also provide more opportunities for coordination and control than do simpler systems. Signal Transduction Pathways Binding to a specific signaling molecule to a receptor in plasma membrane triggers the first step in the signal transduction pathway. This leads to a response within the cell. The signal activated receptor activates another molecules and continues until the protein that produces the final cellular response is activated. Interaction of proteins is a major theme in cell signaling. **original signaling molecule is not passed along signaling pathway, in most cases it doesn’t enter cell. ** Protein Phosphorylation and Dephosphorylation Is a widespread cellular mechanism for regulating protein activity. An enzyme that transfers phosphate groups from ATP to a protein is called a protein kinase. Act on proteins different from themselves. Also most cytoplasmic protein kinases phosphorylate either amino acids, serine or threonine, rather than tyrosine. Responsible for signaling pathways in animals, plants, fungi. Many of the relay molecules in signal transduction pathways are protein kinases and they act on other protein kinases in the pathway. Picture is a pathway containing three different protein kinases that create a phosphorylation cascade. The change in shape(happens because of the interaction of newly added phosphate groups with charged or polar amino acids on the protein being phosphorylated) alters the function of the protein, most often activating it. Some cases, phosphorylation decreases the activity of the protein. Protein kinases regulate cell division. Abnormal activity of kinase causes abnormal cell division causing cancer. Protein phosphates are also important in the phosphorylation cascade, they are enzymes that can quickly remove phosphate groups from proteins, in a process called dephosphorylation. By dephosphorylation and inactivating protein kinases, phosphates provide the mechanism for turning off the signal transduction pathway when the initial signal is not there anymore. Phosphates also make the protein kinases available for reuse, letting the cell respond again to an extracellular signal. The phosphorylation/ dephosphorylation system acts as a molecular switch in the cell, turning activities on or off, up/down too. Activity of protein regulated by phosphorylation depends on balance in the cell b/w active kinase molecules and active phosphatase molecules. Small Molecules And ions as second messengers Many components of signal transduction involve small, nonprotein, water-soluble molecules called second messengers. Can spread throughout the cell by diffusion. Cyclic AMP Earl Sutherland found epinephrine causes glycogen breakdown without passing thru plasma membrane. Searching for second messenger that transmit signal from plasma membrane to metabolic machinery in cytoplasm. Binding of epinephrine to the plasma membrane of a liver cell elevates the cytosolic concentration of a compound called the cyclic adenosine monophosphate, cyclic AMP of cAMP. Enzyme embedded in plasma membrane, adenylyl cyclase, coverts ATP to cAMP in response to extracellular signal. But epinephrine doesnt stimulate adenylyl cyclase directly. When epi outside cell binds to a specific receptor protein. The protein activates adenylyl cyclase which can catalyze the synthesis of many molecules of cAMP. (normal cellular concentration can be boosted 20-fold in a matter of seconds.) cAMP broadcasts signal to the cytoplasm. Then enzyme phosphodiesterase converts cAMP to AMP. Another surge of epi is need to boost the cytosolic concentration of cAMP again. -epi is one of the many hormones and other sign.molecules that trigger formation of cAMP. Other components of cAMP pathways include G proteins, coupled receptors, and protein kinases. -immediate effect of an elevation in cAMP levels is the activation of kinase called protein kinase A. then it phosphorylates other proteins depending on the cell type. Calcium Ions and Inositol Triphosphate (IP3) Many of the signaling molecules that function in animals, induce responses in target cells that increase the cytosolic concentration of calcium ions (Ca2+). -Calcium is used more as a second messenger than cAMP. -This increase in calcium causes many responses in animal cells, including muscle cell contraction, secretion of substances and cell division. - in plant cells, hormonal and envir factors affect calcium concentration, triggering signal pathways like greening in response to light. Triggered by both G protein coupled receptors and receptor tyrosine kinases. -calcium functions as a second messenger bc its concentration in the cytosol is much lower than concentration outside the cell. -Calcium ions are actively transported out of the cell and are actively imported from the cytosol into the ER by protein pumps. As result, the calcium concentration is higher in ER than in the cytosol. Pathways leading to calcium release involve two other second messengers, inositol triphosphate (IP3) and diacylglycerol (DAG). These two messengers are produced by cleavage of a certain kind of phospholipid in the plasma membrane. Concept 11.4 Cell signaling leads to regulation of transcription or cytoplasmic activities Nuclear and Cytoplasmic Responses - Signal transduction pathway leads to regulation of one or more cellular activities. - Response at the end may occur in nucleus or cytoplasm. - Many signaling pathways regulate protein synthesis, by turning specific genes on/off in nucleus. - The final activated molecule in a signaling pathway may function as transcription factor. -Next, this will be translated in the cytoplasm into specific proteins. Other cases show the transcription factor turning a gene off. (transcription factor regulates different genes) Sometimes, signaling pathway may regulate the activity of proteins rather than causing their synthesis by activating gene expression. This directly affects proteins that function outside of the nucleus. Signal receptors, relay molecules, and second messengers participate in a variety of pathways, leading to both nuclear and cytoplasmic responses. -Some pathways lead to cell division. Molecular messengers that initiate cell division pathways include growth factors and certain plant and animal hormones. (malfunctioning of growth factors can also cause cancer) Regulation of the Response It is more complex than just turning ‘on/off’. Rather, the response is regulated in multiple ways. Four aspects of this regulation: 1. Signaling pathways amplify cell’s response to a single signaling event. Degree of amplification depends on the function of the specific molecules in the pathway. 2. The many steps in a multistep pathway provide control points at which cells response can be regulated more, coordinating with other signaling pathways. 3. Overall efficiency of response is enhanced by presence of scaffolding proteins. 4. Crucial point in regulating the response is a termination of the signal. Signaling Amplification Elaborate enzyme cascades amplify cell response to signals. At each step in cascade, the # of activated products can be greater than preceding step. The Specifity of Cell Signaling and Coordination of the Response The response of a particular cell to a signal depends on its particular collection of signal receptor proteins, relay proteins, and proteins needed to carry out the response. Liver and heart cell respond differently to hormone epinephrine. They differ in one or more of the proteins that handle and respond to the signal . Branching of pathways and cross talk (interaction) between pathways are important in regulating and coordinating a cell’s responses to info coming in from different sources in the body. The use of some of the same proteins in more than one pathway allows the cell to exonomize on the # of different proteins it must take. Signaling Efficiency: Scaffolding Proteins and Signaling Complexes In many cases, the efficiency of signal transduction is increased by the presence of scaffolding proteins (which are large relay proteins that are attached to other relay proteins). One scaffolding protein holds 3 protein kinases and carries these kinases with it when it binds to an appropriately activated membrane receptor; thus facilitates a specific phosphorylation cascade. Scaffolding proteins in brain cells that permanently hold together networks of signaling pathway proteins at synapses. This hardwiring enhances the speed and accuracy of signal transfer between cells because rate of protein-protein interaction is not limited by diffusion. -In other words, the scaffolding proteins themselves may activate relay proteins. -Also, the importance of relay proteins that serve as points of branching or intersection in signaling pathways is highlighted by the problems arising when these proteins are defective or missing. Termination of the Signal For a cell to remain capable of responding to signals, each change in its signaling pathway must only last a short time. Ability of a cell to receive new signals depend on the reversibility of changes produced by prior signals. Binding of signaling molecules to receptors is reversible. As concentration of signaling molecules fall, unbound receptors go back to their inactive form. When # of active receptors fall below that threshold, the cellular response ceases. Then relay molecules return to their inactive forms. The cell will be ready to respond to a fresh signal. Concept 11.5 The best understood type of controlled cell suicide is apoptosis. -during process, cellular agents chop up DNA and organelles and other cytoplasmic components. Cell shrinks and becomes lobed, cells parts are packaged up in vesicles that are digested by scavenger cells leaving no trace. Apoptosis protects neighboring cells by not leaking out its contents and infecting others. -signal that triggers this process comes from outside/inside cell. -outside the cell, signaling molecules create pathway for cells that carry out the proteins and genes responsible for cell death. -within cell, series of protein-protein interactions pass along signal that triggers cell death.
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