LQB383 Testbank. Week 8 Cell Communication and Signaling Mechanisms

Transcription

1 LQB383 Testbank Week 8 Cell Communication and Signaling Mechanisms Terms to learn match the terms to the definitions Adaptor Intracellular signaling protein Morphogen Endocrine cell Gap junction Neurotransmitter Hormone Receptor Extracellular signal molecule Signaling cascade Steroid hormone Synaptic signaling Paracrine signaling Second messenger Autocrine signaling Nuclear receptor family Interaction domain Contact-dependent signaling Definitions 1. General term for a protein that binds a specific extracellular molecule (ligand) and initiates a response in a cell. 2. Communicating cell-cell junction that allows ions and small molecules to pass from the cytoplasm of one cell to the cytoplasm of another. 3. Hydrophobic signaling molecule with characteristic four-ringed structure derived from cholesterol. 4. Short-range cell-cell communication via secreted local mediators that act on adjacent cells. 5. Specialised cell that secretes a hormone into the blood 6. The cell responds to its own secreted molecules. 7. Small signaling molecule secreted by the presynaptic nerve cell at a chemical synapse to relay the signal to the postsynaptic cell. 8. Small molecule that is formed in the cytosol, or released into it, in response to an extracellular signal, and that helps to relay the signal in the interior of the cell. 9. Molecule from the outside of the cell that communicates the behaviour or actions of other cells in the environment and elicits an appropriate response. 10. General term for a signal relay chain containing multiple amplification steps. 1 Receptor 2 Gap junction 3 Steroid hormone 4 Paracrine signaling 5 Endocrine cell 6 Autocrine signaling 7 Neurotransmitter 8 Second messenger 9 Extracellular signal molecule 10 Signaling cascade

2 Multiple Choice Questions 1. Which of the following types of chemical signals is secreted in the blood and travels long distances to its target. A) Neurotransmitters B) Cytokines C) Hormones D) Target Proteins E) Kinases 2. Which of the following is a characteristic of a receptor? A) Speed B) Specificity C) Saturation D) Competition E) All except A 3. A steroid hormone is different from other types of hormones in that: A) the steroid hormone is made of amino acids B) the steroid hormone causes a change to how the cell functions C) the steroid hormone binds to intracellular receptor in the target cell D) the steroid hormone usually does not enter target cells E) the steroid hormone directly activates DNA responsive elements 4. The sequence of events involved in cell signaling is A) transduction reception response B) response reception transduction C) reception response transduction D) reception transduction response E) response transduction reception 5. Steroid hormones are carried on specific carrier proteins because the hormones: A) are too unstable to survive in the blood on their own. B) cannot dissolve readily in the blood because they are too hydrophobic. C) cannot find their target cells without them. D) need them in order to pass through the plasma membrane. E) require subsequent binding to specific receptor proteins in the nucleus.

3 6. In paracrine signaling, the signaling molecule A) acts on cells in close proximity to the secreting cell B) acts on target cells far away from the secreting cell C) acts on the same cells that secreted the signaling molecule D) is carried to the target cells by the blood 7. Steriod hormone response elements (HREs) are, which, when bound to, alter gene expession at the level of. A) intron sequences; activated hormone receptor; translation. B) nuclear proteins; hormone; transcription. C) plasma membrane proteins; hormone; transcription. D) sequences in DNA; receptor-hormone complex; replication. E) sequences in DNA; receptor-hormone complex; transcription. 8. Which of the following is not involved in the specificity of signal transduction? A) Interactions between receptor and signal molecules B) Location of receptor molecules C) Structure of receptor molecules D) Structure of signal molecules E) Transmembrane transport of signal molecules by receptor molecules 9. The binding of a signaling molecule to its receptor is mediated by A) covalent bonds B) hydrophobic interactions C) ionic interactions D) B and C E) all of the above 10. Lipid soluble signal molecules, such as testosterone, cross the membranes of all cells but affect only target cells because A) only target cells retain the appropriate DNA segments. B) intracellular receptors are present only in target cells. C) most cells lack the Y chromosome required. D) only target cells possess the cytosolic enzymes that transduce the testosterone. E) only in target cells is testosterone able to initiate the phosphorylation cascade leading to activated transcription factor.

4 11. Most signal molecules A) easily diffuse through the membrane and bind to receptors in the cytoplasm or nucleus. B) bind to membrane receptors and transmit information across a membrane without traversing the membrane. C) carry out functions in the nucleus after binding to a receptor in the cell membrane. D) A and C. E) A, B, and C. 12. A signal molecule is also known as a(n). A) ligand B) protein C) initiator D) key E) receptor Refer to the Figures below and answer the following question by choosing the appropriate letter A, B, C, D or E. Figure 1 Figure In Figure 1, which of these is a signal molecule? A 14. In Figure 1, which of these is a receptor molecule? B 15. In Figure 2, which of these receptors is NOT a membrane receptor? E

5 Short Answer and Thought Questions 1. Describe the differences between endocrine, paracrine, and autocrine signaling. In endocrine signaling, signaling molecules are synthesized by one organ and act on distant target cells. In animals the signaling molecule is carried to target cells by the blood or other extracellular fluids. In paracrine signaling, the signaling molecules are released and affect only target cells in close proximity. In autocrine signaling, the cell that releases the signaling molecule is also affected by the released signaling molecule. 2. What are some of the common structural features of the receptors to which signal molecules bind? The molecule must have a signal-binding site on the extracellular side of the membrane, and must have an intracellular domain. Binding of the ligand to the receptor must induce change into another form (transduced) that affects the shape of the intracellular portion, so the signal can be transmitted. 3. What is a disadvantage of using common molecules for signaling paths? When second messengers are involved in more than one signaling pathway, fine tuning and sensitive responses can result. However, when the cross-talk becomes inappropriate, the signaling paths and responses will be in error. 4. What happens when signaling paths are not terminated properly? The cell will not be able to respond properly to new stimuli. The errant signals may lead to cancer, uncontrolled cell growth, or other problems. 5. Compare and contrast signaling by neurons to signaling by endocrine cells. What are the relative advantages of these two mechanisms for cellular communication? Both types of signaling can occur over long distances: neurons can send action potentials along very long axons (from the spinal cord to the fingers, for example), and hormones are passed through the bloodstream throughout the organism. Neurons secrete large amounts of neurotransmitters into a small, well-defined space at the synapse, yielding a high local concentration. Neurotransmitter receptors, therefore, need to bind to neurotransmitters with only low affinity (high Kd). By contrast, hormones are diluted extensively in the bloodstream, where they circulate at minuscule concentrations; hormone receptors, therefore, generally bind their hormones with extremely high affinity (low Kd). Neuronal signaling is very fast, limited only by the speed of propagation of the action potential and the workings of the synapse. In addition to speed, nerves communicate directly with one or a few cells. Hormonal signaling is slower, limited by blood flow and diffusion over relatively large distances, but it communicates at the same time with all the diverse and widely dispersed target cells in the body.

6 6. Cell communicate in ways that resemble human communication. Decide which of the following forms of human communication are analogous to autocrine, paracrine, endocrine and synaptic signaling by cells. A. A telephone conversation B. Talking to people at a cocktail party C. A radio announcement D. Talking to yourself A. A telephone conversation is analogous to synaptic signaling in the sense that it is a private communication from one person to another, usually some distance away and sometimes very far away. It differs from synaptic signaling because it is (usually) a two-way exchange, whereas synaptic signaling is a one-way communication. B. Talking to people at a cocktail party is analogous to paracrine signaling, which occurs between different cells (individuals) and is locally confined. C. A radio announcement is analogous to an endocrine signal, which is sent out to the whole body (the audience) with only target cells (individuals tuned to the specific radio station) affected by it. D. Talking to yourself is analogous to an autocrine signal, which is a signal that is sent and received by the same cell. 7. How is it that different cells can respond in different ways to exactly the same signaling molecule even when they have identical receptors. Cells with identical receptors can respond differently to the same signal molecule because of differences in the internal machinery to which the receptors are coupled. Even when the entire signaling pathway is the same, cells can respond differently if they express different effector proteins at the ends of the pathways. 8. Two intracellular molecules, A and B, are normally synthesised at a constant rate of 1000 molecules per second per cell. Each molecule of A survives an average of 100 seconds, while each molecule of B survives an average of 10 seconds. A. How many molecules of A and B will a cell contain (at equilibrium)? B. If the rates of synthesis of both A and B were suddenly increased 10-fold to 10,000 molecules per second without any change in their average life spans how many molecules molecules of A and B will be present 1 second after the increase in rate of synthesis? C. Which molecule would be the preferred for rapid signaling? Explain your answer. A. A cell will contain 100,000 molecules of A and 10,000 molecules of B at these rates of synthesis and average lifetimes. The number of molecules equals the rate of synthesis times the average lifetime. For A, the number of molecules = (1000 molecules/sec) x (100 sec). B. After 1 second, the number of A molecules will have increased by 10,000 to a total of about 110,000 molecules per cell a 10% increase over the number present before the boost in synthesis. The number of B molecules will also increase by 10,000 to a total of about 20,000, which represents a doubling of its concentration. (For simplicity, the breakdown in A and B over a one-second interval can be neglected.) C. Because of its larger proportional increase in the short term, molecule B would be the preferred signal molecule. Note that after a sufficiently long time both molecules would increase by a factor of 10 in response to a 10-fold increase in rate of synthesis. For signaling it is the rapidity of the change that is most critical. This calculation illustrates the surprising principle that the time it takes to switch a signal on is determined by the lifetime of the signal molecule.