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12 Cards in this Set
- Front
- Back
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Explain why an alpha helix is especially suitable for a trans-membrane protein segment?
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because the alpha helix structure involves the formation of hydrogen bonds between the amino and carboxyl groups of the peptide bond, it neutralizes the partial charges associated with those groups associated with the amino acids of the helical segment are hydrophobic, the segment can stably associate with the membrane.
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Certain fish living in desert streams alter their membrane-lipid composition in the transition from the heat of the day to the cool of the night. Predict the nature of the changes.
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To maintain fluidity at the lower night-time temperature, the cells of the fish would have to alter their membrane composition to have shorter, less saturated fatty acid chains present in their membrane phospholipids.
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A plasma membrane sodium.glucose symporter is able to transport glucose into the cell against a concentration gradient, yet it is not an ATPase. Explain how this can happen.
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There is sodium concentration gradient with its high end outside the cell. Sodium ions flow into the cell via this symport releasing energy to support the transport into the cell against the concentration gradient. The sodium concentration gradient is established by a nearby sodium pump using energy from the hydrolysis of ATP.
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Describe three ways that water molecules might use to pass through biological membranes.
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Pass through the bilayer (small, high concentration, non-charged)
accompanying ions passing through channels aquaporins(water channels) |
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G proteins are sometimes viewed as "self-regulating"switches. Explain how G-proteins are activated and how they are inactivated.
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The G-protein cycle has two important states, the on and off states. The on state has a GTP bound to it. After triggering downstream events, GTP is hydrolyzed to GDP and Pi, converting the G-protein to the OFF state.
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GTP-gamma-S is an analog of GTP, which cannot be hydrolyzed to GDP and Pi. What is the effect of GTP-gamma-S on the function of the heterotrimeric G-protein?
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If GTP is replaced with the non-hydrolyzable GTP-gamma-S, this ON to OFF conversion will not take place, and the system will remain ON all the time.
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G-proteins usually have GTPase activity capable of hydrolyzing GTP to GDP+Pi. What would happen if this activity is lost?
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This will keep the G-protein at the ON state and the downstream event will continue even after the original signal is no longer present.
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Lipid soluble hormones, such as testosterone, cross plasma membranes of all cells but affect only certain target cells. Explain why this might be true?
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Only cells with specific (cystolic) receptors capable of recognizing this hormone can respond to its presence.
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cAMP is a second messenger for many functions in many cells. How can the same messenger act in different ways in different cells?
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The cAMP responding system is different in different cells, such as different types of kinases responding to cAMP in different cells, thus cAMP can elicit different responses to cAMP. FOr example, cAMP in liver cells decreases the activity of glycogen synthase, but at the same time enhances the activity of protein kinase A, eventually leading to glycogen degredation. In other cells, cAMP is known to regulate ion channel activities.
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How does caffeine increase alertness?
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The structure of caffeine is similar to adenosine, thus it blocks the adenosine effect on reducing arousal.
It is also an inhibitor of phosphodiesterase, thus causing cGMP level to increase, leading to smooth muscle relaxation and dilation of blood vessels. Caffeine is an inhibitor of phosphodiesterase, thus also causing cAMP level to increase, leading to increase in blood sugar level. |
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Both IP3 and DAG are derived from the same original phospholipid molecule but why IPD travels fast in the cytosol and DAG travels fast in the membrane?
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IP3 is highly hydrophobic thus can travel fast in teh aqueous environment of cytosol. DAG is hydrophobic thus moves preferentially in the membrane.
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Some protein kinases are inactive unless they are phosphorylated on key serine and threonine residues. In some cases, active enzymes can be generated by mutating these serine or threonine residues to aspartate or glutamate. Propose an explanation.
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When serine is phosphorylated, its R-group resembles that of an aspartic acid. Similarly, a phosphorylated threonine is similar to glutamic acid.
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