Chapter 13 Membrane Channels and Pumps 8 49.What is the"ball-and-chain"model? Ans:"Ball-and-chain"is a model of inactivation of a pore by occlusion.In this model a group of amino acids forms a"ball,"which is attached by a flexible peptide to the rest of the protein. After the channel opens,the ball moves into a site that blocks the channel,thus quickly inactivating the channel function. Section:13.4 50.Why are gap junctions sealed when high concentrations of calcium ions and protons are present? Ans:These conditions are common when cells are dying or traumatized.Under these conditions the gap junctions close,so unhealthy neighbors do not compromise the healthy cells. Section:13.5 51.What are some of the functions of gap cells? Ans:In addition to intracellular communication,the junctions are important in nourishment of cells,coupling with ion transport,development,and differentiation. Section:13.5 52.Describe how a voltage-gated channel operates. Ans:The voltage-gated channel is composed of segments S1-S6.The pore is formed by S5 and S6,and the "voltage-sensing paddles"that close the channel are composed of S1-S4.The paddles lie in the"down"position below the closed channel.Membrane depolarization pulls these channels through the membrane opening the channel. Section:13.4;Figure 13.23Chapter 13 Membrane Channels and Pumps 8 49. What is the “ball-and-chain” model? Ans: “Ball-and-chain” is a model of inactivation of a pore by occlusion. In this model a group of amino acids forms a “ball,” which is attached by a flexible peptide to the rest of the protein. After the channel opens, the ball moves into a site that blocks the channel, thus quickly inactivating the channel function. Section: 13.4 50. Why are gap junctions sealed when high concentrations of calcium ions and protons are present? Ans: These conditions are common when cells are dying or traumatized. Under these conditions the gap junctions close, so unhealthy neighbors do not compromise the healthy cells. Section: 13.5 51. What are some of the functions of gap cells? Ans: In addition to intracellular communication, the junctions are important in nourishment of cells, coupling with ion transport, development, and differentiation. Section: 13.5 52. Describe how a voltage-gated channel operates. Ans: The voltage-gated channel is composed of segments S1S6. The pore is formed by S5 and S6, and the “voltage-sensing paddles” that close the channel are composed of S1S4. The paddles lie in the “down” position below the closed channel. Membrane depolarization pulls these channels through the membrane opening the channel. Section: 13.4; Figure 13.23