Uniporters and ion channels support facilitated transport across cellular membranes. Although both are examples of facilitated transport, the rates of ion movement via an ion channel are roughly 104- to 105-fold faster than the rates of molecule movement via a uniporter. What key mechanistic difference results in this large difference in transport rate? What contribution to free energy (ΔG) determines the direction of transport?

Uniporters and ion channels support facilitated transport across cellular membranes. Although both are examples of facilitated transport, the rates of ion movement via an ion channel are roughly 104- to 105-fold faster than the rates of molecule movement via a uniporter. What key mechanistic difference results in this large difference in transport rate? What contribution to free energy (ΔG) determines the direction of transport?

Human Physiology: From Cells to Systems (MindTap Course List)

9th Edition

ISBN:9781285866932

Author:Lauralee Sherwood

Publisher:Lauralee Sherwood

Chapter3: The Plasma Membrane And Membrane Potential

Section: Chapter Questions

Problem 2SQE: One of the important uses of the Nernst equation is in describing the flow of ions across plasma...

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One of the important uses of the Nernst equation is in describing the flow of ions across plasma membranes. Ions move under the influence of two forces: the concentration gradient (given in electrical units by the Nernst equation) and the electrical gradient (given by the membrane voltage). This is summarized by Ohms law: Ix=Gx(VmEx) which describes the movement of ion x across the membrane. I is the current in amperes (A); G is the conductance, a measure of the permeability of x, in Siemens (S), which is I/V;Vm is the membrane voltage; and Ex is the equilibrium potential of ion x. Not only does this equation tell how large the current is, but it also tells what direction the current is flowing. By convention, a negative value of the current represents either a positive ion entering the cell or a negative ion leaving the cell. The opposite is true of a positive value of the current. a. Using the following information, calculate the magnitude of Na [ Na+ ]0=145mM,[ Na+ ]i=15mM,Gna+=1nS,Vm=70mV b. Is Na+ entering or leaving the cell? c. Is Na+ moving with or against the concentration gradient? Is it moving with or against the electrical gradient?
Assume that a membrane permeable to Na+ but not to Cl- separates two solutions. The concentration of sodium chloride on side 1 is higher than on side 2. Which of the following ionic movements would occur? a. Na+ would move until its concentration gradient is dissipated (until the concentration of Na+ on side 2 is the same as the concentration of Na+ on side 1). b. Cl- would move down its concentration gradient from side 1 to side 2. c. A membrane potential, negative on side 1, would develop. d. A membrane potential, positive on side 1, would develop. e. None of the preceding is correct.
Uniporters and ion channels support facilitated transport across cellular membranes. Although both are examples of facilitated transport, the rates of ion movement via an ion channel are roughly 104 - to 105 -fold faster than the rates of molecule movement via a uniporter. What key mechanisticdifference results in this large difference in transport rate?What contribution to free energy (ΔG) determines the direction of transport?
In the situations described below, what is the free energy change if 1 mole of Na+ is transported across a membrane from a region where the concentration is 48 μM to a region where it is 110 mM? (Assume T=37∘C.) When the transport is opposed by a membrane potential of 70 mV.
For each type of membrane transport, know the following:– Is a transporter protein required? If so, what type?– Is there an energy requirement, and if so, what is the energy source?– What is the relative rate of solute transport based on molecule type? On concentration gradient?– What are examples of the types of solutes transported by carriers and channels?
Glucose transport across cell membranes varies depending upon blood glucose levels. When glucose levels are high, glucose transport is accomplished via membrane transporters. When glucose concentrations are low, the transport of glucose across the membrane is dependent upon the sodium ion concentration. What types of transport is observed for glucose? A)simple diffusion at high [glucose], secondary active transport at low [glucose] B)facilitated diffusion at high [glucose], secondary active transport at low [glucose] C)simple diffusion at high [glucose], primary active transport at low [glucose] D)facilitated diffusion at high [glucose], primary active transport at low [glucose]
During an investigation on membrane transport, a researcher exposed bacterial cells to different concentrations of two different solutes: A and B. The rate of transport of each solute into cells isrepresented in the graphSolute ASolute BSolute ConcentrationWhich of the following best explains the greater rate of transport for solute A than for solute B at higher solute concentrations?A Solute A is being transported by simple diffusion, which does not rely on membrane proteins to control the rate of transportSolute A is being transported by active transport, which uses ATP and has higher rates of transport than passive transportSolute A is being transported by facilitated diffusion, which uses membrane proteins to increase the rate of transportRate of Transport
Ouabain is a specific inhibitor of the active transport of sodium ions out of the cell and is therefore a valuable tool in studies of membrane transport mechanisms. Which of the following processes in your own body would you expect to be sensitive to inhibition by ouabain? Explain your answer in each case. a) Facilitated diffusion of glucose into a muscle cell b) Active transport of dietary phenylalanine across the intestinal mucosa c) Uptake of potassium ions by red blood cells d) Active uptake of lactose by the bacteria in your intestine
Discuss carrier-mediated transport. How could you experimentally distinguish between the different types of carrier-mediated transport?
Name the three classes of membrane transport proteins. Explain which one or ones of these classes is able to move glucose and which can move bicarbonate (HCO3 −) against an electrochemical gradient. In the case of bicarbonate, but not glucose, the ΔG of the transport process has two terms.What are these two terms, and why does the second not apply to glucose? Why are cotransporters often referred to as examples of secondary active transport?
Name the three classes of membrane transport proteins. Explain which one or ones of these classes is able to move glucose and which can move bicarbonate (HCO3−) against an electrochemical gradient. In the case of bicarbonate, but not glucose, the ΔG of the transport process has two terms. What are these two terms, and why does the second not apply to glucose? Why are cotransporters often referred to as examples of secondary active transport?
Why is the plasma membrane descibed as a semipermeable membrane? why are proteins required to augment the functions of hte phospholipid bilayer that forms the backbone of cell membranes? distinguish between the different types of molecular transport, why are they necessary, how do they enhance cellular functions, why are some cellualar processes free while others require the input of energy?