CHAPTER 12 MEMBRANE TRANSPORT 2009 Garland Science Publishing Principles of Membrane Transport 12 1 Indicate whether the following statements are true or false If a statement is false explain why it is false A CO2 and O2 are water soluble molecules that diffuse freely across cell membranes B The differences in permeability between artificial lipid bilayers and cell membranes arise from variations in phospholipid content C Transporters are similar to channels except that they are larger allowing folded proteins as well as smaller organic molecules to pass through them D Cells expend energy in the form of ATP hydrolysis so as to maintain ion concentrations that differ from those found outside the cell 12 2 Although the extracellular environment has a high sodium ion concentration and the intracellular environment has a high potassium ion concentration both must be neutralized by negatively charged molecules In the extracellular case what is the principal anion a HCO3 b Cl c PO43 d OH 12 3 Circle the molecule in each pair that is more likely to diffuse through the lipid bilayer A amino acids or benzene B Cl or ethanol C glycerol or RNA D H2O or O2 E adenosine or ATP 12 4 We can test the relative permeability of a phospholipid bilayer by using a synthetic membrane that does not contain any protein components Some uncharged polar molecules are found to diffuse freely across these membranes to varying degrees Which of the following has the lowest rate of diffusion across an artificial membrane Why a glucose b water c glycerol d ethanol 12 5 For each of the following sentences fill in the blanks with the best word or phrase selected from the list below Not all words or phrases will be used each word or phrase should be used only once A molecule moves down its concentration gradient by transport but requires transport to move up its concentration gradient Transporter proteins and ion channels function in membrane transport by providing a pathway through the membrane for specific polar solutes or inorganic ions are highly selective in the solutes they transport binding the solute at a specific site and changing its conformation so as to transport the solute across the membrane On the other hand discriminate between solutes mainly on the basis of size and electrical charge active amino acid amphipathic transporter proteins hydrophilic hydrophobic ion channels noncovalent passive Transporters and their Functions 12 6 A hungry yeast cell lands in a vat of grape juice and begins to feast on the sugars there producing carbon dioxide and ethanol in the process C6H12O6 2ADP 2Pi H 2CO2 2CH3CH2OH 2ATP 2H2O Unfortunately the grape juice is contaminated with proteases that attack some of the transport proteins in the yeast cell membrane and the yeast cell dies Which of the following could account for the yeast cell s demise a toxic buildup of carbon dioxide inside the cell b toxic buildup of ethanol inside the cell c diffusion of ATP out of the cell d inability to import sugar into the cell 12 7 Ion channels are classified as membrane transport proteins Channels discriminate by size and charge In addition to Na which one of the following ions would you expect to be able to freely diffuse through a Na channel Explain your answer a Mg2 b H c K d Cl 12 8 Transporters in contrast to channels work by a specific binding to solutes b a gating mechanism c filtering solutes by charge d 12 9 filtering solutes by size Pumps are transporters that are able to harness energy provided by other components in the cells to drive the movement of solutes across membranes against their concentration gradient This type of transport is called a active transport b free diffusion c facilitated diffusion d passive transport 12 10 Indicate whether the statements below are true or false If a statement is false explain why it is false A Facilitated diffusion can be described as the favorable movement of one solute down its concentration gradient being coupled with the unfavorable movement of a second solute up its concentration gradient B Transporters undergo transitions between different conformations depending on whether the substrate binding pocket is empty or occupied C The electrochemical gradient for K across the plasma membrane is small Therefore any movement of K from the inside to the outside of the cell is driven solely by its concentration gradient D The net negative charge on the cytosolic side of the membrane enhances the rate of glucose import into the cell by a uniporter 12 11 It is thought that the glucose transporter switches between two conformational states in a completely random fashion How is it possible for such a system to move glucose across the membrane efficiently in a single direction 12 12 Active transport requires the input of energy into a system so as to move solutes against their electrochemical and concentration gradients Which of the following is not one of the common ways to perform active transport a Na coupled b K coupled c ATP driven d light driven 12 13 The Na K ATPase is also known as the Na K pump It is responsible for maintaining the high extracellular sodium ion concentration and the high intracellular potassium ion concentration What happens immediately after the pump hydrolyzes ATP a Na is bound b ADP is bound c The pump is phosphorylated d The pump changes conformation 12 14 If ATP production is blocked in an animal cell the cell will swell up Explain this observation 12 15 Fill in Table Q12 15 In the type of transport column designate whether the transporter works by uniport symport or antiport mechanisms Table Q12 15 12 16 You have prepared lipid vesicles spherical lipid bilayers that contain Na K pumps as the sole membrane protein All of the Na K pumps are oriented in such a way that the portion of the molecule that normally faces the cytosol is on the inside of the vesicle and the portion of the molecule that normally faces the extracellular space is on the outside of the vesicle Assume that each pump transports one Na ion in one direction and one K ion in the other direction during each pumping cycle see Figure Q12 16 for how the Na K pump normally functions in the plasma membrane Figure 12 16 Predict what would happen in each of the following conditions A The solutions inside and outside the vesicles contain both Na and K ions but no ATP B The solution outside the vesicles contains both Na and K ions the solution inside contains both Na and K ions and ATP C The solution