细胞生物学资料：02课堂思考（9.15—9.23）

1、Testing and Thinking(9-15-2004)10-33 Proteins that form a b-barrel pore in the membrane have several b strands that span the membrane. The pore-facing side of each strand carries hydrophilic amino acid side chains, whereas the bilayer-facing sides carry hydrophobic amino acid side chains. Which of t

2、he three 10-amino acid sequences listed below is the most likely candidate for a transmembrane b strand in a b-barrel pore? Explain the reasons for your choice. (See inside back cover for one-letter amino acid code.) A. A D F K L S V E L T B. A F L V L D K S E TC. A F D K L V S E L T10-25 (True/Fals

3、e) The basic structure of biological membranes is determined by the lipid bilayer, but their specific functions are carried out largely by proteins. Explain your answer.Answer（9-15-2004）10-33 A. In a b strand, adjacent amino acid side chains protrude from opposite sides of the strand; thus, every ot

4、her amino acid side chain will face the same side of the strand. If a b strand is part of a b-barrel pore, its amino acid side chains will alternate between hydrophobic and hydrophilic, so that one side of the strand will be hydrophobic and the other side will be hydrophilic. Only choice A has alter

5、nating hydrophobic and hydrophilic amino acids.10-25 True. The lipid bilayer defines the structure of the membrane and provides a permeability barrier that separates the inside from the outside of the cell. Specific membrane proteins allow particular solutes to enter and leave the cell, bind signali

6、ng molecules, and mediate attachment to the extracellular matrix.Testing and Thinking(9-20-2004)11-3 If a frog egg and a red blood cell are placed in pure water, the red blood cell will swell and burst, but the frog egg will remain intact. Although a frog egg is about one million times larger than a

7、 red cell, they both have nearly identical internal concentrations of ions so that the same osmotic forces are at work in each. Why do you suppose red blood cells burst in water, while frog eggs do not?11-3 Red blood cells have water channels-aquaporins-that make them about 10-fold more permeable to

8、 water than a lipid bilayer. Frog eggs do not express aquaporins and thus their permeability to water is roughly that of a lipid bilayer. However, if there were only a 10-fold difference in permeability to water, wouldnt a frog egg still burst, but just take ten times as long to do so? A part of the

9、 answer lies in the enormous volume difference between a red cell and a frog egg, more specifically in the surface-to-volume ratio. Assuming both are spheres, which of course red cells are not, the 106-fold difference in volume translates into a 100-fold lower surface-to-volume ratio in the egg. (Th

10、is is an underestimate of the difference in the surface-to-volume ratio because the red cell biconcave disc has a larger surface-to-volume ratio than does a sphere of the same volume.) Thus, in the absence of aquaporins the egg should take up water at more than a lO00-fold lower rate than a red cell

11、. If aquaporins are engineered to be expressed into frog eggs, they also swell and burst when placed in water.19. When present in the culture medium, ouabain inhibits the Na+/K+ ATPase in cultured cells, but when the drug is microinjected into cells, it exerts no inhibitory effect. Although ouabain

12、itself possesses no negative charge, ouabain treatment causes the a subunits of the Na+/K+ ATPase to become more negatively charged but does not alter the charge properties of the b subunits.a. On which side of the plasma membrane are the ouabain and ATP-binding sites of the enzyme located?b. During

13、 the transport process, the Na+/K+ ATPase becomes transiently phosphorylated. What do these observations suggest about a likely mechanism for ouabain inhibition of this ion pump?c. Which of the two subunits, a or b, is most likely to be phosphorylated?19a. The ouabain-binding site of the Na+/K+ ATPa

14、se must be located on the exoplasmic face of the plasma membrane, as microinjected ouabain does not inhibit the ATPase. The ATP-binding site is located on the cytoplasmic face of the plasma membrane, where it has access to ATP in the cytosol.19b. The addition of a phosphate group to the enzyme would

15、 cause it to become more negatively charged. In the presence of extracellular ouabain, this negative charge, which normally is temporary, becomes permanent. Most likely, ouabain treatment inhibits the Na+/K+ ATPase by blocking the reaction at the point when phosphate has been added to the enzyme. Th

16、us the phosphorylated form of the enzyme accumulates, blocking further ion transport. 19c. The observation that ouabain treatment results in the a subunit becoming more negatively charged but has no effect on the charge of the b subunit suggests that the a subunit is phosphorylated. This hypothesis

17、could be tested by investigating the effect of a phosphatase on the charge properties of the a subunit following ouabain treatment.Testing and Thinking(9-23-2004)1.(False/True): Because of their rigid structure, proteoglycans can withstand a large amount of compressive force.2.Embryos injected with

18、antibodies to fibronectin show inhibited movement of neural crest cells during development of the nervous system. These experiments show that:a.development of the neural crest involves the expression of antibody genes.b.developing neurons must synthesize fibronectin.C.fibronectin-antibody complexes

19、form pathways for neural migration.d.neurons in embryos must transiently bind to fibronectin during migration.3. Heating of calf type I collagen fibers to 450C denatures the triple helices and separates the three chains from each other. Collagen that has been treated in this way does not renature to form a normal collagen triple helix. Why?Answer:1. False. Proteoglycans can withstand a large amount of compressive force but do not have a rigid structure. Thei