Membrane Structure Chapter 11 Questions in this chapter you should be able to answer: Chapter 11- #s1 - 19 Membrane Structure

Membranes are described as a “2-dimensional liquid”. Why? Jmol membrane model Laser Tweezers Membrane Fluidity Membrane Structure

How can we measure membrane fluidity? FRAP: ‘Fluorescence Recovery After Photobleaching’ GFP: Green fluorescent protein Cell fusion also shows fluidity See Figure 11-30 P 378 FRAP Membrane Structure

What factors determine how fluid a membrane is? Properties of P-lipids: Chain length Saturation Properties of membranes: Cholesterol content Cytoskeleton association Membrane Structure

What are the principal membrane lipids? Phospholipids Glycolipids Other membrane lipids (not phospholipids) Cholesterol Cerebrosides Sphingolipids Ceramide Membrane Structure

How are P-lipids distributed in lipid bilayers? Scramblase vs Flippase P-lipids are synthesized on the ER membrane…. How do they get to other side of membrane? How is asymmetry achieved? Membrane Structure

What are the principal functions of membrane proteins? How are membrane proteins connected to the membrane? Membrane associated (peripheral) Covalent vs Noncovalent Transmembrane (integral) Single-pass Multi-pass Membrane Structure

Why do transmembrane proteins occur as alpha helices and beta-pleated sheets?? Jmol Transmembrane proteins Membrane Structure

How can membrane proteins be purified and studied? -- detergents ‘mimic’ P-lipid structure around proteins Question 11-5 Why is red part hydrophilic and blue part hydrophobic? Membrane Structure

the structure of bacteriorhodopsin? What do we know about the structure of bacteriorhodopsin? Function? Structure? Mechanism? Bacteriorhodopsin Membrane Structure

How is the cell membrane structurally reinforced? -- Cell cytoskeleton -- also influences fluidity Membrane Structure

Why are carbohydrates particularly abundant on the cell surface? Functions: Surface protectant Cell recognition Cell adhesion -- extracellular matrix Membrane Structure

How can protein movement in cell membrane be restricted? Consider challenge of intestinal epithelium… Fig 12-16 p 395 Membrane Structure

After urea wash and centrifugation You have isolated two mutants of a normally pear-shaped microorganism that have lost their distinctive shape and are now round. One of the mutants has a defect in a protein you call A and the other has a defect in a protein you call B. You grind up mutant and normal cells separately and separate the plasma membranes from the cytoplasm by centrifugation. You then wash the membrane fraction with a low concentration of urea ( which disrupts their ability to interact with other proteins) and centrifuge the mixture. The membranes and their constituent proteins form a pellet while the proteins liberated by the urea wash remain in the supernatant. When you check each of the fractions for the presence of A or B, you obtain the results given below. First cell extract After urea wash and centrifugation Membrane Cytosol Supernatant Normal cells A and B no A or B B A Mutant A Mutant B Answer the following statements about your results? (a) Which is an integral and which is a peripheral membrane-associated protein. The results for which cell-type shows this? (b) How does the mutation to protein-A alter its properties? (c) How does the mutation to protein-B alter its properties? (d) Which result(s) most indicate an interaction between A and B? Explain. Membrane Structure