Problem 2
when phospholipids are suspended in water. The edges of these sheets close upon each other and undergo self-sealing to form vesicles (liposomes). a. What properties of lipids are responsible for this property of bilayers? Explain. b. What are the consequences of this property for the structure of biological membranes?
Problem 4
Membrane Proteins What are the three main categories of membrane proteins, and how are they distinguished experimentally?
Problem 5
Location of a Membrane Protein Treatment of disrupted erythrocyte membranes with a concentrated salt solution released an unknown membrane protein, \(X\). Proteolytic enzymes cleaved \(\mathrm{X}\) into fragments. In additional experiments, intact erythrocytes were treated with proteolytic enzymes, washed, then disrupted. Extraction of membrane components yielded intact \(X\). What do these observations indieate about the location of \(X\) in the plasma membrane? Do the properties of X resemble those of an integral membrane protein or a peripheral membrane protein?
Problem 6
You have cloned the gene for a human erythrocyte protein, which you suspect is a membrane protein. You deduce the amino acid sequence of the protein from the nucleotide sequence of the gene. From this sequence alone, how would you evaluate the possibility that the protein is an integral protein? Suppose the protein proves to be an integral protein with one transmembrane segment. Suggest biochemical or chemical experiments that might allow you to determinewhether the protein is oriented with the amino terminus on the outside of the cell or on the inside of the cell.
Problem 8
Molecular Species in the Plasma Membrane The plasma membrane of \(\mathrm{E}\). coli is about \(75 \%\) protein and \(25 \%\) phospholipid by weight. How many molecules of membrane lipid are present for esch molecule of membrane protein? Assume an average protein \(M_{\text {, of }} 50,000\) and an average phospholipid \(M_{\mathrm{r}}\) of 750 . What more would you need to know to estimate the fraction of the membrane surface that is covered by lipids?
Problem 10
Membrane Self-Sealing Cellular membranes are selfsealing - if they are punctured or disrupted mechanically, they quickly and automatically reseal. What properties of membranes are responsible for this important festure?
Problem 11
Lipid Melting Temperatures Membrane lipids in tissue samples obtained from different parts of a reindeer's leg have different fatty acid compositions. Membrane lipids from tissue near the hooves contain a larger proportion of unsaturated fatty acids than those from tissue in the upper leg. What is the significance of this observation?
Problem 12
Flip-Flop Diffusion What is the physical explanation for the very slow movement of membrane phospholipids from one leaflet of a biological membrane to the other? What factors influence this rate?
Problem 13
Bilayer Asymmetry The inner leaflet (monolayer) of the human erythrocyte membrane consists predominantly of phosphatidylethanolamine and phosphatadylserine. The outer leaflet consists predominantly of phosphatidylcholine and sphingomyelin. Although the phospholipid components of the membrane can diffuse in the fluid bilayer, this sidedness is preserved at all times. How?
Problem 14
Scramblase and Flippase Explain the difference between the scramblase enzymes and flippase enzymes based on the membranes with which they are associated, the symmetry of these membranes, and their energy requirements.
