Items 70-72

The following set of items pertains to membrane fluidity. For each, choose the ONE best answer.

70. What of the following is the LEAST LIKELY event?

(A) Polar lipids translocation laterally in the plane on the outer leaflet.
(B) Polar lipids rotate about their long axis.
(C) Fatty acid side chains of phospholipids vibrate like a tuning fork.
(D) Diffusion of integral membrane proteins laterally in the plane of the outer leaflet.
(E) Exchange of phospholipids between the inner and outer leaflets.

71. Which of the following has the most dramatic effect on the rate of diffusion of integral membrane proteins?

(A) protein synthesis inhibitors
(B) ATP synthesis inhibitors
(C) cyanide
(D) lowering temperature from 37°C. to O°C.
(E) lowering cholesterol concentration

72. Which of the following is the MOST ACCURATE statement concerning the rate of diffusion of proteins in the plane of membranes?

(A) Cholesterol concentrations in membrane have little effect on diffusion rate.
(B) Integral membrane protein linkage to the cytoskeleton limits lateral diffusion in living cell membrane.
(C) Proteins diffuse in artificial lipid membranes more slowly than in living natural membranes.
(D) The rate of diffusion of integral membrane proteins in living biological membranes is approximately 10 -8 cm²/sec.

ANSWERS AND TUTORIAL ON ITEMS 70-72

The answers are:70-E;71-D;72-B. The plasma of eukaryotic cells consists of a bilayer of phospholipids and associated integral membrane proteins and peripheral membrane proteins. The phospholipids are polar molecules with hydrophilic (charged groups in phospholipids) and hydrophobic portions (fatty acids). The charged portions associate with water either the extracellular face or the cytoplasmic face of the membrane. The hydrophobic self-associate in the internal domain of the membrane .

Prospholipids are freely mobile in the plane of the membrane. This occurs rapidly at approximately 10 -8 cm²/sec. All other movements described in Item 70 also occur. The energetic to break hydrophibic associations and allow a polar lipid molecule to move between planes of the membrane, however, are the largest. Therefore this movement is least likely. Integral membrane proteins diffuse within membranes without the need for protein synthesis or ATP. Although their mobility would be moderately reduced by decreased cholesterol concentrations, clearly, the most dramatic effect would be from lowering temperature.

The rate of diffusion of integral membrane proteins in artificial membranes is an order of magnitude (or more) greater than that in living membranes, where diffusion occurs at about 10 -11 cm²/sec. The chief factor that limits diffusion of integral membrane protein in living membranes is the anchorage of these proteins to cytoskeletal elements. Field Nature - Portable Microscopes, Fluorescence Microscopes