A. Membrane lipid molecules exchange places with their lipid neighbors every 10^–7 second. A lipid molecule diffuses from one end of a 2-μm-long bacterial cell to the other in about 0.2 seconds. Are these two numbers in agreement (assume that the diameter of a lipid head group is about 0.5 nm)? If not, can you think of a reason for the difference?

B. To get an appreciation for the great speed of molecular diffusion, assume that a lipid head group is about the size of a ping-pong ball (4 cm in diameter) and that the floor of your living room (6 m × 6 m) is covered wall-to-wall with these balls. If two neighboring balls exchanged positions once every 10^–7 second, what would their speed be in kilometers per hour? How long would it take for a ball to move from one side of the room to  opposite side?