18. Membranes from cells grown in media enriched with stearate are less fluid than normal membranes. This is because
(A) there are more transmembrane proteins.
(B) the membranes probably have less sterols.
(C) the membranes have a lower transition temperature.
(D) the membranes have more saturated fatty acids.
(E) the membranes have more unsaturated fatty acids.

57. About cell membrane, which of the following statements is NOT TRUE? 
(A) Cell membranes are fluid mosaics of lipid and proteins, and the phospholipid bilayer is the basic building block of the membrane. 
(B) Most of the phospholipids can shift about laterally in the plane of the membrane, but it is quite rare for phospholipids to flip-flop transversely across the membrane because the hydrophilic part of the molecule must cross the hydrophobic interior of the membrane. 
(C) Phospholipids with saturated hydrocarbon tail pack more tightly, therefore decreasing membrane fluidity and viscosity. 
(D) The presence of unsaturated hydrocarbon tails in phospholipids can enhance membrane fluidity, because unsaturated hydrocarbon tails cannot pack together as closely as saturated carbon tails. 
(E) The steroid cholesterol in the plasma membrane of animal cells can change membrane fluidity and this effect is temperature-dependent.