6. Which of the following statements about lipids in biological membranes is correct?(A) The hydrophobic tails of phospholipids are primarily composed of fatty acid chains.(B) Saturated fatty acids contain double bonds and increase membrane fluidity.(C) Cholesterol increases membrane fluidity at all temperatures.(D) Lipids in biological membranes cannot be modified by sugars and therefore do not participate in cell recognition.(E) Natural unsaturated fatty acids commonly contain trans double bonds.

8. Lipids, sugars, and amino acids can be chiral. Which statement about their stereochemical properties in nature is correct?(A) All naturally occurring amino acids in proteins are chiral and have the L-configuration.(B) Fatty acids in biological membranes are predominantly chiral and exist mainly as L-enantiomers.(C) Naturally occurring sugars and amino acids both predominantly adopt the L-configuration.(D) Most monosaccharides in biological systems have the D-configuration, and glycerol-based membrane lipids are synthesized from sn-glycerol-3-phosphate with defined stereochemistry.(E) Biological systems contain racemic mixtures of sugars, amino acids, and lipids due to spontaneous chemical reactions.

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