Which is a basic unit of a sugar molecule? a)amino acid b)fatty acid c)monosaccharide d)nucleotide which nitrogen base replaces thymine (t) in rna? a)uracil (u) b)cytosine(c) c)adenine (a) d)guanine (g) which is a carbohydrate? a)nucleic acid b)polysaccharide c)amino acid d)nucleotide what kinds of lipids make up the bilayer that forms cell membranes? a)phospholipids b)saturated fats c)steroids d)unsaturated fats how does an enzyme’s active site relate to its substrate? a)one is soluble in water, and the other is insoluble. b)they are identical in chemical makeup. c)their shapes fit snugly together. d)they are both used up by a single chemical reaction.