presence of solutes, eliminating choice (D). Pyrophosphate and inorganic phosphate are small,
charged molecules which are relatively soluble, eliminating choice (A).13 . B
The amino acid in question is aspartic acid, which is an acidic amino acid because it contains an
extra carboxyl group. At neutral pH, both of the carboxyl groups are ionized, so there are two
negative charges on the molecule. Only one of the charges is neutralized by the positive charge
on the amino group, so the molecule has an overall negative charge.
14 . A
As DNA is synthesized, it forms phosphodiester bonds, releasing pyrophosphate, PPi.
Pyrophosphate is an inorganic phosphate-containing molecule, but it is not the single phosphate
group commonly referred to as inorganic phosphate, choice (B). The DNA molecule itself is
referred to as an organic phosphate, choice (D).
15 . B
Phosphoric acid has three hydrogens with pKa values spread across the pH range. This allows
some degree of buffering over almost the entire standard pH range from 0 to 14.