What part of an amino acid acts as an acid? (and don't say the whole thing!!)?

I think you're looking for the side chain of the amino acid. The amine and carboxyl ends are used in formation of a polypeptide, but the side group (functional group) is what gives each amino acid its unique properties (acid, base, etc.).

Each amino acid is a Zwitter ion: it contains an amine group (NH2) and a carboxyl group (COOH). Its the dissociation of the hydrogen from the carboxyl group that actually makes it an acid. Some amino acids also have side chains that involve other reactive groups, some of which are capable of acting as an acid, so its worth a look at their structures.

well all amino acids have a carboxyl group which is acidic in that it is a proton donor.Also certain amino acids like aspartic and glutamic acids have side chains that contain carboxyl groups which also contribute to the acidity

R---CH(NH2)-COOH------ R-CH-(NH3+ ) -COO^-1

all amino acids are NH-R-COOH

NH is the amine group

COOH is the acidic group...

so, the ans is COOH.