Here's why:

* Carboxyl group (COOH): This group has a carbon atom double-bonded to an oxygen atom and single-bonded to a hydroxyl group (-OH). The hydrogen atom in the hydroxyl group can dissociate as a proton (H+), giving the carboxyl group acidic properties.

* Amino group (NH2): This group contains a nitrogen atom bonded to two hydrogen atoms. It's basic in nature and can accept a proton (H+).

So, the carboxyl group is what makes amino acids acidic, while the amino group is what makes them basic. This balance of acidic and basic groups gives amino acids their unique properties and allows them to act as buffers, helping to maintain the pH of biological systems.