Amino acids carry both an amino group and a carboxyl group. Which of the statements below are correct?

• Statement I: In aqueous solution an amino acid exists mainly as a dipolar ion called a zwitterion.
• Statement II: Amino acids are amphoteric, reacting with both acids and bases.
• Statement III: Amino acids that the body cannot synthesise are called essential amino acids.
• Statement IV: Glycine is optically active.