Final answer:
Alanine is a hydrophobic amino acid, characterized by its aliphatic side chain, which makes it insoluble in water and prone to form hydrophobic interactions inside protein structures.
Step-by-step explanation:
The amino acid alanine is classified as hydrophobic. This classification is based on the characteristics of alanine's side chain, which is aliphatic. In simple terms, it lacks polar or charged groups that would make it soluble in water. This property makes alanine typically found in the interior of proteins, where it can avoid contact with water and participate in hydrophobic interactions.
Several amino acids are hydrophobic, and these include, among others, alanine, valine, leucine, and phenylalanine. They are not soluble in water and tend to cluster together within a protein structure, away from the aqueous environment. On the other hand, hydrophilic amino acids, such as lysine and arginine, have side chains that interact favorably with water and are thus often found on the exterior of proteins.