Final answer:
Histidine is found in both essential and non-essential amino acids due to its unique chemical nature and ionization properties. While it is considered essential for the growth and development of young children, adults can synthesize histidine in their bodies.
Step-by-step explanation:
The amino acid histidine is found in both essential and non-essential amino acids because it has a unique chemical nature.
Histidine is considered to be a semi-essential amino acid, meaning that it is essential for certain individuals or in certain situations. While it can be synthesized by the body, there are circumstances where histidine needs to be obtained from the diet.
Histidine has a side chain with an amine group that can be ionized and carries a positive charge at certain pH levels. This property makes histidine unique among the other amino acids and gives it specific roles in protein function and enzyme reactions.
Overall, histidine is classified as an essential amino acid because it is necessary for the growth and development of young children. However, for adults, histidine can be synthesized in the body and is not considered essential under normal conditions.