Final answer:
Rice lacks the essential amino acid lysine.
Step-by-step explanation:
Rice and other grains are considered incomplete proteins because they do not contain adequate amounts of all the essential amino acids. Each protein has a limiting amino acid, the one that is present in the smallest amount, which determines its completeness. In the case of rice, it lacks the essential amino acid lysine. Lysine is an important amino acid for the body as it is involved in protein synthesis and plays a role in immune function and calcium absorption. Therefore, to obtain all the essential amino acids, it is important for vegetarians and individuals who rely heavily on grains to consume a variety of plant protein sources that complement each other.