Final answer:
Protein complementation occurs when incomplete proteins from different plant sources are combined to provide all essential amino acids, like rice and beans combining grains and legumes to create a complete protein profile.
Step-by-step explanation:
Protein complementation is the process of combining incomplete proteins from various plant sources to provide all the essential amino acids that the body cannot produce on its own. A classic example of protein complementation is the combination of grains and legumes, such as rice and beans. This combination is effective because grains typically lack the amino acid lysine, while legumes lack methionine and cysteine. By consuming both throughout the day, even if not at the same meal, the body receives a complete amino acid profile similar to that found in complete proteins from animal sources.