Final answer:
A limiting amino acid is an essential amino acid that is present in the lowest amount relative to the body's requirement for protein synthesis. Proteins play a role in body repair and building, as well as serving as enzymes and hormones. An example of complementary proteins is rice and beans, and a source of vitamin B12 is animal products.
Step-by-step explanation:
A) A limiting amino acid is an essential amino acid that is present in the lowest amount relative to the body's requirement for protein synthesis. Its availability can limit the rate at which proteins are synthesized. For example, if a protein lacks enough of the essential amino acid leucine, leucine is considered the limiting amino acid.
B) Two roles of protein in the body are:
- Body repair and building: Proteins are necessary for the repair and growth of tissues in the body. They contribute to the formation of muscles, skin, hair, nails, and organs.
- Enzymes and hormones: Proteins act as enzymes, which facilitate chemical reactions in the body. They also serve as hormones, which regulate various physiological processes.
C) An example of complementary proteins is rice and beans. Rice is deficient in the amino acid lysine, while beans are deficient in the amino acid methionine. When consumed together, rice and beans provide a complete set of essential amino acids.
D) One source of vitamin B12 is animal products such as meat, fish, eggs, and dairy. Vitamin B12 is important for the production of red blood cells and the maintenance of a healthy nervous system.