Which of the following best describes a complete protein?

• A. A protein that contains all nine essential amino acids in adequate proportions.
• B. A protein that cannot be digested by the human body.
• C. A protein that is found only in animal products.
• D. A protein that is composed entirely of non-essential amino acids.

Answer: (A)

Verified step by step guidance

1. Understand the concept of a complete protein: A complete protein is one that contains all nine essential amino acids in adequate proportions. Essential amino acids are those that the human body cannot synthesize and must obtain from the diet.
2. Review the nine essential amino acids: These include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. These amino acids are crucial for various biological functions such as protein synthesis, enzyme activity, and tissue repair.
3. Eliminate incorrect options: A protein that cannot be digested by the human body is not related to the definition of a complete protein. Similarly, a protein found only in animal products is incorrect because complete proteins can also be found in certain plant-based sources like quinoa and soy. Lastly, a protein composed entirely of non-essential amino acids does not meet the criteria for a complete protein.
4. Focus on the correct option: A complete protein must contain all nine essential amino acids in adequate proportions. This is the defining characteristic of a complete protein.
5. Relate to real-world examples: Examples of complete proteins include animal-based sources like eggs, milk, and meat, as well as plant-based sources like soy and quinoa. This helps reinforce the concept and provides practical context.