Which statement best describes protein?

• A. A large group of organic compounds that include sugars, starch, and cellulose and provide energy.
• B. A naturally occurring inorganic solid with a crystal structure.
• C. A class of nitrogenous organic compounds that consist of long chains of amino acids and are essential components of body tissues, enzymes, and antibodies.
• D. A type of lipid that stores energy.

Answer: (C)

Proteins are nitrogen-containing macromolecules built from long chains of amino acids. This description correctly notes that proteins are made of amino acids and are essential components of body tissues, enzymes, and antibodies. The specific sequence of amino acids and how the chain folds gives proteins their unique shapes and functions, such as forming structural materials like collagen, acting as enzymes to speed up chemical reactions, and serving as antibodies to defend the immune system. The other options describe carbohydrates (sugars, starch, cellulose used for energy and structure), minerals (inorganic solids with crystal structures), and lipids (fats that store energy), which are different classes of biomolecules.