Final answer:
Proteins contain nitrogen, an element that fats and carbohydrates lack, and are composed of amino acids that form the backbone of protein molecules.
Step-by-step explanation:
Proteins are complex organic compounds essential to all living organisms. Unlike carbohydrates and lipids, proteins always contain the element nitrogen (N) in addition to carbon (C), hydrogen (H), and oxygen (O). These building blocks of proteins, known as amino acids, join together to form the backbone of the protein molecule. Each protein molecule is uniquely shaped to perform specific functions within the body, such as forming muscle tissue, enzymes, and antibodies.
The 20 different amino acids combine into long chains, called polypeptides, which form the basis for a wide variety of proteins in living things. While some amino acids can be synthesized by the body, others known as essential amino acids must be obtained through the diet.