Final answer:
Proteins contain nitrogen in addition to the carbon, hydrogen, and oxygen found in carbohydrates and fats, with nitrogen being the distinguishing element that sets proteins apart.
Step-by-step explanation:
Proteins contain nitrogen in addition to carbon, hydrogen, and oxygen, which are the same elements that compose carbohydrates and fats. The building blocks of proteins, called amino acids, have a complex structure that incorporates these four elements. A characteristic feature of amino acids and hence proteins is the amino group (-NH2), which contains the nitrogen essential for the formation of proteins. Proteins can also contain other elements such as sulfur, but nitrogen is the distinguishing element that is not present in carbohydrates or fats.