A Level Biology Practice Exam

Which structural characteristic distinguishes proteins from carbohydrates?

1. The presence of carbon atoms

2. The presence of nitrogen atoms

3. The presence of oxygen atoms

4. The presence of phosphorous atoms

The correct answer is: The presence of nitrogen atoms

Proteins are distinguished from carbohydrates primarily by the presence of nitrogen atoms in their structure. While both proteins and carbohydrates contain carbon, hydrogen, and oxygen, proteins are unique because they include nitrogen as a key element. This nitrogen is found in amino acids, which are the building blocks of proteins. Each amino acid contains an amino group (-NH2) that includes nitrogen, which is absent in carbohydrates, whose basic building blocks are simple sugars like glucose that do not have nitrogen in their structure. The presence of oxygen is not a distinguishing feature between proteins and carbohydrates since both types of molecules contain oxygen—carbohydrates typically have a higher ratio of oxygen, while proteins have nitrogen incorporated. Similarly, phosphorous is not a common characteristic of either proteins or carbohydrates in their standard forms; phosphorous is more commonly associated with nucleic acids, such as DNA and RNA. Therefore, the unique presence of nitrogen atoms in proteins serves as a key distinguishing factor between the two classes of biomolecules.