A Level Biology Practice Exam

What defines a polypeptide?

1. Chains of two or more amino acids

2. Chains of three or more amino acids

3. Proteins made solely of carbohydrates

4. Amino acids that are all the same

The correct answer is: Chains of three or more amino acids

A polypeptide is best defined as a chain of amino acids linked together by peptide bonds. The key characteristic is that it consists of a specific minimum number of amino acids, typically three or more, to be classified as a polypeptide. This chain can vary in length, with longer chains known as proteins, but the minimum length starting from three distinguishes it clearly from dipeptides, which are made of two amino acids. The other choices do not accurately reflect the definition of a polypeptide. For instance, while chains of two amino acids will form a dipeptide, they fall short of the three amino acids needed for the polypeptide classification. Additionally, proteins that are made solely of carbohydrates are not polypeptides, as polypeptides are fundamentally composed of amino acids. Lastly, suggesting that all amino acids in a polypeptide are the same does not hold true for the diverse structure of polypeptides, which typically consist of varying amino acids, contributing to different functional properties.