Which of the following is NOT an example of a protein?

• A. Insulin
• B. Collagen
• C. Hemoglobin
• D. Cellulose

Answer: (D)

Verified step by step guidance

1. Step 1: Begin by understanding the nature of proteins. Proteins are macromolecules composed of amino acids linked together by peptide bonds. They perform a wide range of functions in the body, including structural support, transport, enzymatic activity, and hormonal signaling.
2. Step 2: Examine the examples provided in the question. Insulin, collagen, and hemoglobin are all proteins. Insulin is a hormone that regulates blood sugar levels, collagen is a structural protein found in connective tissues, and hemoglobin is a transport protein that carries oxygen in the blood.
3. Step 3: Consider the correct answer, cellulose. Cellulose is not a protein; it is a carbohydrate. Specifically, it is a polysaccharide composed of glucose monomers linked by β-1,4-glycosidic bonds. It serves as a structural component in the cell walls of plants.
4. Step 4: Compare the molecular composition of proteins and carbohydrates. Proteins are made of amino acids, while carbohydrates like cellulose are made of sugar molecules. This distinction helps identify cellulose as the correct answer.
5. Step 5: Conclude that cellulose is the correct answer because it is a carbohydrate, not a protein, unlike insulin, collagen, and hemoglobin, which are all examples of proteins.