Proteins perform many cellular functions, but not all cellular structures are made primarily of protein. Which structure is NOT made of protein?

• A. Phospholipid bilayer of the plasma membrane
• B. Microtubules
• C. Ribosomes
• D. Collagen fibers

Answer: (A)

Verified step by step guidance

1. Identify the main components of each cellular structure listed: microtubules, ribosomes, collagen fibers, and the phospholipid bilayer of the plasma membrane.
2. Recall that microtubules are part of the cytoskeleton and are primarily composed of protein subunits called tubulin.
3. Understand that ribosomes are molecular machines made up of ribosomal RNA and proteins, so they contain a significant protein component.
4. Recognize that collagen fibers are extracellular matrix components made mostly of collagen, which is a structural protein.
5. Know that the phospholipid bilayer of the plasma membrane is primarily composed of phospholipids, which are lipid molecules, not proteins, making it the structure not made primarily of protein.