Which compound we discussed is a hard, water-resistant protein?

A. mucus: Mucous membranes line body cavities that open to the exterior, such as the respiratory, digestive, and urogenital tracts. They contain goblet cells that secrete mucus.

B. synovial: Synovial membranes line joint cavities and produce synovial fluid, not found in the respiratory tract.

C. serous: Serous membranes line body cavities that do not open to the exterior (e.g., pleura, pericardium, peritoneum) and secrete a watery fluid, not mucus.

D. peritoneal: The peritoneal membrane is a type of serous membrane lining the abdominal cavity, not the respiratory tract.

A. hyaline cartilage: Hyaline cartilage contains fine collagen fibers, but not densely packed ones. It is more glassy and smooth, found in the nose, trachea, and at joint surfaces.

B. areolar cartilage: This is not a valid type of cartilage. “Areolar” refers to loose connective tissue, not cartilage.

C. fibrocartilage: This type of cartilage is rich in dense bundles of collagen, making it very tough and resistant to compression. It’s found in intervertebral discs and the pubic symphysis.

D. elastic cartilage: Elastic cartilage contains more elastic fibers than collagen, allowing flexibility. It is found in the ear and epiglottis.