Function of these reagents: lubrication, coat cell surface to provide favorable environment for diffusion, provide greater adhesion between adjacent cells (glue).

• A. Mucin
• B. Collagen
• C. Keratin
• D. Elastin

Answer: (A)

Mucin is the secreted glycoprotein that forms mucus, coating epithelial surfaces with a water-rich, slippery gel. This hydrated layer provides lubrication, reducing friction and creating an environment where diffusion of ions, nutrients, and gases can occur more readily across the surface. The mucus also acts as a sticky matrix that helps adjacent cells stay together, effectively providing a “glue” that enhances adhesion in moist surfaces. The other proteins—collagen supplies structural strength in connective tissue, keratin gives mechanical durability to epithelial cells, and elastin provides tissue elasticity—do not primarily serve as lubricating coatings for diffusion on cell surfaces.