Enzymes are biological catalysts

Steps:

• Define enzyme as a protein that speeds up chemical reactions in living organisms without being consumed.
• Review each option's primary biological role.
• Match roles to enzyme definition: catalysts act on substrates to form products.
• Identify the option that functions as a catalyst for lipid breakdown.

Why D is correct:

• Lipase is a protein enzyme that catalyzes the hydrolysis of fats into glycerol and fatty acids, per enzyme definition as biological catalysts.

Why the others are wrong:

• A: Chlorophyll is a green pigment that absorbs light in photosynthesis, not a catalyst.
• B: Fibrinogen is a soluble plasma protein that converts to fibrin for blood clotting, functioning as a structural precursor.
• C: Insulin is a peptide hormone that regulates glucose uptake, not a reaction catalyst.

Final answer: D