Osmium tetroxide chemically fixes fat.

• A. True
• B. False
• C. Not enough information
• D. Always

Answer: (A)

Osmium tetroxide fixes fats by chemically reacting with the carbon–carbon double bonds in unsaturated fatty acids. This forms stable osmiated lipid adducts that immobilize fats within the tissue and render them electron-dense, which is why lipids (like membranes) are preserved and visualized well when using this fixative. This lipid-specific action is the reason the statement is correct. In practice it’s often used as a post‑fixative after aldehyde fixation to best preserve membranes and lipids for electron microscopy, though its use is context-dependent rather than universal.