Practice question

Transmission electron microscopy images ultrathin biological sections using electrons rather than photons. Because biological material consists mainly of light atoms that scatter electrons weakly, contrast must be enhanced with heavy metals. Osmium tetroxide fixes lipids and adds electron density, uranyl acetate binds nucleic acids and proteins, and lead citrate stains membranes and ribosomes. These high atomic number atoms scatter incident electrons strongly, creating dark regions in bright-field TEM images. Staining thus reveals bilayer membranes, chromatin texture and viral particles at nanometer resolution unattainable with visible light microscopes.