3.6—Recognising organelles in EM images

Electron microscopes resolve much smaller distances than light microscopes because electrons have a shorter wavelength. A TEM sends electrons through a thin specimen to show high-resolution internal structure; an SEM scans the surface to produce a three-dimensional view.

First decide whether the image is a 2D internal section or a 3D surface. Then use distinctive features—such as a nucleus, double-membraned mitochondrion with cristae, or stacked Golgi sacs—to propose an organelle, and check that the scale and context fit.

A high-resolution 2D image showing internal cristae is consistent with a TEM mitochondrion; a raised surface image is more consistent with an SEM. The image is a dead-specimen snapshot, not a movie of a living cell.

Magnification does not create information that resolution cannot support. Do not call every dark oval a nucleus or infer function from appearance without the structural evidence.