5.2 Microscopy, Magnification and Resolution

Magnification is image size divided by actual size. It states how many times larger the image is than the object. Resolution is the ability to distinguish two nearby points as separate. Enlarging a low-resolution image produces a larger blur; it does not reveal new structure.

Keep units consistent before calculating. If an image measures 30 mm and the object is 6 µm, convert 30 mm to 30 000 µm before dividing, giving ×5000. Scale bars are often safer than printed magnification because resizing a digital or printed image changes both the object image and scale bar together while making the original magnification label incorrect.

Visible-light microscopes have a practical resolution near 200 nm because diffraction limits the separation of detail. They can view living specimens, follow movement and use coloured stains or fluorescent labels. Typical cellular outlines, nuclei, chloroplasts and some bacteria are visible, but ribosomes and membrane bilayers are below the resolution limit.

Electron microscopes exploit the much shorter wavelength associated with accelerated electrons. A transmission electron microscope passes electrons through an ultrathin specimen, producing a two-dimensional image of internal ultrastructure. Regions that scatter more electrons appear darker. A scanning electron microscope detects electrons from a coated surface and emphasizes three-dimensional surface form.

Electron microscopy requires a vacuum and extensive preparation, so living specimens cannot be observed. Fixation, dehydration, sectioning and heavy-metal staining can distort or extract material. Electron micrographs are originally grayscale; colour is added later for interpretation and is not the specimen's natural colour.

Freeze-fracture microscopy splits frozen membranes along weak hydrophobic planes and creates a metal replica of the exposed surface. It helped reveal embedded membrane proteins. Cryogenic electron microscopy rapidly freezes purified particles in vitreous, non-crystalline ice and combines many projections computationally to reconstruct molecular structures without conventional crystallization.

A section through a spherical organelle is circular, but its apparent diameter depends on how close the section passes to the centre. A long mitochondrion cut transversely appears circular and cut longitudinally appears elongated. Several separated vacuole profiles in one section may belong to one continuous three-dimensional vacuole.

Fluorescent labels add molecular specificity. A fluorophore absorbs one wavelength and emits a longer wavelength; attaching fluorophores to antibodies or proteins reveals selected targets. The resulting image maps the label, not necessarily the full boundary of a structure, and excessive signal may make nearby objects appear merged.

Technical advance changes biological knowledge by extending observation. Light microscopy supported cell theory; electron microscopy exposed organelles and membrane ultrastructure; fluorescence enabled dynamic localization; cryo-EM resolves macromolecular complexes. Interpretation still depends on controls, calibration and awareness of preparation.