● Light microscope-uses light as a source of radiation
● First used in beginning of 17th century
● Early 19th century, quality of lenses improved creating dramatic images
● Cytology-study of cells
● By 1900, all cell structures and organelles except lysosomes had been
Units of Measurement in Cell Studies
● International System of Units (SI) only accepted scientific measurement
● Basic unit of measurement – metre (m)

SI Units of Measurement
● 1 micrometre (μm) is a thousandth of a millimetre (mm)
● 1 nanometre (nm) is a thousandth of a micrometre (um)
● Cell sizes range from 5 μm to 40 μm
● Smallest structure visible with only the human eye is about 50-100 μm.
● Smallest organelle, ribosomes, are only about 20 μm in diameter.

Sizes of Biological Structures
● Smallest object visible with eye only – 50-100 μm, about diameter of
sharp end of a pin
● Smallest object visible with a light microscope – 0.2 μm or 200 nm,
about the average size of a bacterium
● Smallest object visible with an electron microscope – 0.5 nm, ribosome,
cell membrane
● Invisible – diameter of a hydrogen atom (smallest atom) 0.04 nm

Measuring Cells
● Using Stage Micrometer this cell measures 1 μm, marked in .1 μm and 1
μm divisions
● 1 cm = 10 mm = 1000 micrometers (μm)

Calibrating Eyepiece Graticule
● Using an eyepiece graticule with arbitrary
scale – must calibrate to stage micrometer to
determine actual measurement.
● Count number of divisions on EG to equal to
10 μm on stage micrometer

Calibrating Eyepiece Graticule
● On lowest power, count the number of divisions on eyepiece graticule
equal to 10 μm on the stage micrometer to calculate length that one
eyepiece division is equal to.
● For example, if 43 divisions are equal to 10 μm, then each division is
equal to 0.233 μm at low power.
● Repeat for medium and high power objectives.
Magnification versus Resolution
● Magnification-number of times larger an image is compared with the
real size of an object.
● Magnification = measured size of magnified image / actual size of

● Ability to distinguish between two separate points
● If two points can’t be resolved, they’ll be seen as one point
● Maximum resolution of light microscope is 200 nm
● The limit of resolution is one half the wavelength of the radiation used to
view the specimen

Electron Microscopy

● Free electrons behave like electromagnetic
radiation – they have a very short wavelength
● Suitable form of radiation for microscopy due

xtremely short wavelengths (think

Negatively charged, focus using

Types of Electron Microscopes
● Transmission

Beam passes through specimen

Only electrons transmitted (through) specimen seen

Advantage-can view inside of cells/structures

Can view thin specimens

● Scanning

Scans surfaces of specimens

Only reflected beam is observed

Advantage – surface structures seen with great depth of field

Disadvantage-resolution not as good as TEM

Overall Difficulties with Electron Microscopy
● Must take place in a vacuum. Air molecules would cause electrons to
● Water boils at room temperature in a vacuum, so all specimens must be
● Only dead material can be examined with EM