Cell Structure

● 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 discovered

Units of Measurement in Cell Studies
● International System of Units (SI) only accepted scientific measurement system
● Basic unit of measurement – meter (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 specimen

● 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

  1. Extremely short wavelengths (think
  2. Negatively charged, focus using
    Types of Electron Microscopes

    ● Transmission
  3. Beam passes through specimen
  4. Only electrons transmitted (through) specimen seen
  5. Advantage-can view inside of cells/structures
  6. Can view thin specimens

    ● Scanning
  7. Scans surfaces of specimens
  8. Only reflected beam is observed
  9. Advantage – surface structures seen with great depth of field
  10. 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


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