2.1.2 Slides and photomicrographs

2.1.2 Slides and photomicrographs

1. Optical microscopes are used to view a wide range of specimens

   • Living organisms like paramecium and amoeba
   • Smear preparations of human blood and cheek cells
   • Thin sections of animal, plant or fungal tissue e.g. bone, muscle, leaf, root or fungal hyphae

2. Observing unstained specimens

   • Many biological structures are colourless and transparent
   • Some microscopes use light interference rather than light absorption = produces a clear image without staining
   • Some microscopes use a dark backdrop against the illuminated specimen
   • Useful for studying living specimens

3. Staining specimens

   • Stains= coloured chemicals that bind to molecules on/in the specimen making it easy to see
   • Methylene blue = all purpose stain
   • Differential staining= some stains bind to specific structures staining them each differently to be identified
   • Acetic orcein = binds to DNA, stains chromosomes dark red
   • Eosin = stains cytoplasm
   • Sudan red= stains lipids
   • Iodine (in potassium iodide solution)= stains cellulose in plant walls yellow and starch granules blue/black

4. Observing prepared specimens

   • Permanently fixed slide are made by experts in labs by:
     • Dehydrating specimens
       • Embedded in wax to prevent distortion when slicing
         • Special instrument makes v. thin slices (sections) which are stained and mounted in a special chemical to preserve them

5. Calculations involving magnification

   • If you know the magnification on a photomicrograph, you can work out the actual size of the strucutres
     • Measure the widest part in mm
       • Convert to um by X1000
         • Divide this by magnification (tells you actual size)
   • If you are told actual size of a structure on a photomicrograph and you measure the image size in um, you can calculate magnification factor
     • M: I/A
       • M= magnification factor
         • I= Image size, um
           • A= actual size
         • If magnification factor is 1000 it must be written as X1000