OCR Unit 1: Cell Structure Basics

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  • Created by: anushiee
  • Created on: 17-02-14 16:29

             Measuring units

1 mm   =   1000 µm


                     1 µm   =    1/1000 mm (1 x 10 -3mm)


Some biological structures are so small that the nanometre (nm) unit has to be used.


                            1 µm   =   1000 nm


                            1 nm   =   1/1000 µm (1 x 10-3 µm or 1 x 10-6mm)


  • To convert millimetres to micrometres, multiply by 1000


  • To convert micrometres to nanometres, multiply by 1000


Calculating Linear Magnification


There is a relationship between actual size of a specimen, the magnification of this actual size and the image size seen as a result.


            Magnification    =    image size/actual size


Usually biologists use µm units in these calculations


After rearrangement of this equation


             Actual size       =    image size/magnification



Using Light Microscopes



In Light Microscopy, the specimen is placed on a microscope slide, in a light beam.  The beam passes through the specimen and then through a series of glass lenses that magnify the specimen


Suitable specimens for light microscopy


  • Living cells/organisms


  • Thin sections of tissues cut from plant or animal tissue


Staining - Background


  • Many biological specimens are colourless or transparent, especially when thin sections are viewed, so a stain /dye is often added to make structures within the specimen easier to see


  • Coloured stains are chemicals that bind to molecules in the specimen


  • Different parts of the cell may absorb a stain more readily than other parts and appear darker.  Many stains are taken up by DNA and reveal the nucleus in a cell


  • Some stains cannot pass through the cell surface membrane of a living cell but will stain dead cells.  This enables cells to be identified as  living or dead


Why use stains in microscopy?


Staining increases the contrast so that cell structures can be seen more clearly



Table Listing Stains Commonly used in Light Microscopy (there is no need to learn the names of these stains but it is useful to know that nuclei/chromosomes take up stains)





Methylene blue


For staining living cells

Dark blue nucleus, light blue cytoplasm

Iodine solution

For staining living plant cells

Dark blue-black starch grains, nucleus stains deeper yellow, the cytoplasm stains a lighter yellow

Acetic orcein

For staining the nucleus and chromosomes



Stains the cytoplasm and some organelles of dead cells only



If a question asks for reasons for staining a specific tissue, indicate specifically what can be seen more clearly in your answer – always avoid vague answers





Animal Cell Structures Visible Using the Light Microscope


  • Thin cell surface membrane (plasma membrane)
  • Granular cytoplasm
  • Nucleus
  • Mitochondria (only at higher magnifications)


The microscopes that students use in the laboratory usually magnify cells to a maximum of x400.  Therefore animal cells


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