aqa biology AS topic


Cell structure


  • Microscopes are instruments that produce a magnified image of an object.
  • A convex glass lens acts as magnifying glass, they are more effective if in pairs like a compound microscope.
  • Long wavelengths of light means a light microscope can distinguish between two objects if they are 0.2μm or further. 
  • This can be overcome by using beams of electrons rather than beams of light
  • With shorter wavelengths, the beam of electrons in the electron microscope can distinguish between two objects only 0.1nm apart


  • material under a microscope = object 
  • appearance of material when viewed under microscope = image 
  • Magnification = (size of image) / (size of real object)
  • Size of real object = size of image/ magnification 
  • Kilometer (km) =103
  • Metre (m) = 1
  • Millimetre (mm) = 10-3
  • Micrometre (μm) =10-6
  • Nanometre (nm) =10-9


  • Resolution or resolving power of microscope is minimum distance apart that two objects can be in order for them to appear as separate items
  • Resolving power depends on wavelength or form of radiation used
  • In light microscope, it is about 0.2μm 
  • Means that any objects which are 0.2μm or more apart will be seen separately but any objects closer than 0.2μm will appear as single item 
  • Greater resolution means greater clarity, this image produced is clearer and more precise 
  • increasing magnification increases size of an image but doesn’t always increase resolution 
  • every microscope has limit of resolution 
  • up to this point increasing the magnification will not do this.
  • Object while appearing larger will be more blurred

Cell Fractionation

  • Process where cells are broken up and the different organelles they contain are seperated. 
  • Before it can take place it must be in
  • Cold - to reduce enzyme activity that might break down the organelles
  • Isotonic - to prevent organelles bursting or shrinking due to osmotic gain or loss of water 
  • Buffered - so pH doesn't fluctuate as it could alter structure of the organelles or affect functioning of enzymes
  • Homogenation - cells broken up by homogeniser (blender) to release organelles from cells. Fluid left is homogenate which is filtered to remove any complete cells and large pieces of debris
  • Ultracentrifugation - process by which fragments in filtered homogenate are separate in machine called centrifuge = spins tubes of homogenate at very high speeds in order create a centrifugal force
  • Filtrate is placed in a centrifuge and spun at slow speed
  • Heaviest organelle sinks to form a pellet and fluid left is supernatant
  • Supernatant is removed and transferred then spun again at a faster speed than before.
  • The next heaviest organelles, mitochondria, are forced to the bottom of the tube 
  • process is continued in this way so that at each increase in speed, next heaviest organelle is sedimented and separated out.
  • Results:  Nuclei 1st at 1,000 revolutions per min. Then mitochondria at 3,500 revolutions per min. Then Lysosomes at 16,500 revolutions per min

The electron microscope

  • Light microscopes have poor resolutions as result of relative;y long wavelength of light 


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