1 Cells & Cell Techniques:

• Cell fractionation 


  Cells are broken open using a homogeniser.

  This breaks cell membranes and allows the organelles to be released.

  Cell debris and whole cells are filtered off
• Ultracentrifugation:


  The suspension is centrifuged at a low speed.

  The most dense organelle, the nucleus, separates out at the bottom of the tube.

  The supernatant is poured into a fresh tube and spun at a higher speed to separate off the chloroplast (if a plant cell) or mitochondria
• Features of prokaryotic cells:


  · Circular DNA, not associated with protein and not in nucleus

  · Form new cells by binary fission· No membrane -bound organelles

  · Has a cell wall made of murein

  · Has smaller ribosomes· May have a capsule, flagella and plasmid
• Features of Viruses:


  · Genetic material - either DNA or RNA

  · Capsid - a protein coat

  · Attachment protein - allow the virus to attach to a host cell (complementary to receptors on host cell membrane
• Magnification


  · The number of times bigger the image appears compared to its actual size
• Resolution:


  · the minimum distance between 2 objects/points at which they can be seen as separate
• How to use an Eyepiece graticule:


  · First calibrated against a stage micrometer scale (of known length) to work out the length that each eyepiece graticule division represents.

  · The eyepiece graticule scale can then be used to measure an object under the microscope.
• Conditions for cell fractionation and ultracentrifugation:

  Cold temperature - reduces enzyme activity, so no digestion of organelles (by lysozymes)

  Buffer - Maintains constant pH, to prevent denaturation of proteins, incl enzymes

  Isotonic - Same water potential as organelles, to prevent osmotic lysis of organelles (or shrinking)
• All cells contains...


  A cell-surface membrane enclosing cell contents

  Cytoplasm

  Genetic material, made of DNA.
• Limitations of optical microscopes:

  Lower resolution as they use light which has longer wavelengths (compared to electrons)
• Transmission Electron Microscope (compared to scanning):

  Higher resolution than SEM

  Requires thin sections

  No 3D images

  Allows details of internal cell structures to be seen (e.g. organelles)
• Scanning Electron Microscope (compared to Transmission)


  Lower resolution than TEM

  Does not require thin sections to be cut

  Can have 3D images

  Usually shows the surface of the object