Studying Cells (OCR A Level Biology)

• Microscopes can be used to observe and investigate cell structure
• Different types of microscope can be used to study cells at different levels of detail, e.g. 
  • light microscopes
  • electron microscopes
• The images generated by the different types of microscope differ significantly

Light microscopes

• Light, or optical, microscopes use light to form an image
• The maximum resolution of a light microscope is around 0.2 micrometres (µm), meaning that the maximum useful magnification of optical microscopes is about ×1500
• Light microscopes can only be used to observe larger structures, e.g.:
  • entire cells
  • nuclei
  • mitochondria and chloroplasts
• While light microscopes have limited resolution, they do have advantages, such as:
  • they are small and relatively cheap
  • specimen preparation can be straightforward enough to perform in a school laboratory
  • they can be used to produce colour images
  • they allow the observation of living specimens

Light microscope image

Light microscope images allow the observation of cell shape, as well as larger internal structures, e.g, here chloroplasts can be seen within a series of plant cells

Kelvinsong, via Wikimedia Commons

Electron microscopes

• Electron microscopes use electrons to form an image
• Electron microscopes have a maximum resolution of around 0.0002 µm, or 0.2 nm, and a maximum magnification that range from around ×1,000,000 up to many millions
• Electron microscopes can be used to observe small structures inside cells, such as:
  • cell membranes
  • ribosomes
  • the endoplasmic reticulum
  • lysosomes
• While electron microscopes are essential tools in the study of cell biology, they do have some limitations
  • They are very large and expensive
  • Specimens must be prepared using a highly complex process
  • Specimens must be viewed in a vacuum, meaning that live specimens cannot be observed
  • Images are always black and white, though they can be artificially coloured during processing
• There are two types of electron microscope:
  • transmission electron microscopes (TEMs)
  • scanning electron microscopes (SEMs)

Transmission electron microscopes

• TEMs use electromagnets to transmit a beam of electrons through a specimen; denser parts of the specimen absorb more electrons, meaning that denser parts appear darker on the final image
• TEMs produce images that:
  • are high-resolution
  • allow the internal structures within cells, and within organelles to be seen
  • are two-dimensional

TEM image

TEM images allow the internal structures within cells and organelles to be studied

Klingm01, via Wikimedia Commons

Scanning electron microscopes

• SEMs pass a beam of electrons across the surface of a specimen and then detect the rate at which the electrons bounce back
• This means that SEMs produce images that:
  • are three-dimensional
  • show the surface of specimens
• SEMs have a lower maximum resolution than TEMs

SEM image

SEM images are three-dimensional and show the surface of objects, e.g. here E. coli bacteria can be seen

NIAID, via Flickr