biology 2.2

• what are the three main types of microscope?

  light microscope scanning electron microscope (SEM) transmission electron microscope (TEM)
• what are the two lenses in the light microscope called and what do they do?


  objective lens - magnifies the image eyepiece lens - magnifies the image further
• what are the benefits of using a compound light microscope rather than a simple light microscope?

  allows a much higher magnification gives less chromatic aberration
• define magnification


  how many times larger the image is than the actual size of the object
• define resolution 


  ability to distinguish seperate objects higher resolution = more detail
• what is a dry mount?


  in a dry mount, the object viewed is placed on a slide and covered with a coverslip
• what is a wet mount?


  in a wet mount, the specimen to be viewed is placed on a slide and a drop of liquid is added. The coverslip is lowered gently, at an angle, to avoid air bubbles
• what is a squash slide?


  with a squash slide, a wet mount slide is prepared and the lens tissue is placed over the coverslip and gently pressed trying not to break the coverslip.
• what is a smear slide?


  a smear slide is where the sample is added to one end of the slide and a second slide is pulled back until it touches the sample. This slide is then pushed across the surface of the lower side to create a thin smear, spreading out the sample in a thin layer
• what are the different methods of staining?

  positive / negative staining gram staining acid fast techniques
• what is positive staining?


  positively charged dyes are attracted to the negatively charged cell structures leading to the staining of cell parts and can be more easily distinguished
• what are some examples of positively charged dyes?


  crystal violet methylene blue
• what is negative staining?


  dyes are negatively charged so are repelled by the cell structures, these dyes then stay outside of the cells leaving them unstained but colour the background so cells can stand out against it
• what are examples of negatively charged dyes?


  congo red nigrosin
• what can different stainings distinguish between?

  different organisms different organelles in the same organism
• what is gram staining?


  used to seperate bacteria into two groups (gram positive and gram negative)
• what is the method for gram staining?

  add crystal violet stain then add iodine (fixes the dye) wash with alcohol counterstain with safranin
• how can you tell if the bacteria is gram positive?


  the bacteria retains the crystal violet stain and appear blue
• how can you tell if the bacteria is gram negative?


  the bacteria loses the crystal violet stain when washed with alcohol so appear red due to the counterstain
• what are acid fast techniques used for?


  used to identify mycrobacterium species from other bacteria as mycrobacteria are red and other bacteria species are blue
• what is the method for detecting acid fast bacteria?

  apply primary stain of carbol fuschin for 30 secs heat fix the cells to the slide using a flame decolourize with acid alcohol for 15-20 secs apply counterstain of methylene blue for 30 secs then rinse excess stain
• what are the risks associated with staining?

  many of the stains used are toxic or irritants so a risk assesment is needed to reduce risk
• what is the purpose of fixing?


  chemicals like formaldehyde are used to preserve specimens in as near a natural state as possible
• what is the purpose of sectioning?

  specimens are dehydrated with alcohols and then mounted in wax to form a hard block. a sharp knife called a microtome is used to cut very thin sections
• what is the purpose of staining?

  specimens are often treated with multiple stains to highlights different structures
• what is the purpose of mounting?


  the specimens are secured onto a microscope slide and a coverslip is placed on top
• why do we stain specimens?


  to provide more contrast and make it easier to distinguish certain parts of the cell.
• what is differential staining?


  using a stain to distinguish between either 2 different organisms or between organelles of a specimen due to preferencial absorbtion of stain
• what is the equation for magnification?


  magnification = image size/ actual size
• how does an electron microscope work?


  a beam of electrons with a wavelength of less than 1nm is used to illuminate the specimen
• why can you see the cell in more detail with an electron microscope?

  more detail of cell ultrastructure can be seen because electrons have a much smaller wavelength than light waves so they can produce images wiht magnifications of up to 500,000x and still have a clear resolution
• what are the disadvantages of using an electron microscope?

  very expensive

  can only be used in a controlled environment in a dedicated space eg a lab and not a school

  specimens can be damaged by the electron beam
• what is the role of the nucleus?


  Contains coded genetic information in the form of DNA
• what is the role of the nucleolus?


  Area within the nucleus and is responsible for producing ribosomes.
• what is the role of the nuclear envelope?

  keeps the contents of the nucleus separate from the cytoplasm of the cell.
• what is the role of the Rough endoplasmic reticulum

  Responsible for the synthesis and transport of proteins
• what is the role of the Smooth endoplasmic reticulum


  Responsible for lipid and carbohydrate synthesis and storage
• what is the role of the Golgi apparatus?

  Modifies proteins and ‘packages’ them into vesicles
• what is the role of ribosomes?

  site of proteinsynthesis
• what is the role of the mitrochondria?


  The site of the final stages of cellular respiration