Studying cells 3.2.1.3

• Advantages and disadvantages of optical microscopes

  Resolving power=0.2 micro metres (lowest)Advantages-colour images-can show living structures-affordable apparatusDisadvantages-2D image-low resolution-cannot show ultra structure
• Describe how optical microscopes work

  1. Lenses focus rays of light and magnify the view of a thin slice of specimen.2. Different structures absorb different amounts and wavelengths of light.3. Reflected light is transmitted to the observer via the objective lens and eyepiece.
• Advantages and disadvantages of scanning electron microscopes (SEMs)

  Resolving power=0.5-4nm (medium)Advantages-3D image-high resolutionDisadvantages-requires a vacuum (can’t see living structures)-no colour image-only shows outer surface
• Describe how a scanning electron microscope works?

  1. Focus a beam of electrons onto a specimen's surface using electromagnetic lenses.2. Reflected electrons hit a collecting device and are amplified to produce an image on a photographic plate.
• Advantages and disadvantages of transmission electron microscopes

  Resolving power=0.2nm (highest)Advantages-high resolution-ultrastructures visible-high magnification (x500,000)Disadvantages-requires a vacuum-cannot show living structures-2D image-extensive preparation-may produce artefacts-no colour image
• Describe how a transmission electron microscope (TEM) works

  1. Pass a high energy beam of electrons through thin slice of specimen. 2. More dense structures appear darker since they absorb more electrons. 3. Focus image onto fluorescent screen or photographic plate using magnetic lenses.
• Describe how temporary mounts are made

  1)add drop of water to glass slide2)thin slice3)put in suitable stain4)add coverslip using mounted needle
• Define magnification and resolution

  Magnification: factor by which the image is larger than the actual specimen.Resolution: smallest separation distance at which 2 separate structures can be distinguished from one another
• Units which can be used for microscopes

  
• How do you use a microscope to find the mean diameter?

  -Use eyepiece graticule to measure diameter-Calibrate using stage micrometer-Repeat and calculate mean
• What is cell fractionation?

  A technique used to isolate particular organelles in a cell to study structure and function
• Stages of cell fractionation

  First needs to be placed in a cold, buffered, isotonic(same water potential) solution1. Homogenisation = vibrating/grinding up cells in a homogeniser to break up the cells and release the organelles2. Filtration = the resulting fluid (homogenate is filtered to remove any debris)3. Ultracentrifugation = the fragments in the filtered homogenate are separated in a centrifuge at increasing speeds
• Results of centrifugation

  
• Why do cells need to be placed in a cold, buffered, isotonic solution?

  Cold=to reduce enzyme activity that could break down organellesBuffered=so the PH doesn’t fluctuate and denature proteinsIsotonic=prevent osmosis to stop cells bursting