3.2.1.3 Methods of studying cells

• Magnification


  is how much bigger the image is than the specimen — calculated using this formula

  
• Specimen


  the sample you're looking at
• Calculating magnification Example

  
• Calculating size of image Example

  
• Calculating size of real object Example

  
• When you're calculating magnification you need to make sure

  all lengths are in the same unit — e.g. all in millimetres

  
• The table shows millimetres are

  ● three orders of magnitude — 10³ or 1000 times ● bigger than micrometres — which are three orders magnitude bigger than nanometres

  
• To convert smaller unit to a bigger unit

  you divide by 1000

  
• Resolution


  is how detailed the image is
• Resolution


  how well a microscope distinguishes between two points that are close together — microscope lens can't separate two so increasing the magnification won't help
• microscope


  can't distinguish between objects that are smaller than its maximum resolution
• Resolution Example


  ● When you look at a car in the dark that’s a long way away you see the two headlights as one light— because your eyes can’t distinguish between the two points at that distance — your eyes produce a low resolution image● When the car gets a bit closer you can see both headlights — a higher resolution image
• in the exam you could be given a micrograph


  with scale bar drawn on it — you can use this bar to work out the size of the object — E.g. if the specimen is 4 bars long, it would measure 4 x 10 = 40 μm

  
• Types of microscope

  ● Optical light microscopes● Electron microscopes — Transmission and Scanning
• Optical (light) microscopes


  use light to form an image

  
• Optical (light) microscopes


  have maximum resolution about 0.2 micrometres µm — means can't use it to view organelles smaller than 0.2 µm— includes ribosomes, the endoplasmic reticulum and lysosomes — You may able to make out mitochondria, not in perfect detail — You can also see the nucleus

  
• In Optical (light) microscopes


  the maximum useful magnification is about × 1500

  
• Features of Light Microscopes

  ● small and easy to carry● vacum — no need● sample preparation — easy● magnification — up to × 2000 magnification ● resolution — 200 nm● specimens — can be living or dead

  
• Features of Electron Microscopes

  ● large — installation means it cant move ● vacum — needed● sample preparation — complicated● magnification — over × 500 000 ● resolution — 0.5 nm● specimens — are dead

  
• Electron microscopes


  use electrons to form an image

  
• Electron microscopes

  ● have a higher resolution than optical microscopes— so give a more detailed image and can be used to look at more organelles ● have maximum resolution of about 0.0002 micrometres µm ● maximum magnification of an electron microscope is about × 1 500 000

  
• Electron microscopes produce

  black and white images— are often coloured by a computer

  
• Comparison of electron and light microscopes

  
• Comparison of light vs electron microscopes

  
• Types of electron microscope

  ● Transmission electron microscopes — TEMs● Scanning electron microscopes — SEMs
• Transmission electron microscopes (TEMs)


  ● use electromagnets to focus a beam electrons — which is transmitted through specimen● Denser parts of the specimen absorb more electrons— which makes them look darker on the image you end up with

  
• Transmission Electron Microscopes are good because

  ● they give high resolution images — so you see the internal structure of organelles like chloroplasts ● But you've got to view the specimen in a vacuum — so no good for looking at living organisms ● can also only be used on thin specimens

  
• Scanning electron microscopes (SEMs)


  ● scan beam of electrons across the specimen — this knocks off electrons from the specimen which gathered in a cathode ray tube— to form an image● The images you end up with show the surface the specimen and they can be 3D

  
• SEMs are good because

  they can be used on thick specimens— but they give lower resolution images than TEMs

  
• Advantages of electron microscopes

  ● TEMs — Give high resolution images, so shows small objects● SEMs — Can be used on thick specimens— Can be 3-D
• Disadvantages of electron microscopes

  ● Transmission Electron Microscopes — Can only be used on thin specimens— Can only be used on non-living specimens● Scanning Electron Microscopes — Give lower resolution images than TEMs— Can only be used on non-living specimens
• Comparison of TEMs and SEMs

  
• If you want to look at specimen with an optical microscope you'll need

  ● to put on a microscope slide first — often done using temporary mount — where the specimen is suspended in drop of liquid e.g. water , oil on the slide

  
• Microscope slide


  strip of clear glass or plastic that protects the specimen
• Temporary Mount


  also known as a wet mount
• Step 1 — Preparing microscope slides


  Start by pipetting a small drop of water onto the centre of the slide

  
• Step 2 — Preparing microscope slides


  ● use tweezers to place thin section of your specimen on top of the water drop— specimen needs to let light through for you to be able to see clearly under the microscope — if you've got quite thick specimen you'll need to take thin slice to use on your slide

  
• Step 3 — Preparing microscope slides

  Add a drop of stain

  
• Stains are used to

  highlight objects in cells
• Step 4 — Preparing microscope slides


  add the cover slip