Microscopes

• Microscopes can be used to analyze cell components and observe organelles
• Magnification tells you how many times bigger the image produced by the microscope is than the real-life object you are viewing
• Resolution is the ability to distinguish between objects that are close together
• Two main types of microscopes are optical (light) microscopes and electron microscopes
• Optical microscopes use light to form an image, limiting their resolution to around 0.2 micrometers
• Electron microscopes use electrons to form an image, greatly increasing resolution compared to optical microscopes
• Electron microscopes have a maximum resolution of around 0.0002 µm or 0.2 nm
• Transmission electron microscopes (TEMs) use electromagnets to focus a beam of electrons through the specimen
• Advantages of TEMs include high-resolution images, allowing internal structures within cells to be seen
• Disadvantages of TEMs include the need for very thin specimens and the inability to observe live specimens
• Scanning electron microscopes (SEMs) scan a beam of electrons across the specimen to produce three-dimensional images
• Advantages of SEMs include the ability to observe thick or 3-D specimens and show the external structure of specimens
• Disadvantages of SEMs include lower resolution images compared to TEMs and the inability to observe live specimens
• Optical microscopes are invaluable for studying tissues, cells, and organelles
• When using an optical microscope, start with the low power objective lens to prevent damage and make it easier to find what you are looking for
• A graticule, a small disc with an engraved scale, is used to take measurements of cells in a microscope
• The stage micrometer scale is used to find out how many micrometers each graticule unit represents
• Biological drawings are line pictures showing specific features observed under a microscope
• Rules/conventions for making a biological drawing:

  • Must have a title
  • Magnification must be recorded
  • Use a sharp HB pencil and good eraser
  • Draw on plain white paper with clear, single lines (no shading)
  • Drawing should take up as much space as possible
  • Well-defined structures should be drawn with proper proportions
  • Label lines should not cross or have arrowheads, connecting directly to the part being labelled
  • Label lines should be kept to one side of the drawing and drawn with a ruler
• When producing a biological drawing, only draw what you see, not what you think you see
• To accurately reflect size and proportions of structures seen under a microscope, use the eyepiece graticule
• Starch grains, storage polysaccharides of plants, can be stained with iodine in potassium iodide solution to make them easier to see
• Iodine in potassium iodide solution turns blue-black in the presence of starch
• Magnification is how many times bigger the image of a specimen observed is compared to the actual size
• Resolution is the ability to distinguish between two separate points; limited by the wavelength of light
• Electron microscopes have higher resolution and magnification than light microscopes due to electrons' smaller wavelength
• Light microscopes are used for specimens above 200 nm, while electron microscopes are used for specimens above 0.5 nm
• Electron microscopes are useful for looking at organelles, viruses, DNA, and whole cells in more detail
• The size of cells is typically measured using the micrometre (μm) scale, with cellular structures measured in either micrometers (μm) or nanometers (nm)
• When doing calculations, all measurements must be in the same units, and it's best to use the smallest unit of measurement shown in the question
• To convert units, multiply or divide depending on whether the units are increasing or decreasing
• Magnification does not have units
• Converting units of measurement:

  • 1000 nanometers (nm) in a micrometre (µm)
  • 1000 micrometres (µm) in a millimetre (mm)
  • 1000 millimetres (mm) in a metre (m)
• Cell fractionation involves three stages:

  • Homogenisation
  • Filtration
  • Ultracentrifugation
• Homogenisation is the breaking up of cells, where the sample of tissue must first be placed in a cold, isotonic buffer solution
• Filtration separates out any large cell debris or tissue debris that were not broken up, leaving a solution (filtrate) containing a mixture of organelles
• Ultracentrifugation involves spinning the filtrate at different speeds to separate out different types of organelles present