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  1. 1-biological-molecules

    1-1-biological-molecules-carbohydrates
    11 主题
  2. 1-2-biological-molecules-lipids
    3 主题
  3. 1-3-biological-molecules-proteins
    5 主题
  4. 1-4-proteins-enzymes
    12 主题
  5. 1-5-nucleic-acids-structure-and-dna-replication
    8 主题
  6. 1-6-atp-water-and-inorganic-ions
    4 主题
  7. 2-cell-structure
    2-1-cell-structure
    7 主题
  8. 2-2-the-microscope-in-cell-studies
    4 主题
  9. 2-3-cell-division-in-eukaryotic-and-prokaryotic-cells
    8 主题
  10. 2-4-cell-membranes-and-transport
    7 主题
  11. 2-5-cell-recognition-and-the-immune-system
    7 主题
  12. 2-6-vaccines-disease-and-monoclonal-antibodies
    6 主题
  13. 3-exchange-and-transport
    3-1-adaptations-for-gas-exchange
    6 主题
  14. 3-2-human-gas-exchange
    10 主题
  15. 3-3-digestion-and-absorption
    5 主题
  16. 3-4-mass-transport-in-animals
    6 主题
  17. 3-5-the-circulatory-system-in-animals
    8 主题
  18. 3-6-mass-transport-in-plants
    6 主题
  19. 4-genetics-variation-and-interdependence
    4-1-dna-genes-and-chromosomes
    7 主题
  20. 4-2-dna-and-protein-synthesis
    6 主题
  21. 4-3-genetic-diversity-mutations-and-meiosis
    7 主题
  22. 4-4-genetic-diversity-and-adaptation
    6 主题
  23. 4-5-species-and-taxonomy
    4 主题
  24. 4-6-biodiversity
    9 主题
  25. 5-energy-transfers-in-and-between-organisms-a-level-only
    5-1-photosynthesis-a-level-only
    5 主题
  26. 5-2-respiration-a-level-only
    7 主题
  27. 5-3-energy-and-ecosystems-a-level-only
    9 主题
  28. 5-4-nutrient-cycles-a-level-only
    4 主题
  29. 6-organisms-respond-to-changes-in-their-environments-a-level-only
    6-1-response-to-stimuli-a-level-only
    12 主题
  30. 6-2-nervous-coordination-a-level-only
    10 主题
  31. 6-3-skeletal-muscles-a-level-only
    6 主题
  32. 6-4-homeostasis-a-level-only
    11 主题
  33. 7-genetics-populations-evolution-and-ecosystems-a-level-only
    7-1-inheritance-a-level-only
    6 主题
  34. 7-2-populations-a-level-only
    3 主题
  35. 7-3-evolution-a-level-only
    5 主题
  36. 7-4-populations-in-ecosystems-a-level-only
    7 主题
  37. 8-the-control-of-gene-expression-a-level-only
    8-1-genetic-mutations-a-level-only
    2 主题
  38. 8-2-regulation-of-gene-expression-a-level-only
    11 主题
  39. 8-3-using-genome-projects-a-level-only
    4 主题
  40. 8-4-gene-technologies-a-level-only
    13 主题
  41. exam-guidance-and-skills
    essay-guidance
    3 主题
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Microscopes

  • Microscopes can be used to analyse cell components and observe organelles

  • Key terms when discussing microscopy include:

    • Magnification: how many times larger the image is than the actual object

    • Resolution: the ability to distinguish two close objects as separate

Optical (light) microscopes

  • Use light to form images

  • Light microscopes are limited by low resolution and magnification

    • Maximum resolution is around 0.2 µm (200 nm)

      • It is possible to view the nucleus, mitochondria and chloroplasts

      • It is not possible to view ribosomes, ER or lysosomes

    • Maximum magnification for light microscopes is around ×1500

  • It is possible to observe live specimens and produce colour images with a light microscope

Electron microscopes

  • Use a beam of electrons to form the image

    • This means a shorter wavelength and higher resolution

    • Maximum resolution is around 0.0002 µm (0.2 nm)

      • It is possible to view smaller organelles (e.g. ribosomes, ER)

    • Maximum magnification is around ×1,500,000

  • Electron microscopes produce black and white images and specimens must be dead

  • There are two types of electron microscopes:

    • Transmission electron microscopes (TEMs)

    • Scanning electron microscopes (SEMs)

Transmission electron microscopes (TEMs)

  • Electrons pass through specimen

  • Gives high-resolution, 2D images of internal structures

  • Limitations include:

    • Specimens must be thin

    • Cannot view live cells

    • May introduce artefacts

Scanning electron microscopes (SEMs)

  • Electrons scan the specimen surface

  • Produces 3D images of external surfaces

  • Limitations include:

    • Lower resolution than TEM

    • Cannot view live specimens

Comparing microscopes

Feature

Light microscope (optical)

Transmission EM (TEM)

Scanning EM (SEM)

Radiation used

light

electrons

electrons

Resolution

~0.2 µm (200 nm)

~0.0002 µm (0.2 nm)

~0.002 µm (2 nm)

Magnification

up to ×1500

up to ×1,500,000

up to ×500,000

Image type

2d, colour

2d, black and white

3d, black and white

Specimen state

living or dead

dead only (due to vacuum)

dead only (due to vacuum)

Preparation

simple

complex, may introduce artefacts

complex, may introduce artefacts

Sample thickness

thick acceptable

must be very thin

can be thick or 3d

Structures visible

nucleus, mitochondria, chloroplasts

internal structures, organelles

surface details, external structures

Cost and availability

inexpensive, common in schools

expensive, specialised

expensive, specialised

Examiner Tips and Tricks

Focus on how each microscope works and be ready to justify which is most suitable in a given scenario.
Early scientists using electron microscopes struggled to tell apart real cell structures and artefacts (e.g. dust, air bubbles or fingerprints)

  • They had to prepare samples in different ways to see if a structure was real or an artefact

  • Over time, improved techniques helped reduce artefacts and increase confidence in identifying organelles