Cells are the fundamental unit of life and all living things are made up of cells or cell products (e.g. hair, nails, etc.)

Every cell can carry out the basic functions of life, which include:

  • Movement:   Living things show movement, either externally or internally
  • Reproduction:   Living things produce offspring, either sexually or asexually
  • Sensitivity:   Living things can respond to and interact with the environment
  • Growth:   Living things can grow or change size / shape
  • Respiration:   Living things use substances from the environment to make energy
  • Excretion:   Living things exhibit the removal of wastes
  • Nutrition:   Living things exchange materials and gases with the environment

There are two main types of cells - prokaryotes and eukaryotes - which give rise to the six kingdoms of life

  • Prokaryotic cells are simpler in structure and lack a nucleus and membrane-bound organelles
  • Eukaryotic cells are more complex and have compartmentalisation of function due to the presence of a nucleus and membrane-bound organelles

The Six Kingdoms of Life


  • Prokaryotic cells are believed to be the originators of life on Earth, with eukaryotes evolving from prokaryotic ancestors via endosymbiosis
  • Endosymbiotic theory proposes that eukaryotic organelles arose from independent prokaryotes being ingested by larger prokaryotes via endocytosis
  • Instead of being digested, the smaller prokaryote formed a symbiotic relationship with the host, over generations becoming an organelle


  • Evidence for the theory of endosymbiosis includes:
    • Mitochondria and chloroplast have their own DNA (circular and naked like bacterial DNA)
    • They have double membranes (consistent with the formation of a vesicle when first ingested)
    • They have ribosomes similar to prokaryotes (70S)
    • They replicate like prokaryotes, have an internal chemistry similar to prokaryotes and are sensitive to certain antibiotic agents

Relative Sizes of Cells

Relative sizes:                                                                 Unit Conversion Table:

  • A molecule = 1 nm
  • Cell membrane thickness = 7.5 nm
  • Virus = 100 nm  (range: 20 - 200 nm) 
  • Bacteria = 1 - 5 um
  • Organelles = <10 um
  • Eukaryotic cells = <100 um

Diagram of the Relative Sizes and Scale of Biological Materials

Cell Size and Scale (Learn Genetics)


  • Because cells are small, microscopes can be used to visualise cell structures in order to discern their functions and properties
  • There are two main types of microscopes used by scientists to view living organisms
    • Light Microscopes:  Visible light is passed through an object and is focused on the eye via a series of lenses
    • Electron Microscopes:  Beams of electrons are passed through an object and focused on a photographic plate using an electromagnetic field
  • Transmission electron miscroscopy (TEM) is used to visualise internal structures while scanning electron microscopy (SEM) is used for surfaces

Comparison of Light and Electron Microscopes

Bamboo Vascular Bundles with Different Microscopy

Calculating Magnification

To calculate the linear magnification of a drawing the following equation should be used:

  • Magnification = Size of image (with ruler) ÷ Actual size of object (according to scale bar)

To calculate the actual size of a magnified specimen the equation is simply re-arranged:

  • Actual size = Size of image (with ruler) ÷ Magnification


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