Cell theory

Understanding: Cell Theory Explains That Living Organisms Are Composed Of Cells

The cell theory states that:

  • Cells are the fundamental building blocks of all living organisms (i.e. all living organisms are composed of cells)
    • Unicellular organisms are composed of one single cell (e.g. bacteria) and therefore are the smallest type of organisms possible.
    • Multicellular organisms (e.g. humans) are composed of many cells
  • The cell is the smallest unit of life à they are the smallest structures capable of surviving on their own
  • Cells arise from pre-existing cells (e.g. new cells arise from cell division and a zygote (the very first cell formed when an organism is produced) arises from the fusion of an egg cell and a sperm cell)

Figure 1.1 .1 – Moss Leaf Cells 


  • Cell Theory states that:
    • All living organisms are made of cells
    • The cell is the smallest unit of life
    • Cells arise from pre-existing cells 

Application: Questioning the cell theory using atypical examples, including striated muscle, giant algae and aseptate fungal hyphae.

There are exceptions/discrepancies to the cell theory, the following are not made up of typical/regular cells:

  • Striated muscle: skeletal muscle is composed of muscle fibres, these are long fibres that can measure 300mm or more and are therefore much larger than regular cells. In addition to this, each muscle fibre contains hundreds of nuclei

Figure 1.1.2 - Striated muscle fibres

  • Giant algae: these organisms are able to grow up to 100 mm in length yet they are unicellular and contain only one nucleus. Due to their size one would expect them to be composed of many cells.

Figure 1.1.3 - Giant Algae

  • Aseptate fungi: fungi are made of thread like structures called hyphae (normal in fungi) divided by septa (figure 1.1.4), however in aseptate fungi these hyphae are not divided into sections, instead they are uninterrupted tube like structures which contain many nuclei (figure 1.1.5). 

 Figure 1.1.4 - Normal septa divided hyphae

  Figure 1.1.5 - Fungi with aseptate hyphae


  • Exceptions to cell theory:
    • Striated muscle - made of 300 mm (or more) long cells with hundreds of nuclei
    • Giant algae - 100 mm organism made of one long cell and containing only one nuclei
    • Aseptate fungi - fungi with long undivided hyphae containing many nuclei