Cell Theory


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•  According to the cell theory, living organisms are composed of cells

Principles of the Cell Theory

The cell theory states that:

1.  All living things are composed of cells (or cell products)

2.  The cell is the smallest unit of life

3.  Cells only arise from pre-existing cells

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•  Questioning the cell theory using atypical examples (striated muscle, giant algae, aseptate hyphae)

Caveats to the Cell Theory

Certain types of cells / tissues do not conform to a standard notion of what constitutes a cell:

  • Striated muscle fibres:
    • Muscle cells fuse to form fibres that may be very long (>300mm)
    • Consequently, they have multiple nuclei despite being surrounded by a single, continuous plasma membrane
    • Challenges the idea that cells always function as autonomous units

  • Aseptate fungal hyphae:
    • Fungi may have filamentous structures called hyphae, which are separated into cells by internal walls called septa
    • Some fungi are not partitioned by septa and hence have a continuous cytoplasm along the length of the hyphae
    • Challenges the idea that living structures are composed of discrete cells

  • Giant Algae
    • Certain species of unicellular algae may grow to very large sizes (e.g. Acetabularia may exceed 7 cm in length)
    • Challenges the idea that larger organisms are always made of many microscopic cells

cell theory exceptions