IB Biology, topic 1

Principles of the Cell Theory

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

Caveats to the Cell Theory

1. Striated muscle fibres

2. Aseptate fungal hyphae:

3. Giant Algae

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 idea that cells always function as autonomous units.

Giant Algae

• Certain species of unicellular algae may grow to very large sizes

• Challenges the idea that larger organisms are always made of many microscopic cells