Understanding:
• 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
Application:
• 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
