Cell Theory Lecture Notes

Overview of Cell Theory

• Cell Theory Definition: Theory stating all living organisms are composed of cells.
• Three Principles of Cell Theory:
  1. Cells are the smallest units of life.
     • Organelles (e.g., nucleus, mitochondria) cannot exist independently.
  2. All living things are composed of cells.
     • Examples: Bacteria (single-cell), plants and animals (multi-cellular).
     • Viruses are non-living, not made of cells.
  3. Cells come from pre-existing cells.
     • Cell formation through division: binary fission in prokaryotes, fission, mitosis, or meiosis in eukaryotes.

Evidence Supporting Cell Theory

• Principles consistently observed and validated.
• Subcellular components (organelles) cannot perform life functions independently.
• Universal presence of cells in tissues from varied organisms (plants, animals, fungi, bacteria, protists).
• Observations of cell division; no spontaneous generation observed.

Historical Development of Cell Theory

• 1590: Compound microscope invented by Zacharias Janssen.
• 1665: Robert Hooke discovers "cells" in cork using a light microscope.
• 1674: Anton van Leeuwenhoek observes "animalcules" in pond water.
• 1838: Matthias Schleiden establishes that all plants are made of cells.
• 1839: Theodor Schwann concludes all animals are made of cells.
• 1855: Rudolf Virchow asserts "all cells come from cells."

Evidence Against Cell Theory

• Atypical Cells:
  • Striated Muscle Fibres:
    • Fusion of multiple cells, possessing multiple nuclei in a single plasma membrane.
  • Aseptate Fungal Hyphae:
    • Large, filamentous structures with continuous cytoplasm, not distinct cells.
  • Unicellular Giant Algae (e.g., Acetabularia):
    • Large single-celled organisms (up to 10cm), complex structures challenging size assumptions.

Conclusion

• Cell theory holds strong as a foundational biological concept, though there are rare exceptions challenging some assumptions.

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