Cell Theory Lecture Notes

Introduction to Cell Theory

• Cell theory states that all living organisms are composed of cells.

Three Principles of Cell Theory

1. Smallest Units of Life

   • Cells are the smallest units of life.
   • Organelles (e.g., nucleus, mitochondria) cannot perform life processes independently; they rely on the cell.

2. Composition of Living Things

   • All living things are made of cells.
   • Examples:
     • Bacteria: single-celled organisms
     • Plants and Animals: multicellular organisms
     • Viruses are not considered alive and are not made of cells.

3. Cells from Pre-existing Cells

   • Cells arise from the division of existing cells.
   • Mechanisms of cell division:
     • Prokaryotes: binary fission
     • Eukaryotes: fission, mitosis, meiosis.

Evidence Supporting Cell Theory

• No observations contradicting the principles.
• Subcellular components cannot perform life functions independently.
• Presence of cells confirmed in tissues across various life forms (plants, animals, fungi, bacteria, protists).
• Observation of cell division has been documented, while spontaneous generation has not.

Historical Development of Cell Theory

• 1590: Zacharias Janssen invented the compound microscope.
• 1665: Robert Hooke discovered "cells" in cork tissue.
• 1674: Anton van Leeuwenhoek observed living organisms in pond water (termed "animalcules").
• 1838: Matthias Schleiden concluded all plants are made of cells (co-credit for first two principles).
• 1839: Theodor Schwann concluded all animals are made of cells (co-credit for first two principles).
• 1855: Rudolf Virchow concluded that all cells come from pre-existing cells (credited for the third principle).

Evidence Against Cell Theory

• Some exceptions exist that challenge cell theory:
  • Striated Muscle Fibers:
    • Formed by fusion of multiple cells, resulting in long fibers with a single plasma membrane and multiple nuclei.
  • Aseptate Fungal Hyphae:
    • Large filamentous structures with incomplete cell separation, creating continuous cytoplasm with multiple nuclei.
  • Unicellular Giant Algae (e.g., Acetabularia):
    • Complex structures, single cells can grow large (up to 10cm) while maintaining unicellularity.

Conclusion

• Cell theory is foundational in biology but is subject to exceptions.
• Understanding these exceptions helps deepen comprehension of cellular organization and function.