Cell Theory Lecture Notes

Introduction to Cell Theory

Smallest Units of Life
- Cells are the smallest units of life.
- Organelles (e.g., nucleus, mitochondria) cannot perform life processes independently.
Composition of Living Things
- All living organisms are made up of cells.
- Examples:
  - Bacteria: single-celled.
  - Plants and Animals: multicellular.
  - Viruses: not considered living, not composed of cells.
Origin of Cells
- Cells arise from pre-existing cells through cell division.
- Types of cell division:
  - Prokaryotes: binary fission.
  - Eukaryotes: fission, mitosis, meiosis.

Consistency of Principles: The principles of cell theory have never been disproven.
Observation of Cell Structures: Tissues from various life forms show one or more cells.
Cell Division Observed: Cells forming from division is a documented process.
Rejection of Spontaneous Generation: No evidence supports the spontaneous generation of cells.

1590: Zacharias Janssen invents the compound microscope.
1665: Robert Hooke discovers cells in cork tissue.
1674: Anton van Leeuwenhoek observes living microorganisms in pond water (termed "animalcules").
1838: Matthias Schleiden states all plants are made of cells.
1839: Theodor Schwann states all animals are made of cells.
1855: Rudolf Virchow concludes all cells arise from pre-existing cells.

Some organisms and cell types do not fit the classical definition of a cell.
Atypical Cells:
- Striated Muscle Fibres: Formed by the fusion of multiple cells, leading to a long fibre with multiple nuclei.
- Aseptate Fungal Hyphae: Large structures with incomplete separation, creating a continuous cytoplasm.
- Unicellular Giant Algae (e.g., Acetabularia): Large single-celled organisms (up to 10cm) with complex structures.

Cell theory is foundational to biology, but exceptions exist that challenge its traditional definitions.

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