INTRODUCTION TO CELL & CELL THEORY – Concise Integrated Notes
- Cell Biology (Cytology): Study of structure, function, and behavior of cells — the basic units of life.
- Combines genetics, biochemistry, and molecular biology.
- Focuses on cell structure, organelles, division, communication, and metabolism.
- Cytology is microscopic cell study used in diagnostics (e.g. cancer screening).
What is a Cell?
- From Latin cellula = “small room.”
- Smallest structural and functional unit of life.
- Each /a single cell is capable of performing all life functions like metabolism, growth, and reproduction.
- Organisms are unicellular- capable of independent existence and perform all necessary life functions (e.g. Amoeba) or multicellular (e.g. humans).
- Average size: 0.1–100 µm; smallest – Mycoplasma (0.1 µm); largest – Caulerpa (30 cm).
- * Note: Anything less than a complete structure of a cell does not ensure independent living.
1. Early Beliefs
Scientists once believed that life arose spontaneously from nonliving matter (spontaneous generation). This view changed as microscopy and experimental biology advanced.
2. Historical Development of the Cell Concept
| Year | Scientist(s) | Contribution / Publication | Key Observation / Concept |
| 1665 | Robert Hooke | Micrographia | Observed dead cork cells; coined the term “cell.” |
| 1673–76 | Antonie van Leeuwenhoek (Holland) | Handcrafted microscopes | First to see living cells—blood cells, bacteria, spermatozoa (“animalcules”). |
| 1820–30 | H. Milne-Edwards & F.V. Raspail (France) | Microscopic studies | Tissues consist of globular units performing physiological roles; Raspail said “Omnis cellula e cellula.” |
| 1820–30 | R.J.H. Dutrochet (France) | Comparative studies | Proposed that the cell is the basic unit of organization and physiology; all life arises from cells; plant and animal cells are similar. |
| 1833 | Robert Brown (Scotland) | Plant studies | Discovered the nucleus as a constant cell component. |
| 1839 | Purkinje (Bohemia) | Animal embryology | Identified protoplasm – the living substance of the cell. |
| 1838 | M.J. Schleiden (Germany) | Studies on plants | “All plants are composed of cells”; wrote the first tenet of cell theory. |
| 1839 | Theodor Schwann (Germany) | Microscopical Researches… | Combined his and Schleiden’s work to form the Classical Cell Theory. |
| 1852 | Robert Remak (Germany) | Embryology | Demonstrated cell division – cells arise from pre-existing cells (before Virchow). |
| 1858 | Rudolf Virchow (Germany) | Cellular Pathology | Popularized “Omnis cellula e cellula” – all cells arise from pre-existing cells. |
Claims and Counterclaims
Credit is often given to Schleiden and Schwann, but similar ideas were proposed earlier by L. Oken (1805), Dutrochet (1824), Purkinje (1834), G.G. Valentin (1834), and F.V. Raspail. Robert Remak (1852) demonstrated cell division before Virchow.
3. Classical Cell Theory (Schleiden & Schwann, 1838–39)
Main Tenets:
- All living organisms are composed of cells and their products.
- The cell is the basic structural and functional unit of life.
- (Virchow, 1858) All cells arise from pre-existing cells (“Omnis cellula e cellula”).
4. Modern Cell Theory
Objective: To explain cellular basis of heredity, energy, and biochemistry.
Postulates:
- All living organisms are made of one or more cells and their products.
- The cell is the basic structural, functional, and genetic unit of life.
- All cells arise only from pre-existing cells (principle of biogenesis).
- Energy flow and metabolism occur within cells.
- Cells contain DNA/RNA (genetic material) transmitted during division.
- Cells of similar species have similar chemical composition.
- The activity of an organism is the sum of activities of its cells.
Supporting Discoveries:
- Electron Microscope (1931): Revealed organelles like mitochondria and ribosomes.
- Watson & Crick (1953): DNA double helix established heredity mechanism.
- Singer & Nicolson (1972): Fluid Mosaic Model of membrane structure.
- Modern tools: CRISPR, genetic engineering, stem cell therapy, fluorescence microscopy.
5. Key Related Concepts
| Concept | Scientist(s) | Description |
| Cytology (Cell Biology) | — | Study of structure, function, and behavior of cells; integrates genetics, biochemistry, and molecular biology. |
| Protoplasm Theory | Purkinje | Cell interior is a living, dynamic fluid. |
| Membrane Theory | Pfeffer (1877) | Plasma membrane acts as a selective barrier. |
| Osmotic Studies | Hamburger (1889) | Demonstrated osmosis and selective permeability in RBCs. |
6. Characteristics of Cells
| Feature | Description / Example |
| Definition | From Latin cellula = “small room”; smallest unit capable of metabolism, growth, and reproduction. |
| Size | 0.1–100 µm; smallest – Mycoplasma (0.1 µm); largest – Caulerpa (30 cm). |
| Shapes | Nerve cells – long, branched (impulse conduction); RBCs – biconcave (O₂ transport); Muscle cells – spindle-shaped (contraction); Plant cells – rectangular (support). |
- Anomalies / Exceptions to Classical Cell Theory
| Organism / Condition | What Classical Theory States | Reality / Exception |
| Viruses | All life functions occur within cells | Acellular; replicate only inside host cells using host machinery. |
| Prokaryotes (Bacteria, Mycoplasma) | Cells have a nucleus | Lack membrane-bound nucleus and organelles. |
| Multinucleate / Coenocytic cells (Vaucheria, Rhizopus, tapetal cells) | Each cell has a single nucleus | Multinucleate cytoplasm without partition walls. |
| Mammalian RBCs, sieve tubes | Every cell has a nucleus | Mature forms lack nuclei but perform limited functions. |
| Striated muscle fibres | Each cell has one nucleus | Multinucleated syncytia act as one unit. |
| Size exception | Cells are microscopic | Acetabularia single cell up to 7 cm long. |
- Comparative Table: Cells vs. Non-cellular Entities
| Feature | Cells | Viruses | Prions | Viroids |
| Nature | Living | Living only in host | Non-living | Non-living |
| Structure | Membrane, cytoplasm, organelles | DNA/RNA + protein coat | Protein only | RNA only |
| Genetic Material | DNA (±RNA) | DNA or RNA | None | RNA |
| Reproduction | By cell division | Only in host | Converts normal proteins | Infects plant cells |
| Example | Bacteria, plants, animals | HIV, Influenza | BSE (Mad Cow) | Potato spindle tuber viroid |
9. Significance of Cell Theory
Cell Theory is the foundation of modern biology and medicine. It explains structure, function, heredity, and disease at the cellular level and unifies understanding of life from microbes to humans. Exceptions such as viruses and coenocytic organisms highlight the diversity and complexity of life beyond traditional cellular definitions.
Summary: Modern Cell Theory integrates structural, physiological, genetic, and biochemical unity of all living organisms, while its exceptions expand the concept of life’s cellular basis.
