1. Discovery of the Cell
| Scientist | Year | Contribution |
|---|---|---|
| Robert Hooke | 1665 | First observed and named cells — saw box-like compartments in cork (dead cells); coined the term "cell" (Latin: cella = small room) |
| Antonie van Leeuwenhoek | 1674 | First observed living cells (free-living cells) — bacteria and protozoa in pond water; called them "animalcules"; developed improved microscopes |
| Robert Brown | 1831 | Discovered the nucleus in plant cells |
| Matthias Schleiden | 1838 | Proposed that all plants are made of cells |
| Theodor Schwann | 1839 | Proposed that all animals are made of cells; together with Schleiden formulated the Cell Theory |
| Rudolf Virchow | 1855 | Added the third tenet: "Omnis cellula e cellula" — every cell arises from a pre-existing cell; completed the Modern Cell Theory |
2. Cell Theory
Original Cell Theory (Schleiden & Schwann, 1838–39)
- All living organisms are composed of cells.
- The cell is the basic unit of life.
Modern Cell Theory (with Virchow's addition)
- All living organisms are composed of one or more cells.
- The cell is the basic structural and functional unit of life.
- All cells arise from pre-existing cells (Omnis cellula e cellula — Virchow, 1855).
- All cells are similar in chemical composition and metabolic activities.
- All energy flow (metabolism and biochemistry) of life occurs within cells.
- Cells contain hereditary information (DNA) which is passed on during cell division.
Exceptions to Cell Theory
- Viruses: Acellular — not made of cells. Considered at the boundary of living and non-living.
- First cell: The very first cell could not have arisen from a pre-existing cell — origin of life is a challenge to Virchow's tenet.
- Multinucleate organisms: Some organisms like Rhizopus (coenocytic fungi) and Vaucheria (algae) are not divided into separate cells despite having multiple nuclei.
3. An Overview of Cell Types
All cells fall into one of two fundamental categories based on their internal organisation:
- Prokaryotic cells (Greek: pro = before; karyon = nucleus) — no membrane-bound nucleus or organelles.
- Eukaryotic cells (Greek: eu = true; karyon = nucleus) — membrane-bound nucleus and organelles present.
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Cell size | Generally small (1–10 µm) | Generally larger (10–100 µm) |
| Nuclear membrane | Absent — nucleoid region (naked DNA) | Present — true nucleus with nuclear envelope |
| Number of chromosomes | Single circular chromosome (+ plasmids) | Multiple linear chromosomes |
| Histone proteins | Absent (DNA not associated with histones) | Present — DNA wrapped around histones |
| Membrane-bound organelles | Absent (no mitochondria, ER, Golgi, etc.) | Present (mitochondria, ER, Golgi, etc.) |
| Ribosome size | 70S (50S + 30S subunits) | 80S (60S + 40S subunits); 70S in organelles |
| Cell wall | Present (peptidoglycan in bacteria) | Present in plants (cellulose), fungi (chitin); absent in animal cells |
| Flagella | Simple — made of flagellin protein | Complex — 9+2 arrangement of microtubules |
| Cell division | Binary fission (no mitosis/meiosis) | Mitosis and meiosis |
| Examples | Bacteria, Archaea, Mycoplasma, Cyanobacteria | Fungi, Protists, Plants, Animals |
4. Prokaryotic Cell — Structure in Detail
The prokaryotic cell is structurally simpler but metabolically diverse. The best-studied prokaryote is Escherichia coli (E. coli). Key structures:
Cell Envelope
The prokaryotic cell envelope consists of three layers (from outside to inside):
- Glycocalyx: Outermost layer — can be a loose slime layer or a rigid capsule. The capsule (e.g., in Streptococcus pneumoniae) protects from phagocytosis and desiccation.
- Cell wall: Made of peptidoglycan (murein) — a polymer of sugars cross-linked by peptide bridges. Provides shape and protection. Absent in Mycoplasma (the smallest living cell).
- Plasma membrane: Innermost layer — phospholipid bilayer with proteins.
Gram Staining — Classification of Bacteria
| Feature | Gram Positive (+) | Gram Negative (−) |
|---|---|---|
| Cell wall thickness | Thick peptidoglycan layer | Thin peptidoglycan layer |
| Outer membrane | Absent | Present (lipopolysaccharide) |
| Stain colour | Purple/violet (retains crystal violet) | Pink/red (does not retain crystal violet) |
| Examples | Staphylococcus, Streptococcus, Bacillus | E. coli, Salmonella, Vibrio cholerae |
Internal Structures of Prokaryotic Cell
| Structure | Description |
|---|---|
| Nucleoid | Region where the single circular DNA chromosome is located — not membrane-bound; no histone proteins |
| Plasmids | Small, circular, extrachromosomal DNA molecules; carry accessory genes (e.g., antibiotic resistance); not essential for survival |
| Ribosomes | 70S ribosomes (50S + 30S subunits); site of protein synthesis; freely distributed in cytoplasm |
| Mesosome | Infoldings of plasma membrane; help in cell wall formation, DNA replication, and respiration (functionally analogous to mitochondria) |
| Inclusion bodies | Reserve materials stored in cytoplasm — phosphate granules, cyanophycean granules, glycogen granules; not membrane-bound |
| Gas vacuoles | Found in aquatic prokaryotes (cyanobacteria); help in buoyancy |
Appendages — Flagella and Pili
- Flagella: Long, whip-like appendages for locomotion. Made of flagellin protein. Much simpler than eukaryotic flagella — no microtubules. Can be monotrichous (1), lophotrichous (tuft at one end), amphitrichous (both ends), or peritrichous (all around).
- Pili (fimbriae): Short, hair-like projections. Help in attachment to surfaces. Sex pili are used for conjugation (transfer of genetic material between bacteria).
5. Eukaryotic Cell — Overview
Eukaryotic cells are more complex, compartmentalised, and generally larger than prokaryotic cells. The defining feature is the presence of a membrane-bound nucleus. All multicellular organisms are eukaryotic.
Plant Cell vs Animal Cell
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Present (cellulose) | Absent |
| Plastids | Present (chloroplasts, chromoplasts, leucoplasts) | Absent |
| Large central vacuole | Present — maintains turgor pressure | Absent (small vacuoles may be present) |
| Centrioles | Absent (in higher plants) | Present — involved in cell division |
| Lysosomes | Rarely present | Present — digestive organelles |
| Glyoxysomes | Present (fat storage seeds) | Absent |
| Plasmodesmata | Present (cytoplasmic connections between cells) | Absent |
| Shape | Fixed (rigid cell wall) | Variable (no cell wall) |
6. The Endomembrane Concept and Compartmentalisation
A key advantage of eukaryotic cells is compartmentalisation — different metabolic processes are segregated in specialised membrane-bound organelles, allowing them to occur simultaneously without interference.
- Nucleus: DNA storage, transcription, ribosome assembly.
- Endoplasmic Reticulum: Protein synthesis (rough ER) and lipid synthesis (smooth ER).
- Golgi apparatus: Modification, sorting, and packaging of proteins.
- Mitochondria: ATP production (cellular respiration).
- Chloroplasts (plants): Photosynthesis.
- Lysosomes: Intracellular digestion.
- Peroxisomes: Oxidative reactions, fatty acid metabolism.
- Vacuoles: Storage (plants: large central vacuole for turgor; animals: small food/contractile vacuoles).
Endosymbiotic Theory
Proposed by Lynn Margulis — mitochondria and chloroplasts were once free-living prokaryotes that were engulfed by ancestral eukaryotic cells and formed a symbiotic relationship. Evidence includes: both have their own circular DNA, 70S ribosomes, double membranes, and divide by binary fission — all prokaryotic characteristics.
7. Sizes of Cells and Microscopy
| Cell/Structure | Approximate Size | Visible with |
|---|---|---|
| Mycoplasma (smallest cell) | 0.1–0.3 µm | Electron microscope |
| Most bacteria | 1–10 µm | Light microscope |
| Most eukaryotic cells | 10–100 µm | Light microscope |
| Human RBC | ~8 µm diameter | Light microscope |
| Largest cell (ostrich egg) | ~170 mm (yolk) | Naked eye |
| Longest cell (nerve cell) | Up to 1 metre | Naked eye (length) |
| Acetabularia (largest single-celled organism) | Up to 10 cm | Naked eye |
