PRELIMS
Cell Biology Foundations
Cell is the basic structural and functional unit of life. All living beings are composed of one or more cells.
The Discovery and Size of Cells
- Discovery: The word 'cell' was coined by Robert Hook (1665) while observing thin slices of cork.
- Cell Sizes:
- Smallest Cell: Mycoplasma gallisepticum.
- Longest Cell: Neuron (Nerve cell).
- Biggest Cell: Egg of Ostrich.
Cell Theory
Proposed by Schleiden and Schwann (1839).
- All organisms are composed of one or more cells.
- Cell is the structural and functional unit.
- Viruses are an exception to Cell Theory.
Later, Rudolf Virchow (1855) added the third principle — "Omnis cellula e cellula" (all cells arise from pre-existing cells), refuting spontaneous generation.
Mitosis vs. Meiosis (Cell Division)
Mitosis
- Occurs in: Somatic (body) cells
- Divisions: One
- Daughter cells: 2, genetically identical to parent
- Chromosome number: Same as parent (2n → 2n)
- Purpose: Growth, repair, asexual reproduction
Meiosis
- Occurs in: Germ cells (during gamete formation)
- Divisions: Two (Meiosis I and II)
- Daughter cells: 4, genetically distinct (due to crossing over)
- Chromosome number: Halved (2n → n)
- Purpose: Sexual reproduction; maintains chromosome number across generations
Prokaryotic vs. Eukaryotic Cells
Prokaryotic Cells
- Nucleus: Undefined (Nucleoid)
- Cell Organelles: Absent (Membrane-bound)
- Ribosome: 70S type
- DNA: Circular, naked
- Division: Amitosis
- Examples: Bacteria, Cyanobacteria
Eukaryotic Cells
- Nucleus: Well-defined with nuclear membrane
- Cell Organelles: Present (Mitochondria, Golgi, etc.)
- Ribosome: 80S type
- DNA: Linear, with histones (Chromatin)
- Division: Mitosis or Meiosis
- Examples: High plants and animals
Main Components of Cell
- Cell Wall: Found in plant cells; rigid, made of Cellulose.
- Cell Membrane (Plasma Membrane): Semi-permeable; made of Lipid bilayer.
- Cytoplasm: Jelly-like substance between nucleus and membrane.
UPSC Relevance
Cell biology is the most foundational part of GS-III Biology.
- Exceptions: The fact that Viruses do not obey cell theory is a standard Prelims fact.
- Cell Comparison: Differentiating between prokaryotic and eukaryotic life forms is essential for biotechnology concepts.
- Organelles: Understanding the roles of Mitochondria (Powerhouse) vs Ribosomes (Protein factory) follows this foundational structural knowledge.