Cell - The Fundamental Unit of Life
Introduction
The cell is the basic structural and functional unit of all living organisms. ICSE Class 9 covers the discovery of cells, cell theory, types of cells, cell organelles, and an introduction to cell division.
Discovery of the Cell
- Robert Hooke (1665): Observed cork under a microscope and saw small box-like compartments, which he named 'cells'
- Anton van Leeuwenhoek (1674): First observed living cells (bacteria and protozoa)
Cell Theory
Proposed by: Matthias Schleiden (1838) and Theodor Schwann (1839), later expanded by Rudolf Virchow (1855).
Postulates:
- 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)
Prokaryotic vs Eukaryotic Cells
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Nucleus | Absent (nucleoid region) | Present (bound by nuclear membrane) |
| Size | Small (1-10 μm) | Large (10-100 μm) |
| Organelles | No membrane-bound organelles | Membrane-bound organelles present |
| Ribosomes | 70S | 80S |
| Cell wall | Present (peptidoglycan) | Present in plants (cellulose) |
| Examples | Bacteria, Blue-green algae | Plants, animals, fungi, protists |
Plant Cell vs Animal Cell
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Present (cellulose) | Absent |
| Chloroplasts | Present | Absent |
| Vacuoles | Large central vacuole | Small, temporary vacuoles |
| Shape | Fixed (rectangular) | Irregular (round) |
| Centrioles | Absent | Present |
| Cytoplasmic division | Cell plate formation | Cleavage furrow |
Cell Organelles
Plasma Membrane (Cell Membrane)
- Outermost boundary in animal cells
- Composition: Lipid bilayer with embedded proteins
- Function: Regulates entry and exit of substances (selectively permeable)
- Fluid mosaic model: Proposed by Singer and Nicolson
Cell Wall
- Outermost layer in plant cells
- Composition: Cellulose, hemicellulose, pectin
- Function: Provides structural support and protection
Nucleus
- Nuclear membrane: Double membrane with pores
- Nucleoplasm: Fluid inside the nucleus
- Chromatin: DNA-protein complex (condenses to form chromosomes during division)
- Nucleolus: Site of ribosome synthesis
- Function: Controls all cellular activities; contains genetic material
Cytoplasm
- Jelly-like substance between cell membrane and nucleus
- Contains organelles and inclusions
- Site of many metabolic reactions
Endoplasmic Reticulum (ER)
- Rough ER: Has ribosomes on surface; involved in protein synthesis
- Smooth ER: No ribosomes; involved in lipid synthesis and detoxification
Golgi Apparatus
- Stack of flattened membrane sacs (cisternae)
- Function: Modifies, packages, and transports proteins
Mitochondria
- Powerhouse of the cell
- Has double membrane (outer + inner folded into cristae)
- Contains its own DNA
- Function: ATP production through cellular respiration
Chloroplasts (in plant cells only)
- Contains chlorophyll (green pigment)
- Site of photosynthesis
- Has double membrane and internal thylakoid stacks (grana)
Lysosomes
- Suicide bags of the cell
- Contain digestive enzymes
- Function in intracellular digestion and waste removal
Ribosomes
- Site of protein synthesis
- 70S in prokaryotes; 80S in eukaryotes
Cell Division
Mitosis
- Purpose: Growth, repair, asexual reproduction
- Result: Two identical daughter cells (2n → 2n)
- Stages: Prophase, Metaphase, Anaphase, Telophase (PMAT)
- Occurs in somatic (body) cells
Meiosis
- Purpose: Formation of gametes (sex cells)
- Result: Four non-identical daughter cells (2n → n)
- Stages: Meiosis I (reduction division) and Meiosis II (equational division)
- Occurs in reproductive cells
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | 1 | 2 |
| Daughter cells | 2 | 4 |
| Chromosome number | Same as parent (2n) | Half of parent (n) |
| Genetic variation | No | Yes (crossing over) |
| Purpose | Growth, repair | Gamete formation |
Common Mistakes With Fixes
| Mistake | Correction |
|---|---|
| All cells have a nucleus | Prokaryotes do NOT have a nucleus |
| Plant cells do not have mitochondria | Plant cells DO have mitochondria |
| Cell wall is present in animal cells | Cell wall is ONLY in plant cells, fungi, bacteria |
| Mitochondria and chloroplasts have 70S ribosomes | They have 70S like prokaryotes (endosymbiotic theory) |
ICSE Exam Focus
| Topic | Marks (approx.) | Frequency |
|---|---|---|
| Cell organelles and functions | 4-5 marks | Very common |
| Plant vs animal cell | 3-4 marks | Very common |
| Prokaryotic vs eukaryotic | 3-4 marks | Common |
| Cell division (mitosis vs meiosis) | 4-5 marks | Frequently asked |
Self-Test
Q1: State the three postulates of cell theory.
Q2: Differentiate between plant cells and animal cells (4 points).
Q3: What are the functions of: (i) Mitochondria (ii) Golgi apparatus (iii) Lysosomes
Q4: Distinguish between mitosis and meiosis.
Q5: Why are mitochondria called the powerhouse of the cell?
