Cell — The Unit of Life
'The cell is the basic unit of structure and function in all living organisms.' — Cell Theory
1. Chapter Overview
The CELL is the FUNDAMENTAL structural and functional unit of life. This chapter covers the CELL THEORY, the difference between PROKARYOTIC and EUKARYOTIC cells, the structure and FUNCTION of each CELL ORGANELLE, the FLUID MOSAIC MODEL of the cell membrane, and the CYTOSKELETON. Understanding cells is ESSENTIAL — all life processes ultimately happen AT the cellular level.
2. Cell Theory
Historical Milestones
- Robert Hooke (1665): Observed cork cells under microscope (first to use 'cell')
- Anton van Leeuwenhoek: Observed LIVING cells (bacteria, protozoa)
- Schleiden & Schwann (1838-39): CELL THEORY
- Schleiden: Plants are made of cells
- Schwann: Animals are made of cells
- Rudolf Virchow (1855): Omnis cellula e cellula — all cells ARISE from pre-existing cells
Modern Cell Theory
- ALL living organisms are composed of cells
- Cells are the FUNDAMENTAL unit of structure and function
- All cells arise from PRE-EXISTING cells
3. Prokaryotic vs Eukaryotic Cells
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | ABSENT (nucleoid) | PRESENT (nuclear membrane) |
| DNA | Circular, NAKED | Linear, with HISTONES |
| Organelles | NO membrane-bound | MANY (mitochondria, ER, Golgi) |
| Ribosomes | 70S | 80S (cytoplasmic) |
| Cell division | Binary fission | Mitosis/Meiosis |
| Size | 1-10 μm | 10-100 μm |
| Cell wall | Present (peptidoglycan) | Plants: cellulose; Fungi: chitin |
Prokaryotic Cell Structure
- Cell envelope: Glycocalyx (capsule/slime layer) → Cell wall → Cell membrane
- Cytoplasm: No cytoskeleton, 70S ribosomes, mesosomes, inclusion bodies
- Genetic material: Single circular DNA (nucleoid), PLASMIDS (extrachromosomal)
- Flagella: For motility (different from eukaryotic flagella)
4. Eukaryotic Cell — Organelles
Cell Membrane (Plasma Membrane)
- Fluid Mosaic Model (Singer and Nicolson, 1972)
- Structure: PHOSPHOLIPID BILAYER + PROTEINS (integral + peripheral)
- Selective permeability: Small, non-polar molecules pass FREELY
- Transport: Passive (simple diffusion, facilitated, osmosis) and Active (requires ATP)
Cell Wall (Plant cells only)
- Structure: Middle lamella (pectin) → Primary wall (cellulose) → Secondary wall (lignin)
- Plasmodesmata: Cytoplasmic CONNECTIONS between adjacent cells
Endomembrane System
| Organelle | Structure | Function |
|---|---|---|
| Endoplasmic Reticulum (ER) | Network of membranous tubules | RER: Protein SYNTHESIS (ribosomes); SER: Lipid SYNTHESIS |
| Golgi Apparatus | Stack of FLAT cisternae | PACKAGING, modifying proteins; Lysosome FORMATION |
| Lysosomes | Single-membrane vesicles | INTRACELLULAR DIGESTION (hydrolase enzymes). 'SUICIDE BAGS' |
| Vacuoles | Membrane-bound sacs | STORAGE (water, nutrients, waste). Plant cell: LARGE central vacuole |
Mitochondria (Powerhouse of the Cell)
- Double membrane: Outer + Inner (cristae — FOLDED for surface area)
- Matrix: Contains DNA, ribosomes (70S), enzymes for Krebs cycle
- Function: ATP production (cellular respiration)
- Semi-autonomous: Own DNA and ribosomes
Plastids (Plant cells only)
| Type | Pigment | Function |
|---|---|---|
| Chloroplasts | Chlorophyll (green) | PHOTOSYNTHESIS |
| Chromoplasts | Carotenoids (red/yellow) | COLOUR to flowers/fruits |
| Leucoplasts | None (colourless) | STORAGE (amyloplasts — starch) |
Ribosomes
- Composition: rRNA + proteins (two subunits)
- Prokaryotic: 70S (50S + 30S). Eukaryotic: 80S (60S + 40S)
- Function: PROTEIN SYNTHESIS
Centrosome and Centrioles (Animal cells)
- Centrosome: Organising centre for MICROTUBULES
- Centrioles: 9+0 arrangement of microtubules (9 triplets)
Nucleus
- Nuclear envelope: Double membrane with NUCLEAR PORES
- Nucleolus: rRNA SYNTHESIS (dark-staining region)
- Chromatin: DNA + HISTONE proteins. Condenses to CHROMOSOMES during division
- Nucleoplasm: Fluid matrix
5. Cytoskeleton
| Filament | Diameter | Function |
|---|---|---|
| Microfilaments (Actin) | 6 nm | CELL MOVEMENT, shape, cytokinesis |
| Intermediate filaments | 10 nm | STRUCTURAL support (keratin, lamin) |
| Microtubules (Tubulin) | 25 nm | CELL shape, cilia/flagella, spindle fibres |
Cilia and Flagella
- Structure: 9+2 arrangement of microtubules (9 outer doublets + 2 central)
- Cilia: SHORT, many per cell (movement of substances)
- Flagella: LONG, one/few per cell (cell locomotion)
6. Common Mistakes
- All cells have a cell membrane, NOT all have a cell wall: Animals have ONLY the membrane
- Ribosomes are NOT membrane-bound: They are simply RNA-protein complexes
- 70S ribosomes ≠ 70S weight: S = Svedberg units (sedimentation rate, NOT mass)
- Mitochondria and plastids are SEMI-AUTONOMOUS: They have their own DNA and ribosomes but DEPEND on nuclear genes for many proteins
- ER, Golgi, and lysosomes form an ENDOMEMBRANE SYSTEM: They are INTERCONNECTED functionally and physically
7. CBSE Exam Focus
- Cell theory — contributions of scientists (1/3-mark)
- Prokaryotic vs eukaryotic cell comparison (3/5-mark)
- Fluid mosaic model of cell membrane (3-mark)
- Functions of cell organelles (mitochondria, ER, Golgi, nucleus) (3/5-mark)
- Cilia/flagella structure — 9+2 arrangement (3-mark)
8. Self-Test (5+ Q&A)
Q1: Who proposed the fluid mosaic model? Describe the structure of the cell membrane. A: SINGER and NICOLSON (1972). Phospholipid BILAYER with PROTEINS embedded. Phospholipids have hydrophilic heads (outer) and hydrophobic tails (inner). Proteins are integral (transmembrane) or peripheral.
Q2: Differentiate between RER and SER. A: RER: Has RIBOSOMES attached; involved in PROTEIN synthesis and secretion. SER: No ribosomes; involved in LIPID synthesis, detoxification, calcium storage.
Q3: What is the 9+2 arrangement? Where is it found? A: 9 OUTER doublet microtubules + 2 CENTRAL singlet microtubules. Found in the AXONEME of CILIA and FLAGELLA.
Q4: Why are mitochondria called 'semi-autonomous' organelles? A: They have their OWN DNA and 70S ribosomes, can synthesise some proteins independently. However, many proteins are still CODED by nuclear DNA → semi-autonomous.
Q5: How does the nucleolus differ from the nucleus? A: The NUCLEOLUS is a DENSE, dark-staining region within the nucleus, rich in rRNA. It is the site of rRNA SYNTHESIS and ribosome ASSEMBLY.
9. Conclusion
The cell is the BUILDING BLOCK of life. Prokaryotic cells are SIMPLER (no nucleus, no organelles) while eukaryotic cells are COMPARTMENTALISED (organelles allow specialised functions). The cell membrane CONTROLS what enters and exits. Mitochondria GENERATE energy. The ER, Golgi, and lysosomes form a synthesis-processing-recycling NETWORK. The nucleus HOUSES the genetic material. Understanding cell structure is the FOUNDATION for cell biology, genetics, and physiology — in short, for ALL of modern biology.
