๐ Class 9 Science ( Chapter: Cell โ The Fundamental Unit of Life )
๐ถ Introduction to Cells
The cell is the basic structural, functional, and biological unit of all living organisms.
All living beings are made up of cells (unicellular or multicellular).
Cells perform all essential life functions like respiration, digestion, reproduction, etc.
๐งช Discovery of Cell
| Scientist | Contribution | Year |
|---|---|---|
| Robert Hooke | Discovered dead cells in cork; coined term “cell” | 1665 |
| Anton van Leeuwenhoek | First observed living cells using a microscope | 1674 |
๐ถ Cell Theory
Proposed by: Schleiden & Schwann
Modified by: Rudolf Virchow
๐ Postulates:
All living organisms are composed of cells.
The cell is the basic unit of life.
All cells arise from pre-existing cells.
๐ถ Types of Organisms
| Type | Description | Examples |
|---|---|---|
| Unicellular | One cell performs all functions | Amoeba, Paramecium |
| Multicellular | Made of many specialized cells | Humans, Plants |
๐ถ Types of Cells
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Nucleus | No true nucleus (nucleoid) | True nucleus with nuclear membrane |
| Cell Size | Small (1โ10 ฮผm) | Larger (10โ100 ฮผm) |
| Organelles | No membrane-bound organelles | Membrane-bound organelles present |
| Example | Bacteria | Plants, Animals |
๐ถ Structure of a Cell
๐น 1. Plasma Membrane / Cell Membrane
Thin, flexible membrane surrounding the cell.
Made of lipid bilayer and proteins (Fluid Mosaic Model โ 1972).
Semi-permeable: allows selective movement of substances.
๐ Functions:
Controls entry/exit of materials.
Maintains internal environment (homeostasis).
Helps in communication and signaling.
๐น 2. Cell Wall (Only in Plant Cells)
Rigid outer layer made of cellulose.
Provides support, strength, and shape.
Fully permeable.
๐น 3. Cytoplasm
Jelly-like substance between membrane and nucleus.
Contains all organelles.
Site of many metabolic reactions.
๐น 4. Nucleus
Control center of the cell.
Enclosed by nuclear membrane.
Contains:
Nucleolus (makes ribosomes)
Chromatin (DNA + protein; becomes chromosomes during division)
๐ Functions:
Controls cell activities.
Carries genetic material.
Helps in cell division.
๐ถ Cell Organelles and Their Functions
| Organelle | Structure & Function | Present In |
|---|---|---|
| Mitochondria | Double membrane; produces energy (ATP) โ โPowerhouseโ | Plants & Animals |
| ER (Endoplasmic Reticulum) | Rough ER (ribosomes): Protein synthesis Smooth ER: Lipid synthesis & detox | Plants & Animals |
| Golgi Apparatus | Modifies, packages & transports materials | Plants & Animals |
| Lysosomes | Digestive enzymes; breaks down waste โ โSuicide bagโ | Mostly in animals |
| Plastids | Chloroplasts contain chlorophyll โ site of photosynthesis | Only in plants |
| Vacuoles | Storage of food, water, waste (Large in plants) | Both (large in plants) |
| Ribosomes | Site of protein synthesis | All cells |
| Centrosome/Centrioles | Help in cell division | Only in animals |
๐ถ Comparison Between Plant and Animal Cells
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell Wall | Present (cellulose) | Absent |
| Vacuole | Large and central | Small and many |
| Plastids | Present (e.g., chloroplasts) | Absent |
| Shape | Regular (rectangular) | Irregular (round or oval) |
| Centrosome | Absent | Present |
๐ถ Important Cell Processes
โ Diffusion
Movement of molecules from higher to lower concentration.
Passive process (no energy needed).
โ Osmosis
Movement of water molecules across a semi-permeable membrane.
From lower solute to higher solute concentration.
โ Endocytosis & Exocytosis
| Process | Description |
|---|---|
| Endocytosis | Cell engulfs materials from outside (e.g. food) |
| Exocytosis | Cell releases waste or substances |
๐ถ Diagrams (To Practice Drawing)
Plant Cell (Labelled)
Animal Cell (Labelled)
Fluid Mosaic Model of Cell Membrane
๐ Summary Points
The cell is the foundation of life.
Cell membrane is selectively permeable and controls substance movement.
Nucleus controls all activities and contains DNA.
Organelles perform specialized tasks.
Plant and animal cells differ in structure and organelles.