Class 9 Science Notes
Chapter 2 – Cell: The Building Block of Life
1. Introduction
- All living organisms are made up of tiny units called cells.
- A cell is the basic structural and functional unit of life.
- Some organisms have only one cell (unicellular) while others have many cells (multicellular).
Examples
- Unicellular: Amoeba, Bacteria, Yeast
- Multicellular: Humans, Mango tree, Fish
2. Discovery of the Cell
| Scientist | Contribution |
|---|---|
| Robert Hooke (1665) | First observed cells in cork using a microscope. |
| Matthias Schleiden | Stated that all plants are made of cells. |
| Theodor Schwann | Stated that all animals are made of cells. |
| Rudolf Virchow | Proposed that every new cell comes from an existing cell. |
3. Cell Theory
The Cell Theory states that:
- All living organisms are made of one or more cells.
- The cell is the basic unit of structure and function.
- New cells arise only from pre-existing cells.
4. Microscope
Cells are too small to be seen with the naked eye.
A microscope helps us to:
- Observe tiny cells
- Study cell structure
- Understand cell functions
Types:
- Light Microscope
- Electron Microscope (more powerful)
5. Main Parts of a Cell
Every cell has three basic parts:
(a) Cell Membrane
- Thin outer covering
- Protects the cell
- Controls entry and exit of substances
- Selectively permeable
(b) Cytoplasm
- Jelly-like fluid
- Contains all cell organelles
- Site of many life processes
(c) Nucleus
- Control centre of the cell
- Contains DNA and chromosomes
- Controls growth and reproduction
6. Cell Wall
Present in:
- Plant cells
- Fungi
- Bacteria
Absent in:
- Animal cells
Functions:
- Gives shape
- Provides strength
- Protects the cell
- Freely permeable
7. Osmosis
Definition:
Movement of water through a selectively permeable membrane from higher water concentration to lower water concentration.
Examples:
- Raisins swell in water.
- Potato shrinks in concentrated salt solution.
8. Diffusion
Movement of particles from higher concentration to lower concentration.
Examples:
- Smell of perfume spreads in a room.
- Oxygen enters body cells.
9. Prokaryotic and Eukaryotic Cells
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | Not well-defined | Well-defined |
| Organelles | Absent | Present |
| Size | Small | Large |
| Example | Bacteria | Plants and Animals |
10. Cell Organelles
(a) Nucleus
Function:
- Controls activities of the cell
- Stores genetic material (DNA)
Remember:
Nucleus = Brain of the Cell
(b) Ribosomes
Function:
- Prepare proteins
Remember:
Protein Factory
(c) Endoplasmic Reticulum (ER)
Rough ER
- Has ribosomes
- Makes proteins
Smooth ER
- No ribosomes
- Makes fats (lipids)
(d) Golgi Apparatus
Functions:
- Modifies proteins
- Packages materials
- Sends them to different parts of the cell
Remember:
Packing and Dispatch Centre
(e) Lysosomes
Functions:
- Digest waste
- Destroy damaged organelles
- Keep the cell clean
Remember:
Clean-up Team
(f) Mitochondria
Functions:
- Produce energy
- Carry out cellular respiration
- Produce ATP
Remember:
Powerhouse of the Cell
(g) Plastids (Plant Cells Only)
Chloroplast
- Green colour
- Contains chlorophyll
- Performs photosynthesis
Chromoplast
- Gives red, yellow or orange colour
Leucoplast
- Stores food
(h) Vacuole
Plant Cell:
- One large vacuole
- Stores water, food and waste
- Maintains firmness
Animal Cell:
- Small or absent
11. Plant Cell vs Animal Cell
| Plant Cell | Animal Cell |
|---|---|
| Cell wall present | Cell wall absent |
| Chloroplast present | Chloroplast absent |
| Large vacuole | Small vacuoles |
| Fixed shape | Irregular shape |
12. Cell Division
Cell division helps in:
- Growth
- Repair
- Replacement of damaged cells
- Reproduction
Mitosis
- Produces 2 identical cells.
- Chromosome number remains the same.
- Occurs in body cells.
Functions:
- Growth
- Healing wounds
- Repair
Meiosis
- Produces 4 cells.
- Chromosome number becomes half.
- Occurs in reproductive organs.
Functions:
- Formation of gametes
- Creates variation
13. DNA, Genes and Chromosomes
DNA:
Stores hereditary information.
Genes:
Small sections of DNA that control traits.
Chromosomes:
Structures made of DNA and proteins.
14. Cell Culture
Cells can be grown outside the body under suitable laboratory conditions.
Uses:
- Vaccine production
- Medicine
- Scientific research
- Biotechnology
15. Cancer
Cancer develops when cells divide uncontrollably.
Normal Cells:
- Grow in a controlled way
- Die naturally
Cancer Cells:
- Continue dividing
- Form tumours
16. Important Differences
Cell Membrane vs Cell Wall
| Cell Membrane | Cell Wall |
|---|---|
| Living | Non-living |
| Selectively permeable | Freely permeable |
| Present in all cells | Present only in plants, fungi and bacteria |
Rough ER vs Smooth ER
| Rough ER | Smooth ER |
|---|---|
| Ribosomes present | Ribosomes absent |
| Protein synthesis | Fat synthesis |
Mitosis vs Meiosis
| Mitosis | Meiosis |
|---|---|
| 2 daughter cells | 4 daughter cells |
| Same chromosome number | Half chromosome number |
| Growth and repair | Formation of gametes |
17. Keywords
- Cell
- Cell membrane
- Cell wall
- Cytoplasm
- Nucleus
- DNA
- Gene
- Chromosome
- Osmosis
- Diffusion
- Mitochondria
- Chloroplast
- Ribosome
- Vacuole
- Lysosome
- Golgi apparatus
- Endoplasmic reticulum
- Mitosis
- Meiosis
- Cell Theory
18. One-Line Revision
- Cell is the basic unit of life.
- Hooke discovered cells.
- Cell membrane is selectively permeable.
- Cell wall provides strength.
- Nucleus controls the cell.
- Ribosomes make proteins.
- Mitochondria produce energy.
- Chloroplast performs photosynthesis.
- Vacuole stores materials.
- Mitosis forms two identical cells.
- Meiosis forms four reproductive cells.
- DNA carries hereditary information.
- Cancer is caused by uncontrolled cell division.
Exam Tips
✅ Learn all organelle functions.
✅ Practice plant cell and animal cell diagrams.
✅ Remember important differences in table form.
✅ Revise keywords daily.
✅ Understand osmosis and diffusion with examples instead of memorising definitions.