Class 9 Science – Chapter 5: The Fundamental Unit of Life (Full Notes Based on Textbook
+ PDF)
🔹 1. What are Living Organisms Made Up Of?
● Cell: The structural and functional unit of life.
● Definition: A cell is the smallest unit of life capable of performing all life functions.
● Discovery: Robert Hooke (1665) observed cork under a microscope.
● Leeuwenhoek (1674): Observed living cells.
● Robert Brown (1831): Discovered the nucleus.
● Purkinje (1839): Coined the term protoplasm.
● Schleiden & Schwann (1838-39): Proposed cell theory.
● Rudolf Virchow (1855): "Omnis cellula e cellula" – all cells arise from pre-existing cells.
Cell Theory Includes:
1. All living organisms are made up of cells.
2. Cell is the basic unit of life.
3. All cells arise from pre-existing cells.
🔹 2. What is a Cell?
● A living structure enclosed by a membrane, having cytoplasm and nucleus.
● Performs essential functions like respiration, excretion, etc.
Types of Organisms:
● Unicellular: Made of a single cell (e.g., Amoeba, Paramecium, Bacteria)
● Multicellular: Made of many cells (e.g., Plants, Humans)
🔹 3. Structure of a Cell
📌 Plasma Membrane / Cell Membrane
● Definition: The thin outer boundary of the cell that controls the movement of substances
in and out.
● Nature: Selectively permeable.
● Made of: Lipids and proteins.
● Functions: Protection, transport, exchange of substances.
Transport Across Membrane:
● Diffusion: Movement of molecules (like CO₂, O₂) from high to low concentration.
● Osmosis: Movement of water through a semi-permeable membrane.
Types of Osmosis:
Type External Solution Water Movement Effect on Cell
Hypotonic Dilute (more water outside) Enters cell Cell swells (Endosmosis)
Hypertonic Concentrated (less water Leaves cell Cell shrinks (Exosmosis)
outside)
Isotonic Equal concentration No net movement Cell remains same
📌 Endosmosis
● Definition: Movement of water into the cell from a hypotonic solution.
● Result: Cell swells.
📌 Exosmosis
● Definition: Movement of water out of the cell into a hypertonic solution.
● Result: Cell shrinks.
📌 Plasmolysis
● Definition: Shrinkage of cytoplasm from cell wall due to water loss in hypertonic solution.
● Can be reversed if placed in hypotonic solution.
📌 Cell Wall (only in plant cells)
● Definition: Rigid outer layer made of cellulose.
● Nature: Fully permeable.
● Functions: Gives shape, support, and protection to plant cells.
🔹 4. Nucleus – The Brain of the Cell
● Definition: Spherical structure that controls cell activities and carries genetic material.
● Covered by nuclear membrane with pores.
● Contains:
○ Nucleoplasm – Jelly-like substance
○ Nucleolus – Makes RNA
○ Chromatin – Becomes chromosomes during division
Types of Cells:
Type Nucleus DNA
Prokaryotic No true nucleus (nucleoid) Circular, naked
Eukaryotic True nucleus Enclosed in nuclear
membrane
🔹 5. Cytoplasm
● Definition: Jelly-like fluid inside the cell between nucleus and membrane.
● Contains cell organelles.
● Site of many biochemical processes.
🔹 6. Cell Organelles (Definitions + Functions)
📌 Endoplasmic Reticulum (ER)
● Definition: A network of membranes continuous with nuclear membrane.
● Rough ER: Has ribosomes; makes proteins.
● Smooth ER: Makes lipids; detoxifies poisons and drugs.
📌 Golgi Apparatus
● Definition: A stack of flattened membranes (cisternae).
● Function: Modifies, sorts, and packages proteins/lipids. Forms lysosomes.
📌 Lysosomes
● Definition: Small vesicles with digestive enzymes.
● Function: Digests old organelles, waste, and foreign material. Known as suicide bags.
📌 Mitochondria
● Definition: Double membrane-bound organelle that produces ATP.
● Function: Provides energy through cellular respiration.
● Has own DNA (semi-autonomous).
📌 Plastids (Plant cells only)
● Definition: Double membrane organelles.
● Types:
○ Chloroplasts: Green; do photosynthesis.
○ Chromoplasts: Colored; provide color to fruits/flowers.
○ Leucoplasts: Colorless; store food.
📌 Vacuoles
● Definition: Fluid-filled sac used for storage.
● Plant cells: Large, central.
● Animal cells: Small.
● Function: Maintains turgidity; stores food/waste.
📌 Ribosomes
● Definition: Small, round, non-membranous organelles.
● Function: Site of protein synthesis.
● Present in both prokaryotes and eukaryotes.
🔹 7. Differences
✅ Prokaryotic vs Eukaryotic Cells
Feature Prokaryotic Eukaryotic
Nucleus Absent Present
DNA Circular Linear in chromosomes
Organelle Not Membrane-bound
s membrane-bound present
Size Small (1–10 µm) Large (5–100 µm)
✅ Plant vs Animal Cells
Feature Plant Cell Animal Cell
Cell Wall Present Absent
Plastids Present Absent
Vacuole Large, central Small, many
Shape Rectangular Irregular
Centrioles Absent Present
🔹 8. Cell Division
📌 Mitosis
● Definition: Cell division in which one cell divides to form two identical daughter cells.
● Purpose: Growth and repair.
📌 Meiosis
● Definition: Cell division forming gametes with half the chromosome number.
● Purpose: Sexual reproduction.
🔹 9. Important Terms Recap (With Definitions)
● Diffusion: Movement of particles from high to low concentration.
● Osmosis: Movement of water through semi-permeable membrane.
● Endosmosis: Entry of water into cell in hypotonic solution.
● Exosmosis: Exit of water from cell in hypertonic solution.
● Plasmolysis: Contraction of cell due to loss of water.
● Turgidity: Swollen state due to water intake.
● ATP: Adenosine Triphosphate – energy currency.
● Cristae: Inner folds in mitochondria.
● Endocytosis: Engulfing of material by cell (e.g., Amoeba).
🔹 10. Possible Questions (Subjective & MCQ Style)
1 Marks:
● Who discovered the cell?
● Define osmosis.
● What is plasmolysis?
● Which cell organelle is called the powerhouse?
● Name the organelle responsible for photosynthesis.
3 Marks:
● Differentiate between Prokaryotic and Eukaryotic cells.
● What is the function of the nucleus?
● Explain the structure and function of mitochondria.
● What happens when a plant cell is placed in:
○ (a) Hypotonic solution
○ (b) Hypertonic solution
○ (c) Isotonic solution
5 Marks:
● Describe the structure and functions of plasma membrane.
● Explain osmosis and its importance in plant cells.
● Describe the structure and role of any three organelles.
● Explain the difference between plant and animal cells with diagram.
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