Study Notes – The Fundamental Unit of Life (Class 9 Science) PDF

1. Discovery of Cells

• 1665 – Robert Hooke: Observed cork slice, coined the term “cells” (looked like honeycomb).
• 1674 – Leeuwenhoek: Discovered free-living cells in pond water.
• 1831 – Robert Brown: Discovered nucleus.
• 1839 – Purkinje: Coined the term protoplasm.
• 1838 – Schleiden & 1839 – Schwann: Proposed Cell Theory:
  • All living beings are made of cells.
  • Cell is the basic unit of life.
• 1855 – Virchow: Added “All cells arise from pre-existing cells”.
  • 1940 – Electron Microscope allowed observation of cell organelles in detail.

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2. Unicellular vs. Multicellular Organisms

• Unicellular organisms: e.g. Amoeba, Paramecium, Chlamydomonas, bacteria. A single cell performs all functions.
• Multicellular organisms: Have many cells with division of labour (specialisation of cells for different tasks).

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3. Structural Organisation of a Cell

(i) Plasma Membrane / Cell Membrane

• Outermost boundary of the cell (in animal cells); selectively permeable.
• Functions:
  • Allows exchange of materials (selective entry/exit).
  • Transport processes ― diffusion & osmosis.
    • Diffusion: Movement of gases (O₂, CO₂) from high to low concentration.
    • Osmosis: Movement of water across selectively permeable membrane.
      • Hypotonic solution: cell swells (water enters).
      • Isotonic solution: no net movement; cell stays same.
      • Hypertonic solution: cell shrinks (water leaves).
• In Amoeba, plasma membrane helps in endocytosis (engulfing food).

(ii) Cell Wall (only in plants, fungi & bacteria)

• Provides strength & rigidity, made of cellulose.
• Prevents bursting in hypotonic medium.
• Plasmolysis: Shrinkage of cytoplasm away from cell wall due to water loss.

(iii) Nucleus

• Surrounded by nuclear membrane with pores.
• Contains:
  • Chromatin (uncoiled DNA + proteins) → condenses to form chromosomes.
  • DNA carries genetic information; functional units = genes.
• Functions:
  • Controls cellular activities.
  • Involved in reproduction.
• Types of cells:
  • Prokaryotes → no true nucleus, only nucleoid (e.g., bacteria).
  • Eukaryotes → true nucleus with nuclear membrane.

(iv) Cytoplasm

• Fluid content between plasma membrane & nucleus.
• Contains cell organelles.
• In prokaryotes, no membrane-bound organelles present.
• Viruses lack cytoplasm & membrane-bound organelles (non-living outside host).

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4. Cell Organelles (in eukaryotes)

(a) Endoplasmic Reticulum (ER)

• Network of membrane-bound tubes and sacs.
• Rough ER (RER): has ribosomes; site of protein synthesis.
• Smooth ER (SER): synthesises lipids; detoxifies poisons/drugs in liver.
• Functions: transport (materials between nucleus & cytoplasm), membrane biogenesis.

(b) Golgi Apparatus

• Stacks of membrane-bound vesicles (cisterns).
• Functions: modification, storage, packaging of proteins/lipids into vesicles; formation of lysosomes.

(c) Lysosomes

• Membrane-bound sacs with digestive enzymes.
• Function: breakdown of old organelles, foreign material (bacteria/food particles).
• Known as ‘suicide bags of cell’ (enzymes can digest own cell if lysosome bursts).

(d) Mitochondria

• Double membrane organelles.
• Inner membrane folded into cristae (increase surface area for reactions).
• Function: produces ATP (energy currency of cell) through respiration.
• Has its own DNA & ribosomes → can make proteins & divide independently.

(e) Plastids (in plants only)

• Two main types:
  • Chromoplasts (coloured plastids).
  • Leucoplasts (colourless plastids; store starch, oils, proteins).
• Chloroplasts (a type of chromoplast): contain chlorophyll for photosynthesis.
• Like mitochondria, chloroplasts also have DNA & ribosomes.

(f) Vacuoles

• Storage sacs.
• Small in animal cells; large central vacuole in plant cells (can occupy 50–90% volume).
• Contains cell sap (salts, sugars, amino acids, wastes).
• Provides turgidity and rigidity to plant cells.

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5. Cell Division

• Necessary for: growth, replacement of old/dead cells, reproduction.
• Two main types:
  • Mitosis:
    • Produces 2 identical daughter cells (same chromosome number as parent).
    • Responsible for growth & repair.
  • Meiosis:
    • Produces 4 daughter cells with half the chromosome number.
    • Occurs in reproductive organs to form gametes.

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6. Key Differences

Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cell	Eukaryotic Cell
Size	1–10 μm	5–100 μm
Nucleus	Nucleoid, no membrane	True nucleus with membrane
Chromosomes	Single, circular	Multiple, linear
Organelles	Membrane-bound absent	Membrane-bound present

Plant vs. Animal Cells

Feature	Plant Cell	Animal Cell
Cell wall	Present (cellulose)	Absent
Plastids	Present	Absent
Vacuoles	Large & central	Small or absent
Shape	Regular, fixed	Irregular, flexible

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7. Important Questions (for quick revision)

1. Who discovered the cell?
2. Why is the cell the structural and functional unit of life?
3. What is the difference between the plasma membrane and the cell wall?
4. Why are lysosomes suicide bags?
5. Which organelles have their own DNA and ribosomes?
6. Define osmosis with examples.
7. What is the difference between mitosis and meiosis?
8. Explain endocytosis in Amoeba.
9. Functions of mitochondria, ER, and Golgi apparatus.
10. Differences between prokaryotic and eukaryotic cells.