Class 9 Science – Chapter 5: The Fundamental Unit of Life (Detailed Notes)

Class 9 Science – Chapter 5: The Fundamental Unit of Life (Detailed Notes)

1. Discovery of Cell

• Cell was discovered by Robert Hooke in 1665 while observing cork slices under a microscope.
• He observed small box-like structures and named them "cells".

2. Cell Theory

• Proposed by Schleiden and Schwann.
• Later modified by Rudolf Virchow.

Modern Cell Theory:

• All living organisms are made up of cells.
• Cell is the basic structural and functional unit of life.
• All cells arise from pre-existing cells.

3. Types of Organisms

• Unicellular: Single-celled organisms (e.g., Amoeba, Paramecium)
• Multicellular: Made up of many cells (e.g., Humans, Plants)

4. Shape and Size of Cells

• Cells have different shapes – spherical, spindle-shaped, branched, etc.
• Size may vary from 0.1 µm (bacteria) to 170 mm (ostrich egg).

5. Structure of a Cell

A typical cell has three main parts:

(a) Plasma Membrane / Cell Membrane

• Outer covering of the cell.
• Made up of proteins and lipids.
• Functions:
  • Protects the cell.
  • Controls the entry and exit of substances (selectively permeable).
  • Helps in diffusion and osmosis.

Osmosis and Diffusion

• Diffusion: Movement of molecules from high to low concentration.
• Osmosis: Movement of water through a semi-permeable membrane from a region of higher to lower water concentration.

(b) Cell Wall (Only in Plant Cells)

• Rigid outer covering made of cellulose.
• Provides strength and protection to plant cells.

(c) Cytoplasm

• Jelly-like substance present between cell membrane and nucleus.
• Contains cell organelles.

6. Nucleus

• Control center of the cell.
• Surrounded by nuclear membrane with pores.
• Contains:
  • Nucleolus: Synthesizes RNA.
  • Chromatin: Thread-like structures made of DNA. Condense to form chromosomes during cell division.
• Functions: Controls cellular activities, stores genetic information.

7. Prokaryotic and Eukaryotic Cells

• Prokaryotic Cells: No defined nucleus (e.g., Bacteria)
• Eukaryotic Cells: Have well-defined nucleus (e.g., Plant and Animal cells)

8. Cell Organelles

(a) Endoplasmic Reticulum (ER)

• Rough ER (RER): Has ribosomes, helps in protein synthesis.
• Smooth ER (SER): No ribosomes, helps in lipid synthesis and detoxification.

(b) Golgi Apparatus

• Modifies, sorts, and packages proteins and lipids.
• Helps in formation of lysosomes.

(c) Lysosomes

• Contain digestive enzymes.
• Break down waste materials (also called "suicide bags").

(d) Mitochondria

• Powerhouse of the cell.
• Site of cellular respiration and energy (ATP) production.

(e) Plastids (only in Plant Cells)

• Chloroplasts: Contain chlorophyll, help in photosynthesis.
• Others: Chromoplasts (color), Leucoplasts (storage).

(f) Vacuoles

• Storage sacs for water, food, and wastes.
• Large in plant cells, small in animal cells.

9. Differences between Plant and Animal Cells

Feature	Plant Cell	Animal Cell
Cell Wall	Present	Absent
Chloroplasts	Present	Absent
Vacuoles	Large and Central	Small or absent
Centrioles	Absent	Present