Science Class 9 — Chapter 5
The Fundamental Unit of Life
NCERT Syllabus Aligned Revision Notes: Cell Structure, Discovery, and Basic Classification.
1. What is a Cell?
All living organisms are made up of basic units called cells. Robert Hooke discovered cells in 1665 using a
primitive microscope, observing thin slices of cork. Anton van Leeuwenhoek later discovered free-living cells in
pond water with improved microscopes. Robert Brown discovered the nucleus in 1831, and Purkinje coined the
term 'protoplasm' in 1839.
The Cell Theory
Formulated by Schleiden (1838) and Schwann (1839), and later expanded by Virchow (1855):
All plants and animals are composed of cells, and the cell is the basic unit of life.
All cells arise from pre-existing cells.
2. Types of Organisms Based on Cell Number
Organisms can be classified into two groups based on the number of cells they contain:
Unicellular Organisms: Single-celled organisms capable of independent existence and carrying out all life
processes (e.g., Amoeba, Paramecium, Chlamydomonas, bacteria).
Multicellular Organisms: Organisms composed of millions of cells performing specialized tasks through
division of labor (e.g., fungi, plants, human beings, animals).
Class 9 Biology: The Fundamental Unit of Life Page 1 of 5
, 3. Structural Organization of a Cell
Every cell has three major features: the plasma (cell) membrane, the nucleus, and the cytoplasm. These
components enable the cell to perform fundamental life functions.
A. Plasma Membrane (Cell Membrane)
The plasma membrane is the outermost covering of the cell that separates its contents from the external
environment. It is a living, flexible, and selectively permeable membrane composed of organic molecules called
lipids and proteins.
Key Transport Mechanisms across Plasma Membrane
Diffusion: Spontaneous movement of substances (like carbon dioxide or oxygen) from a region of high
concentration to a region of low concentration across the cell membrane.
Osmosis: The passage of water molecules from a region of higher water concentration through a
selectively permeable membrane to a region of lower water concentration.
B. Cell Wall
Plant cells possess an outer rigid layer surrounding the plasma membrane called the cell wall, primarily
composed of cellulose. It provides structural strength, shape, and protection against extreme environmental
variations. When a living plant cell loses water through osmosis, shrinkage or contraction of the contents away
from the cell wall occurs, a phenomenon known as plasmolysis.
Class 9 Biology: The Fundamental Unit of Life Page 2 of 5
The Fundamental Unit of Life
NCERT Syllabus Aligned Revision Notes: Cell Structure, Discovery, and Basic Classification.
1. What is a Cell?
All living organisms are made up of basic units called cells. Robert Hooke discovered cells in 1665 using a
primitive microscope, observing thin slices of cork. Anton van Leeuwenhoek later discovered free-living cells in
pond water with improved microscopes. Robert Brown discovered the nucleus in 1831, and Purkinje coined the
term 'protoplasm' in 1839.
The Cell Theory
Formulated by Schleiden (1838) and Schwann (1839), and later expanded by Virchow (1855):
All plants and animals are composed of cells, and the cell is the basic unit of life.
All cells arise from pre-existing cells.
2. Types of Organisms Based on Cell Number
Organisms can be classified into two groups based on the number of cells they contain:
Unicellular Organisms: Single-celled organisms capable of independent existence and carrying out all life
processes (e.g., Amoeba, Paramecium, Chlamydomonas, bacteria).
Multicellular Organisms: Organisms composed of millions of cells performing specialized tasks through
division of labor (e.g., fungi, plants, human beings, animals).
Class 9 Biology: The Fundamental Unit of Life Page 1 of 5
, 3. Structural Organization of a Cell
Every cell has three major features: the plasma (cell) membrane, the nucleus, and the cytoplasm. These
components enable the cell to perform fundamental life functions.
A. Plasma Membrane (Cell Membrane)
The plasma membrane is the outermost covering of the cell that separates its contents from the external
environment. It is a living, flexible, and selectively permeable membrane composed of organic molecules called
lipids and proteins.
Key Transport Mechanisms across Plasma Membrane
Diffusion: Spontaneous movement of substances (like carbon dioxide or oxygen) from a region of high
concentration to a region of low concentration across the cell membrane.
Osmosis: The passage of water molecules from a region of higher water concentration through a
selectively permeable membrane to a region of lower water concentration.
B. Cell Wall
Plant cells possess an outer rigid layer surrounding the plasma membrane called the cell wall, primarily
composed of cellulose. It provides structural strength, shape, and protection against extreme environmental
variations. When a living plant cell loses water through osmosis, shrinkage or contraction of the contents away
from the cell wall occurs, a phenomenon known as plasmolysis.
Class 9 Biology: The Fundamental Unit of Life Page 2 of 5
