The Cell: The Fundamental Unit of Life
Introduction to the Cell
The cell is considered the fundamental and smallest unit of all living organisms. It is the structural unit of
life because the entire body of an organism is composed of cells. It is also the functional unit of life
because all essential life functions occur within the cell and are regulated by it.
Discovery of the Cell
1665: Robert Hooke observed a thin slice of cork under a self-designed microscope and discovered
compartments he called "cells," a Latin word for "a little room."
1674: Anton Van Leeuwenhoek built a simple microscope.
1831: Robert Brown discovered the nucleus in the cell.
1839: Purkinje coined the term "protoplasm" for the fluid substance within the cell.
1838-1839: Schleiden and Schwann presented the cell theory, stating that all plants and animals are
composed of cells, and the cell is the basic unit of life.
1855: Virchow expanded on cell theory, proposing that all cells arise from pre-existing cells.
1940s: The electron microscope was discovered, allowing for the observation of finer cellular
structures.
Unicellular vs. Multicellular Organisms
Unicellular Organisms:
Composed of a single cell.
The single cell performs all essential life functions (e.g., eating, reproduction, waste removal,
movement).
Examples: Amoeba, bacteria.
Multicellular Organisms:
Composed of more than one cell.
Cells are specialized to perform different life functions (e.g., nerve cells, muscle cells).
Examples: Humans, plants.
Division of Labour in Organisms
In multicellular organisms, different parts of the body perform specific functions (e.g., the heart pumps
blood, the stomach digests food). This is known as division of labour.
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, In unicellular organisms, cell organelles perform specialized functions. Each organelle has a specific role,
such as synthesizing new materials, clearing waste, or providing energy. A cell's ability to live and function
depends on these organelles.
Key Features of a Cell
All cells, regardless of their function or the organism they belong to, share three main features:
1. Plasma Membrane: The outermost covering that separates the cell's contents from its external
environment.
2. Nucleus: The control center of the cell, containing the genetic material.
3. Cytoplasm: The gel-like substance filling the cell, enclosing the organelles.
These features enable all activities within the cell and its interactions with the environment.
Plasma Membrane
The plasma membrane is the outermost covering of the cell.
Function: It regulates the passage of substances into and out of the cell, making it a selectively
permeable membrane. It prevents the movement of some materials while allowing others.
Composition: It is flexible and made up of organic molecules called lipids and proteins. Its structure
can only be observed with an electron microscope.
Flexibility: The flexibility of the plasma membrane allows the cell to engulf food and other materials
from its external environment through processes like endocytosis (e.g., how Amoeba obtains its
food).
Transport Mechanisms:
Diffusion: The spontaneous movement of a substance from a region of high concentration to a
region of low concentration. This process is used for the movement of gases like oxygen (O2 )
and carbon dioxide (CO2 ).
Osmosis: The passage of water across a semi-permeable membrane from a region of high
water concentration to a region of low water concentration.
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Introduction to the Cell
The cell is considered the fundamental and smallest unit of all living organisms. It is the structural unit of
life because the entire body of an organism is composed of cells. It is also the functional unit of life
because all essential life functions occur within the cell and are regulated by it.
Discovery of the Cell
1665: Robert Hooke observed a thin slice of cork under a self-designed microscope and discovered
compartments he called "cells," a Latin word for "a little room."
1674: Anton Van Leeuwenhoek built a simple microscope.
1831: Robert Brown discovered the nucleus in the cell.
1839: Purkinje coined the term "protoplasm" for the fluid substance within the cell.
1838-1839: Schleiden and Schwann presented the cell theory, stating that all plants and animals are
composed of cells, and the cell is the basic unit of life.
1855: Virchow expanded on cell theory, proposing that all cells arise from pre-existing cells.
1940s: The electron microscope was discovered, allowing for the observation of finer cellular
structures.
Unicellular vs. Multicellular Organisms
Unicellular Organisms:
Composed of a single cell.
The single cell performs all essential life functions (e.g., eating, reproduction, waste removal,
movement).
Examples: Amoeba, bacteria.
Multicellular Organisms:
Composed of more than one cell.
Cells are specialized to perform different life functions (e.g., nerve cells, muscle cells).
Examples: Humans, plants.
Division of Labour in Organisms
In multicellular organisms, different parts of the body perform specific functions (e.g., the heart pumps
blood, the stomach digests food). This is known as division of labour.
1/5
, In unicellular organisms, cell organelles perform specialized functions. Each organelle has a specific role,
such as synthesizing new materials, clearing waste, or providing energy. A cell's ability to live and function
depends on these organelles.
Key Features of a Cell
All cells, regardless of their function or the organism they belong to, share three main features:
1. Plasma Membrane: The outermost covering that separates the cell's contents from its external
environment.
2. Nucleus: The control center of the cell, containing the genetic material.
3. Cytoplasm: The gel-like substance filling the cell, enclosing the organelles.
These features enable all activities within the cell and its interactions with the environment.
Plasma Membrane
The plasma membrane is the outermost covering of the cell.
Function: It regulates the passage of substances into and out of the cell, making it a selectively
permeable membrane. It prevents the movement of some materials while allowing others.
Composition: It is flexible and made up of organic molecules called lipids and proteins. Its structure
can only be observed with an electron microscope.
Flexibility: The flexibility of the plasma membrane allows the cell to engulf food and other materials
from its external environment through processes like endocytosis (e.g., how Amoeba obtains its
food).
Transport Mechanisms:
Diffusion: The spontaneous movement of a substance from a region of high concentration to a
region of low concentration. This process is used for the movement of gases like oxygen (O2 )
and carbon dioxide (CO2 ).
Osmosis: The passage of water across a semi-permeable membrane from a region of high
water concentration to a region of low water concentration.
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