The cell is the basic unit of structure and life. All organisms are composed of one or more cells. Robert Hooke first observed cells in 1665 when examining cork under a microscope. The cell theory states that all organisms are made of similar basic units called cells, cells come only from pre-existing cells, and all living things have similar chemical components. The modern cell theory includes that organisms are made of cells, cells are the basic functional units of life, and cells reproduce through division of pre-existing cells.

1. Concept of life cell as a basic unit
of life cell theory
By
Akanksha Jain
Ph.D. Biotechnology

2. CELL AS BASIC UNIT OF LIFE
The cell is the smallest unit of living matter.
The cell is the smallest unit having all the properties of life.
All organisms are composed of structural and functional units of life
called cells.
The body of some organisms like bacteria, protozoans and some algae is
made up of a single cell ( Unicellular organisms) while the body of fungi,
plants and animals are composed of many cells (Multicellular
organisms)
Cells vary in size and structure as they are specialized to perform
different
functions.
But the basic components of the cell are common to all cells.

3. Robert Hooke (1665), an English scientist, discovered and coined the term cell
while examining a thin slice of cork under a self-designed compound microscope.
The term cell was derived from a Latin word cellular (meaning little room or
chamber).
In 1672, Antony Van Leeuwenhoek observed bacteria, sperms and red blood
corpuscles, all of which were cells.
In 1831, Robert Brown, an Englishman observed that all cells had a centrally
positioned body which he termed the nucleus.
The crude microscope employed by Robert Hook(1665)
MILESTONE IN DISCOVERY OF CELL
Thin slice of cork showing chambers/cells

4. WHAT IS A CELL ? ( DEFINITION OF CELL )
Cell is the structural and functional unit of living organisms, enclosing a bit of
protoplast consisting of numerous micro and macro organic and
molecules, dissolved or suspended in colloidal or watery matrix.
The molecules are busy in reactions with release or uptake of energy. The
protoplasm of the cell is limited by a membrane.
All types of cells include three major components
1.Cell membrane
2.Cytoplasm and
3. DNA naked in prokaryotes (Bacteria,Archaea and BGA) and covered
by a membrane in all other organisms.
Mature Plant cells have in their cytoplasm large vacuole containing non-living
inclusions like crystals, pigments etc.
The bacteria have neither organelles nor a well formed nucleus.

5. Hooke in 1663, observed cork
(plant): named the cell
Schwann in 1800’s states:
all animals are made of cells
Pasteur’s work with bacteria
~ 1860 disproved idea of
spontaneous generation (living things arise from nonliving matter)
Modern cell theory emerged by 1900
DEVELOPMENT OF THE CELL THEORY

6. • The Cell Theory
states that all
organisms are
composed of
similar units of
organization
called cells.
The Cell Theory states that all organisms are composed of
similar units of organization called cells.
The cell theory was jointly put forward by Schleiden and
Schwann (1839) in their paper “Microscopic investigations on
the similarity of structure and growth in animals and plants” :
As follows
1) All organisms are composed of one or more cells .
2)Cells are the basic living units within organisms and the
chemical reactions of life take place within cells.
3)All cells arise from pre existing cells by division and are not
formed de novo
In 1855, a German medical doctor named
Rudolph Virchow observed, under the
microscope, cells dividing. He reasoned that
all cells come from other pre-existing cells by
cell division
Rudolph Virchow
Schleiden
(PLANT CELL)
Schwann
(ANIMAL CELL)

7. FRANCISCO REDI PROVED THAT
MACROORGANISMS DO NOT
SPONTANEOUSLY GENERATE
People thought
that maggots came
from meat.
Redi disproved
this in a controlled
experiment

8. PASTURE PROVED THAT
MICROOGANISMS DO NOT
SPONTANEOUSLY GENERATE

9. Haeckel (1866) established that nucleus stores and transmits hereditary
traits.
Cell theory was modified accordingly and was
renamed as cell doctrine or cell principle
Ernst Haeckel (1834-1919)
Modern cell theory includes following features
1. The cell is the fundamental unit of structure and function in living organisms.
2.New cells arise from pre-existing cells through division. All new cells contain the
same amount and degree of genetic information as contained in the parent cell
3. Energy flow occurs within cells.
4.Cells contain hereditary information (DNA) which is passed from cell to cell during
cell division.
5.All cells are basically the same in chemical composition in organisms of similar
species.

10. 6. All known living things are made up of one or more cells.
7.Some organisms are made up of only one cell and are known as unicellular
organisms. All life begins as a single cell.
8. Others are multicellular, composed of a number of cells.
9.The activity of an organism depends on the total activity of independent cells.
10.Depending upon specific requirement, the cells get modified, e.g.
elongated in muscle and nerve cells, loss of nucleus in RBCs or cytoplasm in
outer skin cells.

11. PRINCIPLES OF MODERN CELL
THEORY
All organisms composed of cells and cell
products.
A cell is the simplest structural and functional
unit of life. There are no smaller subdivisions of a
cell or organism that, in themselves, are alive.
An organism’s structure and all of its functions
are ultimately due to the activities of its cells.
Cells come only from preexisting cells, not from
nonliving matter. All life, therefore, traces its
ancestry to the same original cells.
Because of this common ancestry, the cells of all
species have many fundamental similarities in
their chemical composition and metabolic
mechanisms.