THE CELL
THEORY
What level of complexity is
necessary for life?

Aristotle (384 – 322BC)
Unorganised
material
(non-living)
Matter
Homogeneous
(tissues)
Organised
material (living)
Heterogeneous
(organs)
© 2010 Paul Billiet ODWS
Tissues and Organs
Image Credit Muscle tissue (surloin
steak)
Image Credit Kidney longitudinal section
What level of complexity is
necessary for life?
C17th microscopists discovered tissues
were made of cells (Hooke 1665 and
Leeuwenhoek 1677)
© 2010 Paul Billiet ODWS
Image Credit Cork cells
Cells
C18th and C19th
showed that
tissues were
made of cells
The cells of a
particular tissue
had a common
structure.
Image Credit Liver cells
What is a cell?
Taken to its simplest form
 A plasma membrane…
 Surrounding cytoplasm…
 Containing hereditary material.

© 2010 Paul Billiet ODWS
What level of complexity is
necessary for life?



Xavier Bichat (17711802): An organ is
composed of different
tissues
Several organs can be
grouped together as an
organ system (e.g. the
digestive system)
An idea of hierarchy of
structure developed:
© 2010 Paul Billiet ODWS
Organism
Organ-system
Organ
Tissue
Cell
What level of complexity is
necessary for life?
Purkinje (1835) Observed a fertilised hen's
egg (a single cell) could develop into an
embryo (many specialised cells in a
compact mass)
 C19th botanists showed that plant tissues
consist of many different types of cells.

© 2010 Paul Billiet ODWS
THE CELL THEORY
Matthias Schleiden (1838) & Theodor
Schwann (1839)
“The cell is the basic unit of living
tissue”
 The cell is an autonomous unit (“a citizen”)
grouped together to form an organism
(“the society”).

© 2010 Paul Billiet ODWS
« Omnis cellula e cellula »
Rudolf Virchow (1858)
noted that:
“all cells come from
pre-existing cells”
Image Credit Cell division
THE ORGANISMAL THEORY
The counter arguments:
Reichert a morphologist: Argued that an
organism has a structured plan
Image Credit Frog embryo
Image Credit Frog embryo fate map
Plasmodesmata
Strasberger a cytologist:
Cells are connected in
an organism sometimes
by cytoplasmic
bridges
Image Credit Black sapote (Diospyros) fruit
Acellular organisms
Some organisms do
not have cellular
compartments
Image Credit Common field mushroom (Agaricus bisporus)
Image Credit Fungal hyphae
Homeostasis
Sherrington and
Pavlov
neurophysiologists:
Cells communicate
with one another and
they are co-ordinated
in their actions
Image Credit Spiney dendrites of the hippocampus
region of the brain. Red dots show the spines
associated with synapses.
Unicellular organisms
Some organisms only consist of a single
cell
 But these do usually have the components
of cells (nucleus, membrane etc)

Image Credit Paramecium
Cellular components
Some cells lack the basic components
 But as a result their functions are affected.

Image Credit Red blood cells
Tissue culture
Cells can be cultured away from a body
 But this often requires elaborate support
systems

Image Credit Tissue culture hood U of Wisconsin
SUMMARY
THE CELL THEORY
THE ORGANISMAL
THEORY
1. Multicellular organisms develop
from a single fertilised germ cell (the
zygote)
1. Some organisms are not divided
into cellular compartments
= non-cellular
2. The basic components of the cell
are repeated in every cell
2. Certain cells lack the basic
components
Cells in multicellular organisms are
highly specialised
Unicellular organisms have a
cytoplasm that is not subdivided
Should be considered as acellular
© 2010 Paul Billiet ODWS
SUMMARY
THE CELL THEORY
3. All cells come from cells
Cells can be taken from organisms
and cultured away from the body
THE ORGANISMAL
THEORY
3. Remove cells from complete
multicellular organisms requires
elaborate life support systems to keep
them alive
New individuals can be cultured from
isolated cells
Regeneration capacity = totipotence
4. Homeostatic control and coordination is required to maintain the
whole organism whether it is
unicellular or multicellular.
© 2010 Paul Billiet ODWS
Cell theory or organismal
theory?
That the cell is the basic unit of living
organisms is accepted
 That unicellular organisms carry out all the
functions of life is accepted
 BUT multicellular organisms are not simply
a mass of similar building blocks

© 2010 Paul Billiet ODWS
More is different!
As a multicellular organism grows and
develops it follows a structured plan
 The cells specialise (differentiate)
 The whole organism shows homeostatic
control
 A developing multicellular organism shows
emergent properties
 It is not just the sum of the parts

© 2010 Paul Billiet ODWS