1. The cell theory states that the cell is the basic unit of all living things and that multicellular organisms develop from single cells. However, the organismal theory argues that multicellular organisms cannot be viewed simply as a collection of cells, as they develop according to a structured plan and cells specialize and coordinate in their functions. 2. While some organisms like bacteria are unicellular, more complex multicellular organisms require elaborate life support systems to keep isolated cells alive in culture and maintain homeostasis across the entire organism. 3. Both theories acknowledge that cells are the basic functional units of life, but the organismal theory emphasizes that organisms exhibit properties that cannot be reduced to the properties of individual cells alone

1. THE CELL
THEORY

2. What level of complexity is
necessary for life?
 Aristotle (384 – 322BC)
Matter
Organised
material (living)
Unorganised
material
(non-living)
Heterogeneous
(organs)
Homogeneous
(tissues)
© 2010 Paul Billiet ODWS

3. Tissues and Organs
Image Credit
Muscle tissue (surloin steak)
Image Credit Kidney longitudinal section

4. What level of complexity is
necessary for life?
C17th microscopists discovered tissues
were made of cells (Hooke 1665 and
Leeuwenhoek 1677)
Image Credit Cork cells
© 2010 Paul Billiet ODWS

5. 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

6. What is a cell?
 Taken to its simplest form
 A plasma membrane…
 Surrounding cytoplasm…
 Containing hereditary material.
© 2010 Paul Billiet ODWS

7. What level of complexity is
necessary for life?
 Xavier Bichat (1771-
1802): 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:
Organism
Organ-system
Organ
Tissue
Cell
© 2010 Paul Billiet ODWS

8. 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

9. 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

10. « Omnis cellula e cellula »
Rudolf Virchow (1858)
noted that:
“all cells come from
pre-existing cells”
Image Credit Cell division

11. 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

12. Plasmodesmata
Strasberger a
cytologist: Cells are
connected in an
organism sometimes by
cytoplasmic bridges
Image Credit Black sapote (Diospyros) fruit

13. Acellular organisms
Some organisms
do not have cellular
compartments
Image Credit Common field mushroom (Agaricus bisporus)
Image Credit Fungal hyphae

14. 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 brai

15. 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

16. Cellular components
 Some cells lack the basic components
 But as a result their functions are affected.
Image Credit Red blood cells

17. 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

18. SUMMARY
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
2. The basic components of the cell
are repeated in every cell
1. Some organisms are not divided
into cellular compartments
= non-cellular
1. Multicellular organisms develop
from a single fertilised germ cell (the
zygote)
THE ORGANISMAL
THEORY
THE CELL THEORY
© 2010 Paul Billiet ODWS

19. SUMMARY
4. Homeostatic control and co-
ordination is required to maintain the
whole organism whether it is
unicellular or multicellular.
3. Remove cells from complete
multicellular organisms requires
elaborate life support systems to keep
them alive
3. All cells come from cells
Cells can be taken from organisms
and cultured away from the body
New individuals can be cultured from
isolated cells
Regeneration capacity = totipotence
THE ORGANISMAL
THEORY
THE CELL THEORY
© 2010 Paul Billiet ODWS

20. 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

21. 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