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THE CELL THEORY. Unorganised material (non-living). Homogeneous (tissues). Organised material (living). Heterogeneous (organs). What level of complexity is necessary for life?. Aristotle (384 – 322BC). Matter. © 2010 Paul Billiet ODWS. Image Credit Muscle tissue (surloin steak).

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what level of complexity is necessary for life

Unorganised material (non-living)

Homogeneous (tissues)

Organised material (living)

Heterogeneous (organs)

What level of complexity is necessary for life?
  • Aristotle (384 – 322BC)


© 2010 Paul Billiet ODWS

tissues and organs

Image Credit Muscle tissue (surloin steak)

Image Credit Kidney longitudinal section

Tissues and Organs
what level of complexity is necessary for life4

Image Credit Cork cells

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 Liver cells


C18th and C19th showed that tissues were made of cells

The cells of a particular tissue had a common structure.

what is a cell
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 life7






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 anorgan system (e.g. the digestive system)
  • An idea of hierarchy of structure developed:

© 2010 Paul Billiet ODWS

what level of complexity is necessary for life8
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 theory9
  • 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

Image Credit Cell division

« Omnis cellula e cellula »

Rudolf Virchow (1858) noted that: “all cells come from pre-existing cells”

the organismal theory

Image Credit Frog embryo

Image Credit Frog embryo fate map


The counter arguments:

Reichert a morphologist: Argued that an organism has a structured plan


Image Credit Black sapote (Diospyros) fruit


Strasberger a cytologist: Cells are connected in an organism sometimes by cytoplasmic bridges

acellular organisms

Image Credit Common field mushroom (Agaricus bisporus)

Image Credit Fungal hyphae

Acellular organisms

Some organisms do not have cellular compartments


Image Credit Spiney dendrites of the hippocampus region of the brain. Red dots show the spines associated with synapses.


Sherrington and Pavlov neurophysiologists: Cells communicate with one another and they are co-ordinated in their actions

unicellular organisms

Image Credit Paramecium

Unicellular organisms
  • Some organisms only consist of a single cell
  • But these do usually have the components of cells (nucleus, membrane etc)
cellular components

Image Credit Red blood cells

Cellular components
  • Some cells lack the basic components
  • But as a result their functions are affected.
tissue culture

Image Credit Tissue culture hood U of Wisconsin

Tissue culture
  • Cells can be cultured away from a body
  • But this often requires elaborate support systems




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




4. Homeostatic control and co-ordination is required to maintain the whole organism whether it is unicellular or multicellular.


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

3. Remove cells from complete multicellular organisms requires elaborate life support systems to keep them alive

© 2010 Paul Billiet ODWS

cell theory or organismal theory
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
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