THE CELL THEORY. What level of complexity is necessary for life? Aristotle (384 – 322BC) Matter Organised material (living) Unorganised material (non-living)

THE CELL THEORY

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

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)

Image Credit Cork cells© 2010 Paul Billiet ODWS

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 (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

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 organismsSome 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

2. Certain cells lack the basic componentsCells in multicellular organisms are highly specialisedUnicellular 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

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

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