THE CELL THEORY © 2016 Paul Billiet ODWS

What level of complexity is necessary for life? Aristotle (384 – 322BC) Organised material (living) Unorganised material (non-living) Heterogeneous (organs) Homogeneous (tissues) Matter © 2016 Paul Billiet ODWS

Tissues and Organs Muscle tissue (surloin steak) Kidney longitudinal section © 2016 Paul Billiet ODWS

What level of complexity is necessary for life? C17th microscopists discovered tissues were made of cells (Hooke 1665 and Leeuwenhoek 1677) Cork cells © 2016 Paul Billiet ODWS

Cells C18th and C19th showed that tissues were made of cells. The cells of a particular tissue have a common structure. Liver cells © 2016 Paul Billiet ODWS

What is a cell? Taken to its simplest form A plasma membrane… surrounding cytoplasm… containing hereditary material. © 2016 Paul Billiet ODWS

Unicellular organisms Some organisms only consist of a single cell yet carry out all life functions But these do usually have the components of cells (nucleus, membrane etc) Paramecium © 2016 Paul Billiet ODWS

What are life functions? Nutrition Metabolism Growth Response Excretion Homeostasis Reproduction. © 2016 Paul Billiet ODWS

Multicellular organisms Development of a chick embryo © 2016 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 into: Organism Organ-system Organ Tissue Cell © 2016 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 So all living organisms seem to be composed of cells © 2016 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”). © 2016 Paul Billiet ODWS

« Omnis cellula e cellula » Cell division Remak & Virchow (1858) noted that: “all cells come from pre-existing cells” © 2016 Paul Billiet ODWS

Is the cell really that autonomous? Cells in multicellular organisms © 2016 Paul Billiet ODWS

Cellular components Some cells lack the basic components But as a result their functions are affected. Red blood cells © 2016 Paul Billiet ODWS

Plasmodesmata Strasberger observed: Cells are connected in an organism sometimes by cytoplasmic bridges. Not all cells seem to be completely contained in a plasma membrane. Black sapote (Diospyros) fruit © 2016 Paul Billiet ODWS

Acellular organisms Some organisms do not have cellular compartments Common field mushroom (Agaricus bisporus) Fungal hyphae © 2016 Paul Billiet ODWS

Giant algal cells Acetabularia © 2016 Paul Billiet ODWS http://www.coexploration.org/bbsr/coral/assets/images/acetabularia.jpg Acetabularia © 2016 Paul Billiet ODWS

Acellular tissues: Striated muscle fibres Striated muscle viewed at x400 © 2016 Paul Billiet ODWS

Homeostasis 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 They are co-ordinated in their actions © 2016 Paul Billiet ODWS

Tissue culture Cells can be cultured away from a body But this often requires elaborate support systems Tissue culture hood U of Wisconsin © 2016 Paul Billiet ODWS

THE ORGANISMAL THEORY The counter arguments: Reichert a morphologist: Argued that a multicellular organism has a structured plan Frog embryo Frog embryo fate map © 2016 Paul Billiet ODWS

SUMMARY THE ORGANISMAL THEORY THE CELL 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 Where they are absent or in multiples this affects life processes e.g. anucleate red blood cells cannot reproduce e.g. muscles cells contain many nuclei but form from many cells that fuse 2. Certain cells lack the basic components e.g. red blood cells Cells in multicellular organisms are highly specialised Unicellular organisms have a cytoplasm that is not subdivided Should they be considered as acellular? © 2016 Paul Billiet ODWS

SUMMARY THE ORGANISMAL THEORY THE CELL THEORY 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 3. Remove cells from complete multicellular organisms requires elaborate life support systems to keep them alive 4. Homeostatic control and co-ordination is required to maintain the whole organism whether it is unicellular or multicellular. © 2016 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 independent building blocks. © 2016 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. © 2016 Paul Billiet ODWS