THE CELL THEORY © 2016 Paul Billiet ODWS What level of complexity is necessary for life? • Aristotle (384 – 322BC) Unorganised material (non-living) Matter Homogeneous (tissues) Organised material (living) Heterogeneous (organs) © 2016 Paul Billiet ODWS Tissues and Organs Muscle tissue (surloin steak) © 2016 Paul Billiet ODWS Kidney longitudinal section What level of complexity is necessary for life? C17th microscopists discovered tissues were made of cells (Hooke 1665 and Leeuwenhoek 1677) © 2016 Paul Billiet ODWS Cork cells Cells C18th and C19th showed that tissues were made of cells. The cells of a particular tissue have a common structure. © 2016 Paul Billiet ODWS Liver cells 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) © 2016 Paul Billiet ODWS Paramecium What are life functions? Nutrition Metabolism Growth Response Excretion Homeostasis Reproduction. © 2016 Paul Billiet ODWS Multicellular organisms © 2016 Paul Billiet ODWS Development of a chick embryo 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 into: © 2016 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 • 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 » Remak & Virchow (1858) noted that: “all cells come from pre-existing cells” © 2016 Paul Billiet ODWS Cell division 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. © 2016 Paul Billiet ODWS Red blood cells 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 http://www.coexploration.org/bbsr/coral/assets/images/acetabularia.jpg © 2016 Paul Billiet ODWS Acellular tissues: Striated muscle fibres © 2016 Paul Billiet ODWS Striated muscle viewed at x400 Homeostasis Sherrington and Pavlov neurophysiologists: Cells communicate with one another They are co-ordinated in their actions Spiney dendrites of the hippocampus region of the brain. Red dots show the spines associated with synapses. © 2016 Paul Billiet ODWS Tissue culture • Cells can be cultured away from a body • But this often requires elaborate support systems © 2016 Paul Billiet ODWS Tissue culture hood U of Wisconsin 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 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 e.g. red blood cells 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 Cells in multicellular organisms are highly specialised © 2016 Paul Billiet ODWS Unicellular organisms have a cytoplasm that is not subdivided Should they be considered as acellular? SUMMARY THE CELL THEORY THE ORGANISMAL THEORY 3. All cells come from cells 3. Remove cells from complete multicellular organisms requires Cells can be taken from organisms elaborate life support systems to and cultured away from the body keep them alive New individuals can be cultured from isolated cells 4. Homeostatic control and coordination 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
