2.1 Spontaneous Generation
- The theory of spontaneous generation states that life arose from nonliving matter. It was a long-held belief dating back to Aristotle and the ancient Greeks.
- Experimentation by Francesco Redi in the 17th century presented the first significant evidence refuting spontaneous generation by showing that flies must have access to meat for maggots to develop on the meat. Prominent scientists designed experiments and argued both in support of (John Needham) and against (Lazzaro Spallanzani) spontaneous generation.
- Louis Pasteur is credited with conclusively disproving the theory of spontaneous generation with his famous swan-neck flask experiment. He subsequently proposed that “life only comes from life.”
2.2 Foundations of Modern Cell Theory
- Although cells were first observed in the 1660s by Robert Hooke, cell theory was not well accepted for another 200 years. The work of scientists such as Schleiden, Schwann, Remak, and Virchow contributed to its acceptance.
- Endosymbiotic theory states that mitochondria and chloroplasts, organelles found in many types of organisms, have their origins in bacteria. Significant structural and genetic information support this theory.
- The miasma theory of disease was widely accepted until the 19th century, when it was replaced by the germ theory of disease thanks to the work of Semmelweis, Snow, Pasteur, Lister, and Koch, and others.
2.3 Unique Characteristics of Prokaryotic Cells
- Prokaryotic cells differ from eukaryotic cells in that their genetic material is contained in a nucleoid rather than a membrane-bound nucleus. In addition, prokaryotic cells generally lack membrane-bound organelles.
- Prokaryotic cells of the same species typically share a similar cell morphology and cellular arrangement.
- Most prokaryotic cells have a cell wall that helps the organism maintain cellular morphology and protects it against changes in osmotic pressure.
- Outside of the nucleoid, prokaryotic cells may contain extrachromosomal DNA in plasmids.
- Prokaryotic ribosomes that are found in the cytoplasm have a size of 70S.
- Some prokaryotic cells have inclusions that store nutrients or chemicals for other uses.
- Some prokaryotic cells are able to form endospores through sporulation to survive in a dormant state when conditions are unfavorable. Endospores can germinate, transforming back into vegetative cells when conditions improve.
- In prokaryotic cells, the cell envelope includes a plasma membrane and usually a cell wall.
- Bacterial membranes are composed of phospholipids with integral or peripheral proteins. The fatty acidcomponents of these phospholipids are ester-linked and are often used to identify specific types of bacteria. The proteins serve a variety of functions, including transport, cell-to-cell communication, and sensing environmental conditions. Archaeal membranes are distinct in that they are composed of fatty acids that are ether-linked to phospholipids.
- Prokaryotic cell walls may be composed of peptidoglycan (bacteria) or pseudopeptidoglycan (archaea).
- Gram-positive bacterial cells are characterized by a thick peptidoglycan layer, whereas gram-negative bacterial cells are characterized by a thin peptidoglycan layer surrounded by an outer membrane.
- Some prokaryotic cells produce glycocalyx coatings, such as capsules and slime layers, that aid in attachment to surfaces and/or evasion of the host immune system.
- Some prokaryotic cells have fimbriae or pili, filamentous appendages that aid in attachment to surfaces. Pili are also used in the transfer of genetic material between cells.
- Some prokaryotic cells use one or more flagella to move through water.