Development of Cell Theory

• Aristotle proposed the theory of spontaneous generation


  350 BCE
• Spontaneous generation


  Life emerged spontaneously from non-living matter
• Spontaneous generation


  • Thought that frogs would spontaneously appear in mud
• Aristotle had no experiments to prove his theory of spontaneous generation; it was just accepted to be true
• Francesco Redi performed an experiment to disprove the theory of spontaneous generation


  1668
• Redi's experiment


  1. Three beakers: one flask had meat and opened to the air, the second flask was sealed with meat inside, and the third flask had meat and was covered by gauze

  2. The only flask to produce flies was the one opened to the air
• Despite Redi's evidence, the theory of spontaneous generation still continued to thrive
• Louis Pasteur performed an experiment that finally disproved the theory of abiogenesis


  1864
• Pasteur's experiment


  1. Manipulated variable: access to dust and organisms

  2. Responding variable: ability to grow bacteria in the broth

  3. Control variables: broth type, light, temp., flask type

  4. The S-bend would catch micro-organisms and other particles therefore preventing (bacteria) growth in the broth

  5. If the solution was tipped so the broth could reach the S-bend then bacteria would grow in the broth
• Pasteur provided evidence that micro-organisms are found in the air
• Biogenesis


  Living organisms can only arise from other living organisms
• With development of lens/microscope technology, scientists were able to develop cell theory


  1830s
• Brown identified the nucleus as an opaque granular spot within the cell


  1831
• Brownian motion


  Random motion of particles
• Schleiden observed that all plants are composed of cells and that the nucleus was responsible for cell development


  1838
• Schwann observed that animal cells were similar to plant cells


  1839
• Schleiden and Schwann proposed that all plant and animals are composed of cells and that cells are the basic unit of all organisms
• Virchow extended the cell theory that all cells arise only from pre-existing cells


  1859
• Cell Theory


  • All living things are made of cells
  • Cells are the smallest unit of life
  • All cells are produced from pre-existing cells through the process of cell division