Bio Notes-Chapter 4 pt. 2

4.2

  • During the European Renaissance (14th to 17th centuries), most people thought that at least some forms of life arose repeatedly from inanimate or decaying matter by %%spontaneous generation%%
    • Think that living organisms just appear
  • Francesco Redi first disproved spontaneous generation in 1668
  • Experiments by %%Louis Pasteur%% showed that microorganisms can arise only from other microorganisms
  • But these experiments did not prove that spontaneous generation had never occurred
  • Eons ago, conditions on Earth and in the atmosphere were vastly different
  • About 4 billion years ago, chemical conditions, including the presence of water, became just right for life

  ### Two of the theories on the origin of life:

  

  1. ^^Life came from outside Earth^^
    • in 1969, fragments of a meteorite were found to contain molecules unique to life, including purines, pyrimidines, sugars, and ten amino acids
    • evidence from other meteorites suggest that living organisms could possibly have reached earth within a meteorite
  2. ^^Life arose on Earth through chemical evolution^^
    • Chemical evolution: conditions on primitive Earth led
  • Miller and Urey (1950s) set up an experiment with:
    • gases (inorganic) thought to have been present in Earths early atmosphere
    • an electric spark to simulate lightning (energy)
    • ^^After several days, inorganic molecules had formed, including amino acids^^
    • In another experiment, Miller filled tubes with NH3, HCN, and water. After 27 years at -78, they contained %%amino acids%% and %%nucleotide bases%%
  • The Miller and Urey experiments sparked decades of research

4.3 How Did the Large Molecules of Life Originate?

  • Conditions in which polymers might have been first synthesized:
    • ^^Solid mineral surfaces^^- silicates within clay may have been catalysts
    • ^^Hydrothermal vents^^- metals as catalysts
    • ^^Hot pools at ocean edges^^- concentrated monomers favored polymerization (the “primordial soup”)
  • The chemical reactions of metabolism and replication could %%NOT%% occur in a %%dilute%% aqueous environment
  • The compounds involved must have been %%concentrated%% in a compartment
  • Today, living %%cells%% are separated from their environment by a %%membrane%%

4.4 How Did the First Cells Originate?

  • In water, fatty acids will form %%a lipid bilayer%% around a compartment
  • These %%protocells%% allow small molecules such as sugars and nucleotides to pass through
  • Protocells may be a reasonable model for the evolution of cells:
    • They are %%organized%% systems of parts with substances interacting
    • They have an interior that is %%distinct%% from the exterior environment
    • They can %%self-replicate%%
  • In the 1990s, evidence of cells in rocks 3.5 billion years old was found in Australia
  • The cells were probably cyanobacteria (blue-green bacteria) that could perform %%photosynthesis%%
  • Photosynthesis uses CO2, and leaves a specific ratio of carbon isotopes (13C:12C), which were could in the fossils
  • It is plausible that it took about 500 million to a billion years from the formation of the Earth until the appearance of the first cells