HL 2 Biology

D.1c Origin of Life: Endosymbiosis

<<Prokaryotes and oxygen>>

Photosynthesis in cynaobacteria; produces oxygen

  • this oxygen eventually started to be released in atmosphere
  • later, thought that eukaryotes and chloroplasts together further increased atmospheric oxygen
  • great increase in oxygen => led many prokaryotes to go extinct while others adapted by developing mechanisms to deal with oxygen (e.g. aerobic cellular respiration)


  • theory of endosymbiosis
    • partly explains evolution of eukaryotic cells
    • develops how chloroplasts & mitochondria were once free living organisms that lost independence (ending up by living inside other cells)



1) Belief is a thought or acceptance of an idea that does not have a clear evidence to support the claim where as knowledge is known to be a true or justified belief. 

2) Margulis’ belief is falsifiable to a great extent due to the lack of supporting evidence that can justify her claim but her claim may be appropriate with her own scientific reasonings.  Although Margulis’ hypothesis is well supported with her knowledge about the endosymbiosis theory, it can be falsified as her hypothesis has not yet been carried out in an experiment to be proven right. This hypothesis may be her own scientific claim but it cannot be proven to be fully correct without experimental justification to conclusively prove it to be right. Thus, Margulis’ belief is falsifiable as her belief could be claimed incorrect without further justification done experimentally to prove her belief is true.

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