D.1c Origin of Life: Endosymbiosis

26/08/2013 § Leave a comment

So prokaryotes were the first living organisms on Earth – before eukaryotes – and because they started using water a source of hydrogen in photosynthesis, they also started releasing oxygen into the atmosphere as a waste product. As photosynthetic prokaryotes continued to produce oxygen as a waste product, the amount of oxygen in the atmosphere accumulated to a point where certain prokaryotic organisms could start using it for aerobic cell respiration. The evidence of oxygen in the atmosphere at that time is found through banded iron formation on old rocks found in Greenland.

One of the biggest differences between a prokaryote and a eukaryote (as we should know, hahahaha) is the content of each cell; while a eukaryote has both mitochondria and chloroplasts, a prokaryote doesn’t. However, since we assume that eukaryotes arose from prokaryotes, then the mitochondria and chloroplasts must’ve gotten there somehow and this can be explained through the endosymbiotic theory. What happened was mitochondria and chloroplasts probably evolved from independent prokaryotic cells which were then quite literally sucked into larger heterotrophic cells through endocytosis. The independent prokaryotic cells (early mitochondria and chloroplasts) were allowed to live and so continued their lives from within a larger cell.

Mitochondria and chloroplasts hold evidence that supports this theory, such as:

  • they can both grow and divide like cells
  • they have their own naked loop of DNA, just as prokaryotes do
  • they synthesise their own proteins using 70S ribosomes
  • they both have double membranes, necessary for endocytosis

(Some scientists also say flagella and cilia have an endosymbiotic origin, too, but let’s not get ahead of ourselves now.)

DATA BASED QUESTIONS

Thinking about science: the theory of endosymbiosis, page 310

1. What is the distinction between belief and knowledge?

Knowledge stems from proof and, normally, scientific fact; things researches and humans have seen and proved through equations, experiments, and by simply seeing it with their own eyes – there is proof of the thing. Belief stems from faith and, normally, has little to no scientific or visible proof – an individual simply trusts that the thing is true.

2. To what extent is Margulis’ belief falsifiable? In other words, to what extent is it a scientific claim?

Margulis presents a probable hypothesis about the endosymbiosis theory with the spirochete bacterium but says directly that “the concept that cilia evolved from spirochetes has not been proved” which is not justified. Because of this, Margulis’ belief can be falsifiable at a great extent since she has no real evidence to back up her hypothesis.

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