HL Bio - UNIT #1: Endosymbiosis and Multicellularity

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12 Terms

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2 Main Theories for the Origin of Eukaryotic Cells

  1. Infoldings of the prokaryotic cell membrane

  2. Endosymbiosis

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Endosymbiosis

A type of symbiosis in which one organism lives inside the other, the two typically behaving as a single organism

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How Eukaryotic cells got Mitochondria’s

Early eukaryotic cell engulfed aerobic bacteria through endocytosis but didn’t digest it

  • This led to the origin of the mitochondria (symbiotic relationship)

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How Eukaryotic cells got Chloroplasts

Then the eukaryotic cell with mitochondria engulfed photosynthetic bacteria through endocytosis but again didn’t digest it

  • And through endosymbiosis the the bacteria works as the cells chloroplasts

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Physical Evidence of Endosymbiosis: With Mitochondria and Chloroplasts (4)

  • Have the bacterial structure

  • Their own membranous envelopes (similar to a cell membrane)

  • Approximately the size of a prokaryote

  • Have 70S ribosomes

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Genetic Evidence of Endosymbiosis: With Mitochondria and Chloroplasts (2)

  • Circular naked DNA (like plasmids)

  • DNA share common sequences with modern prokaryotes

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Functional Evidence of Endosymbiosis: With Mitochondria and Chloroplasts (3)

  • Move freely within the cell

  • Reproduce on their own (can’t be produced from the nucleus)

    • Through binary fission (mainly in prokaryotes)

  • Inhibited by antibiotics

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Origin of Multicellularity

After the first eukaryotic cells a great range of unicellular organism evolved

  • Some of these unicellular organisms continued to evolve into multicellular forms

They did that by dividing and forming groups

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2 steps to Multicellularity

  1. Formation of cellular clusters from single cells

  2. Differentiation of the cells to form specialized functions

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2 Hypothesis for how Multicellularity came about

  1. Independent cells came together to make a cluster

  2. Or when the unicellular organisms divides the other cell failed to separate resulting in a group (aggregate) of identical cells

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Predation

A selective pressure that is hypothesized to have lead to multicellularity

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Advantage of Multicellularity

Organisms tend to survive longer

  • This is since if one cell were to die the whole organism won’t due and the cell can be replaced