The First Cells on Earth

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The First Cells on Earth
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The First Cells

• Life originated on Earth between 3.5 and 4
  billion years ago
• No oxygen was available (ANAEROBIC)
• First cells resemble cyanobacteria, a type of
  prokaryote

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Cyanobacteria

• Perform photosynthesis (conversion of CO2 to
  sugar, with oxygen and water as by-products)
• Oxygen accumulated in the atmosphere
  (2.7-2.2bya)
• AEROBIC organisms evolved

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Cyanobacteria
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What is a prokaryote?

• Pro before (in Greek)
• Karyon kernel
• No membrane-bound nucleus
• No membrane-bound organelles

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Prokaryote Structure

• 3 main shapes - cocci, rods and spirals

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Prokaryote Size

• A typical prokaryotic cell is 1-10 µm in size
• micron (µm) micrometer
• 1 µm 0.000001 m
• 1x10-6 m
• microscopic

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Bacteria on a plate

• Spread bacteria on the surface of an agar plate
• Each cell will form a COLONY
• Each colony contains millions of cells

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Cell with cytoplasm
Nucleoid - one large naked circle of DNA
Plasmids - small circles of DNA
Ribosomes - protein factories
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Bacterial Cell Walls

• Two main types Gram positive and Gram negative
• Based on a 4-step staining procedure
• Gram purple
• Gram - pink

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Bacterial Cell Walls
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Gram negative
Gram positive

• thick peptidoglycan layer
• plasma membrane

• outer membrane
• thin PG layer
• plasma membrane

Bacillus anthracis
E. coli O157H7
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Bacillus anthracis

• Spore-forming bacterium
• Spores are a thick-walled resting stage
• Induced by stress to the cell
• If inhaled, anthrax spores can be fatal

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E. coli O157H7

• O157 refers to signature molecules found in the
  outer membrane
• H7 refers to signature molecules on the flagella

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Pili (sing. Pilus)

• attachment of the cell to surfaces
• bacterial sex (the transfer of genes via
  conjugation)

• are shorter than flagella
• are NOT used for motility

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Capsule (slime layer)

• Some bacteria produce a thick layer of slime
• Helps them stick to surfaces
• Xanthan gum

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Extremophiles (Archaebacteria)

• Some bacteria like it hot!
• Thermophiles in hot springs
• Some bacteria like it hot and acidic!!
• Thermoacidophiles

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Extremophiles (Archaebacteria)

• Some bacteria like it really salty!
• Halophiles

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What should you know?

• When anaerobic and aerobic prokaryotes appear in
  the fossil record, and why
• What cyanobacteria are, and how they are
  different from most bacteria
• What defines a prokaryote size, shape
  structures
• Difference between Gram positive and Gram
  negative bacteria, with examples
• Extremophiles