1.5 The Origin of Cells

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1.5 The Origin of Cells

• Cells come from pre-existing cells
• The first cells arose from non-living materials
• Endosymbiosis

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Cell Division The Origin of Cells

• All cells are formed by the division of
  pre-existing cells.
• You are made of trillions of cells.
• Any cell that is produced by your body is the
  product of cell division from another cell.
• You originally started life as a single cell, a
  zygote, that underwent numerous cell divisions to
  produce the trillions of cells that comprise you
  today.

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• Even that single celled zygote came from
  other cells the combination your
  fathers sperm cell and your
  mothers egg cell.
• We can trace the origins of all the cells in our
  parents back to the zygotes from which they
  developed, to our human ancestors before them, to
  humans pre-existing ancestral species, all the
  way back to the earliest cells on Earth.

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• We, and all other living things on this planet,
  are descendants from the first cell.
• But if all cells come from pre-existing cells,
  where did the first cell come from?
• How did life start on this planet?

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Problems for starting life on Earth

• How could the lifeless ball of rock that the
  planet Earth was 3.5 billion years ago, become
  home to such lush vegetation and a wide variety
  of bacteria, fungi, protists, and animals that we
  see today?
• There are 4 problems which needed to be overcome
  for life on Earth to exist.

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Requirements to start life

1. Production of Simple Organic Compounds
2. The assembly of organic compounds into polymers
3. Development of a mechanism for inheritance.
4. Formation of membranes

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1. Production of Simple Organic Compounds

• Life as we know it is based on organic compounds
  (compounds containing carbon and hydrogen), such
  as amino acids (the building blocks of proteins)
• But early Earth only had inorganic matter rocks,
  minerals, gases, water.

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• Abiogenesis had to occur.
• abiogenesis the creation of organic matter from
  inorganic matter)
• It is believed that organic molecules were formed
  in the shallow waters of the oceans as the
  products of chemical reactions between compounds
  in the atmosphere and the water.

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Miller and Urey

• Scientists Stanley Miller
  and
  Harold Urey
  
  performed a ground-
  
  breaking experiment in 1953
• They recreated the conditions of early Earth and
  proved that organic compounds could be
  synthesized from inorganic compounds.

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N2 nitrogen gas H2O water H2 hydrogen
gas NH3 ammonia gas CO2 carbon dioxide
gas CH4 methane gas
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The Experimental Design

• The apparatus included an oceanic compartment
  and an atmospheric compartment
• The H2O in the oceanic compartment was heated to
  evaporate and cooled to condense thereby
  recreating the H2O cycle.
• Since early Earth did not have an ozone layer,
  they kept the system at a warm temperature and
  exposed it to UV radiation
• Generated electric sparks to simulate lightning

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The Results

• After 1 week
• 15 of the carbon was now found in organic form!
• 13 of the 20 amino acids had formed inside the
  primordial soup!
• Sugars had formed!
• The nitrogenous base adenine (a component of DNA
  and ATP) had formed!!!

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The Conclusion

• Early Earth had the proper conditions for
  abiogenesis.

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2. Assembly of these molecules into polymers

• Organisms are organized!
• Simple organic molecules would have needed to
  undergo a process of polymerization to form the
  larger more complex organic chemicals required by
  cells.

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Deep-Sea Vents

• Organic molecules could have first formed around
  hydrothermal vents places where hot water
  emanated from beneath the ocean floor.
• Form when cracks in the crust of the seabed
  expose sea water to rocks below which are heated
  by magma

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• As the hot water rises it picks up countless
  minerals along the way.
• Hydrothermal vents are sometimes referred to as
  black smokers because the water coming out of
  them contains so many dark minerals it looks like
  smoke.
• The chemicals and source of energy in this
  environment could be suitable for the formation
  of biological polymers.

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3. The development of a mechanism for inheritance

• Today, most organisms use DNA as its
  molecule for heredity.
• To replicate DNA and pass it on to the next
  generation, enzymes are required
• However, enzymes cannot be made without DNA.
• Therefore, it is unlikely that DNA was the early
  molecule for heredity

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Ribozymes

• However, the small sequences of the molecule RNA
  can act as enzymes and replicate itself.
• These are called ribozymes
• Thus, RNA may be the early
  molecule for hereditary.

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4. Formation of Membranes

• Water is important to life but tends to
  depolymerize (break down)molecules
• Many compounds dissolve in water, making it
  difficult to organize into polymers
• The formation of closed membranes is likely an
  early and important event in the origin of
  cellular life
• It allows for the development of an internal
  chemistry different from the external environment

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Coacervates

• Coacervate - a microscopic sphere that forms
  from lipids in water.
• Forms spontaneously due to the hydrophobic forces
  between the water and lipid molecules.
• Can maintain an internal chemical environment
  different from the surrounding environments.
• Coacervates can be selectively permeable

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Coacervates

• Coacervates (lipids)

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• Although they are not living organisms,
  coacervates are a significant step toward the
  formation of cells.
• They solve the problem of protecting polymers
  from their destructive environments.
• Could be primitive versions of the first cell
  membranes

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• PROTOBIONTS the first precursors to cells, were
  likely coacervate droplets which included
  polynucleotides (DNA or RNA)
• (remember our cell membranes are lipid based)
• Overtime, true cell membranes evolved and other
  characteristics of cells developed.
• Cellular respiration
• Asexual reproduction

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Spontaneous Generation

• While spontaneous generation must of occurred
  billions of years ago to give rise to the first
  cells, spontaneous generation of cells and
  organisms does not now occur on Earth.
• This is supported by experiments conducted by
  Louis Pasteur in 1864.

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Spontaneous Generation

• Before Pasteur, many people believed spontaneous
  generation possible and believed it explained why
  mould would grow seemingly out of nowhere, and
  why fruit flies would appear in a room that
  didnt have any previously.
• Many people believed that spontaneous generation
  could occur as long as there was access to air.

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Pasteurs Experiments

• He created a nutrient broth by boiling water
  containing yeast and sugar, thereby killing the
  organisms
• When the broth was kept in a sealed flask it
  remained unchanged no fungi or organisms
  appeared.

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• When exposed to air that travelled through a swan
  neck, it also remained unchanged because the
  apparatus prevented microorganisms from entering
  the broth
• When the swan neck was removed, microorganisms
  could enter the broth and proliferate

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Where did all the oxygen come from?

• 1/5 of the air you are breathing right now is
  oxygen.
• However, there was none at all present 4 billion
  years ago.
• The earliest life forms on Earth were bacteria
  and they lived in an environment with an
  atmosphere of mostly CO2
• Thus, early life forms were anaerobic cells (did
  not require oxygen)

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• These single-celled organisms would consume
  organic molecules (i.e. simple sugars) that were
  forming from chemical reactions on Earth
• The more they reproduced, the more food that was
  consumed.

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• After million of years, their population would
  have reached such large numbers that food began
  to be scarce.
• In this food shortage, bacteria that could make
  their own food would have an advantage.

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• 3.5 billion years ago, bacteria (that is
  believed to be related to todays
  cyanobacteria)developed the ability to
  photosynthesize.
• Must have contained a form of chlorophyll

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• Development of photosynthesis was one of the most
  significant evens in the history of Earth
• Gives bacteria a source of energy (sunlight) to
  survive
• Created a mass pollution of the atmosphere
• Pollution of oxygen!!!

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• Oxygen gas is toxic to the kinds of bacteria
  which preceded photosynthetic ones, so this
  pollution would have eventually killed off large
  populations of anaerobes.
• Anaerobic bacteria that survived would live in
  mud of places protected from the new oxygen-rich
  atmosphere.

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• The ability of an organism to make its own food
  gives it a distinct advantage over those that
  cannot.
• As a result, photosynthetic bacteria proliferated
  and produced more and more oxygen

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Endosymbiosis

• 3.8-2 bya, bacteria (prokaryotic cells) were the
  only organisms on Earth
• The first fossils of cells with a nucleus
  (eukaryotes) is from around 2 bya.
• How did prokaryotes develop into eukaryotes?
• Endosymbiosis is the most popular theory

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Endosymbiosis

• ENDOSYMBIOSIS when one organism lives within the
  other and they both benefit
• Ex bacteria that live inside our digestive tract

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Endosymbiotic Theory

• The chloroplasts and mitochondria that are found
  inside eukaryotic cells today were once
  independent prokaryotic cells.
• They were engulfed by a bigger prokaryotic cell.
• Rather than being digested, the prokaryotes were
  kept alive inside the host cell in exchange for
  their services

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Endosymbiotic Theory

• The host cell would provide protection for the
  smaller prokaryotic cell
• The engulfed cell would be beneficial to the host
  if
• it was photosynthetic like the chloroplast
  (providing food) for the host or
• able to metabolize food efficiently and produce
  energy for the host like the mitochondrion

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Evidence for Endosymbiosis

• Mitochondria and chloroplasts are very similar to
  prokaryotic cells. The many similarities suggest
  that they were once independent prokaryotic cells.

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Evidence of Endosymbiosis

1. Chloroplasts and mitochondria are surrounded by a
   double membrane (like the prokaryotic plasma
   membranes)
2. Mitochondria and bacteria (a prokaryote) have a
   similar size

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Evidence of Endosymbiosis

• Mitochondrial and bacterial ribosomes are very
  similar in size and shape.
• Mitochondria and chloroplasts have their own DNA
  which is circular like bacteria.
• Mitochondria divide in a process similar to
  binary fission like bacteria

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Problems with Endosymbiosis

• The ability to engulf another cell and have it
  survive in the cytoplasm does not guarantee that
  the host cell can pass it on to its offspring the
  genetic code to synthesize the newly acquired
  organelle
• When chloroplasts or mitochondria are removed
  from a cell, they cannot survive on their own.