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Archaea
 outline the evidence that indicates how the first cellular
organisms (Archaea) may have developed and describe their
mode of respiration (anaerobic fermentation)
 outline the role of chemosynthesis in providing a suitable
energy source for early organisms
Overview
The evolution of life is thought to have
occurred in the following order:
formation of organic molecules
membrane formation
heterotrophic procaryotes
autotrophic procaryotes
eucaryotic cells
colonial cells
multicellular organisms
Membrane formation
Lipids (the main chemical in cell membranes) are
hydrophobic – they are repelled by water.
Experiments have shown that hydrophobic lipids can
group together into microspheres in order to reduce
their contact with water.
It is thought at some point that some of the organic
molecules necessary for life, including nucleic acids,
may have become trapped inside a microsphere.
How these organic microspheres developed into the
first cells is still poorly understood, but is seen as the
most likely pathway given our current understanding
of the earliest cells.
Comparison of procaryotic and
eucaryotic cells
Procaryotes
No nucleolus
No membranebound organalles
Unicellular
1-2 micrometres
diameter
Eucaryotes
Nucleolus present
Membrane-bound
organelles present
May be multicellular
10-100 micrometres
in diameter
Procaryotic cells
Procaryotic cells were the first to evolve and are
distinguished by the lack of a nuclear membrane, i.e.
no true nucleus, and the lack of cell organelles.
Procaryotes are bacteria, and include archaebacteria.
Heterotrophic bacteria receive their nourishment from
organic molecules which occur outside their cell
structure.
Autotrophic bacteria can make their own food either
through photosynthesis or through chemosynthesis.
Archaebacteria
 Archaebacteria literally means ‘ancient bacteria’ and
today they still survive in hostile locations that resemble
their early environment.
 Methanogens are anaerobic and release methane as a
by-product of metabolism. They live in the digestive
tracts of animals, swamps and deep in the ocean.
 Halophiles (salt-loving) are found in extremely salty
environments such as salt lakes.
 Thermophiles (heat-loving) require high temperatures
and commonly live in and around hot springs and
volcanoes, including fumaroles.
Respiration & ATP
Respiration is the chemical breakdown
of ‘foods’ inside a cell in order to release
energy.
This energy is transferred to the
molecule adenosine triphosphate (ATP).
ATP is the principal ‘energy carrier’ of
cells.
Respiration may be either aerobic (in
the presence of oxygen) or anaerobic
(without oxygen).
Anaerobic fermentation
 Early cells lived in an environment with no free
oxygen.
 To release energy the Archaebacteria (and other
anaerobes) have to use fermentation, which is much
less efficient than aerobic respiration.
 Copy and compare Figures 1.5.4 and 1.5.5 on p. 63.
This is the same process that converts the simple
sugars in grapes into the alcohol in wine.
Aerobic fermentation often occurs in the muddy
bottom of swamps, and can release a ‘rotten egg’
smell (sulfur) when disturbed.
Questions
1) Answer questions 1 to 4, p. 61
2) Answer questions 2, 3 & 7, p. 63