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 Eucaryotes Nucleolus present Membrane-bound organelles present May be multicellular 10-100 micrometres in diameter Procaryotes No nucleolus No membrane-bound organalles Unicellular 1-2 micrometres 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 Answer questions 1 to 4, p. 61 Answer questions 2, 3 & 7, p. 63