A 2.1 ORIGINS OF CELLS

• what were the conditions on earth before life

  higher atmospheric temperatures, UV radiation
• higher atmospheric temperatures


  early atmosphere contained higher levels of carbon dioxide and methane than present atmosphere
• how did methane and carbon dioxide increase the temperature

  powerful greenhouse gases which trapped infrared radiation coming from the surface of earth and preventing it from escaping
• UV radiation


  atmosphere of early earth lacked free oxygen which prevented the ozone from forming, thus allowing UV rays to pass through into earth
• how does UV rays protect life on earth

  absorbs damaging UV rays and preventing it from penetrating to the surface of earth
• What are the effects of UV radiation

  can damage DNA and increase the rate at which mutations occur
• how did carbon compounds occur on earth

  spontaneously by chemical processes that do not currently occur - UV radiation could have catalysed to formation of larger polymers from the simpler molecules
• What was the Miller-Urey experiment?


  a chemical experiment that simulated the conditions thought at the time to be present on the early Earth, and tested the chemical origin of life under those conditions.
• what was the result of the Miller-Urey experiment


  analysis showed that a variety of carbon compounds formed including over 20 different amino acids thus showing that it was possible for carbon compounds to form spontaneously
• what are some of the conditions that were not kept right in the experiment

  methane availability and the energy source and nucleotides
• methane availability


  it was believed the early atmosphere had high levels of methane at the time of the experiment, however, we not believe that it may have been in low supply
• nucleotides

  Miller and Urey were unable to generate nucleotides in their experiment; thus not all compounds/molecules needed for life were produced
• what are cells


  considered to be the smallest unit of life
• What features do all cells have in common?

  cell membrane, cytoplasm, DNA, ribosomes
• what features do you need to be alive

  metabolism, nutrition, excretion, sensitivity, growth
• why are viruses considered to be non-living

  lack cell structure and organelles thus do not carry out metabolic reactions, do not require nutrition and can only survive and replicate when in another living organism (host cell)
• what do we know about the origin of cells

  can only form by the division of pre-existing cells thus we assume that they originated from pre-existing, non-living components
• what do we believe happened with these non-living components

  inorganic molecules were synthesised to make organic molecules which were assembled into polymers creating compartments
• what does evidence suggest for the origin of life


  species originated from a common ancestor
• how does DNA provide evolutionary relationships between species

  Organisms with similar DNA sequences are more closely related than those with very different DNA sequences
• LUCA


  last universal common ancestor
• evidence for a common ancestry shared by all living organisms include:

  same biochemistry, DNA bases and genetic code and same shared amino acids forming protein molecules in all organisms
• how do fossils help

  Fossils can provide evidence about the history of life on earth and are often used to determine the timescale across which evolutionary events have occurred
• radiometric dating


  It measures the relative proportions of certain radioactive substances (such as carbon-14 to carbon-12) in a sample
• what are the 2 ways of determining timescale

  analysing genome and radiometric dating
• analysing the genome


  By estimating the average time for DNA mutations to occur, the relative date when species branched from a common ancestor can be determined based on the number of mutations that have occurred between them
• molecular clock


  can be used to determine the date of when life on Earth originated
• Evidence for LUCA


  hydrothermal vents deep in ocean which provided opportunities for organisms to generate energy by chemosynthesis
• what is chemosynthesis

  synthesis of organic compounds by bacteria or other living organisms using energy released by reactions involving inorganic chemicals
• what characteristic might LUCA have had?

  Anaerobic, therefore able to survive in the absence of oxygenConverted carbon dioxide into glucoseUsed hydrogen as an energy source, instead of sunlightConverted nitrogen into ammonia for the synthesis of amino acidsSurvived in environments of very high temperature (thermophilic)
• How did Earth form?

  gravity caused gas and dust in early solar system to come together
• what were the concentrations of gases like in the early atmosphere

  less oxygen, more methane and carbon dioxide concentrations
• why were there traces of oxygen in the early atmosphere

  because it reacted with other elements (e.g. with iron to form iron oxide)
• methane concentration early atmosphere

  higher due to intensive volcanic activity and meteorite bombardment
• carbon dioxide concentrations in early atmosphere

  higher due to emissions from volcanoes
• temperatures in early atmosphere

  likely to have been higher due to carbon dioxide and methane trapping greenhouse gases; comets and asteroid impacts also raised temperatures
• how much energy was the sun emitting during the early atmosphere

  20% less energy
• What is stratospheric ozone?

  protects earth by absorbing UV radiation
• UV radiation in early atmosphere

  stratospheric ozone did not exist thus more solar UV penetrated the earths surface
• what does UV radiation provide

  activation energy for chemical reactions