Formation of Carbon Compounds (HL) (HL IB Biology)

Conditions on early Earth and the origin of carbon compounds

• The conditions on early Earth were not able to support life, but are believed to have been instrumental in the origin of biological compounds that made life possible

• Higher atmospheric temperatures
  • The early atmosphere contained higher levels of carbon dioxide and methane than our present atmosphere
    • Methane and carbon dioxide are powerful greenhouse gases
    • They trap infrared radiation coming from the surface of the earth and prevent it from escaping into space, which in turn increases surface temperatures; this is called the greenhouse effect
  • The higher levels of carbon dioxide and methane in the early atmosphere led to the surface temperatures of Earth being much higher than it is today

• UV radiation
  • The atmosphere of early Earth lacked free oxygen, which wasn't only a problem for sustaining life but also prevented ozone from forming
    • Ozone (O₃) is formed when ultraviolet (UV) radiation from the sun interacts with oxygen molecules (O₂)
  • Ozone absorbs damaging UV radiation and protects life on Earth
  • This lack of free oxygen, and therefore ozone, in the early atmosphere, led to UV radiation penetrating to the surface of Earth
    • UV radiation cause DNA damage and increases the rate at which mutations occur; this can be harmful to living organisms

• These conditions on early Earth may have resulted in the spontaneous formation of carbon compounds by chemical processes that do not currently occur
  • Adding energy, such as heat or UV radiation, to the mixture of gases that would have been present in the early atmosphere could have led to the formation of organic molecules such as amino acids, simple sugars, nucleotides and fatty acids
  • These organic molecules would have formed the building blocks of early cells
  • The scientists Alexander Oparin and JBS Haldane both proposed this idea as the 'primordial soup' hypothesis to explain the origin of biological molecules
• It is possible that the high levels of UV radiation on early Earth could then have catalysed the formation of larger polymers, such as proteins, complex sugars, mRNA and phospholipids, from these simpler molecules

The possible formation of the first organic molecules diagram

Heat and radiation could allow the synthesis of inorganic molecules into simple organic molecules which could then have assembled into polymers to provide the building blocks of life

The Miller-Urey experiment

• The scientists Miller and Urey recreated the conditions thought to have existed on Earth prior to life using a specific piece of apparatus
• The apparatus allowed them to:
  • Boil water to produce steam, recreating the early primordial soup evaporating in the high temperatures that existed on Earth
  • Mix the steam with a mixture of gases (including methane, hydrogen and ammonia) that recreated the atmosphere
  • Add electrical discharges to the gases to stimulate lightning (one of the sources of energy available at the time)
  • Cool the mixture (representing the condensation of water in the atmosphere)
• After a week Miller and Urey analysed the condensed mixture and found traces of simple organic molecules, including amino acids

The apparatus used by Miller and Urey diagram

Miller and Urey used laboratory apparatus to simulate conditions on early Earth

Evaluating the Miller-Urey experiment

• Methane availability
  • At the time Miller and Urey performed their experiment, it was believed that they simulated the early atmosphere accurately by including high levels of methane
  • It is now believed however, that methane may have been in low supply in the atmosphere of early Earth
• The energy source
  • Miller and Urey used an electrical discharge as a source of energy instead of UV light
  • For the synthesis of organic molecules, however, carbon dioxide, nitrogen and water require nuclear and UV radiation along with electrical discharges
• The presence of water
  • In a watery environment amino acids tend to remain as monomers rather than joining to form proteins
  • Given that water is needed to form the 'primordial soup', this contradicts the idea that complex molecules could have formed in this environment
•  Nucleotides
  • Miller and Urey were unable to generate nucleotides with their experiments
  • Nucleotides have since been chemically synthesised using a different approach