a2.1

• early earth

  - unable to support life

  - higher atmospheric temperatures

  - UV radiation
• early earth

  explain the higher atmospheric temperatures

  - higher levels of CO2 and CH4, powerful GG

  - trap infrared radiation from the earth's surface

  - this increases surface temperatures

  - the greenhouse effect

  - led to surface temperatures being much higher than today
• early earth

  explain the UV radiation

  - lack of free oxygen

  - so, unable to sustain life and it prevented ozone formation

  - ozone forms then UV radiation from sun interacts with O2

  - ozone absorbs damaging UV radiation, protects life on earth

  - UV radiation penetrated to earth's surface

  - causes DNA damage and increases rate of mutations
• primordial soup hypothesis

  - conditions caused carbon compounds to spontaneously form

  - adding energy (heat/UV) to gas mixture could have led to the formation of organic molecules

  - e.g. amino acids, simple sugars, nucleotides, fatty acids

  - these formed building blocks of cells

  - high UV could have catalysed large polymer formation

  - e.g. proteins, complex sugars, mRNA, phospholipids
• theory of spontaneous generation

  Life arises spontaneously from non-living material
• theory of special creation

  god
• theory of panspermia

  life exists and is distributed throughout the universe in germ/spore form --> develops in the right environment
• Louis pasteur

  what was he against

  what was he for

  against spontaneous generation

  against abiogenesis

  for biogenesis
• abiogenesis

  life spontaneously arose from non-living cells
• biogenesis

  all cells come from division of pre existing cells
• Louis Pasteur's experiment

  - place broth/culture medium (for bacteria) in flasks

  - sterilised the broth in some flasks to kill microbes

  - microbes did not appear in flasks of boiled broth

  - after flask necks were snapped, boiled broth grew microbes (cloudy)

  - because microbes from air contaminated boiled broth

  curved necks allowed indirect exposure to air but prevent microbe entry
• how did the first cells originate

  from non-living components that made up the primordial atmosphere at the time

  - CATALYSIS: simple organic compounds were synthesised from inorganic molecules

  - SELF ASSEMBLY: simple organic compounds were assembled into polymers

  - SELF-REPLICATION: certain polymers formed the capacity to be duplicated (enabled inheritance)

  - COMPARTMENTALISATION: membranes surrounded the polymers, created compartments with unique internal chemistry
• what are the 3 theories that aim to explain the possible origins of cells

  - protocelll first theory

  - gene first theory

  - metabolism first theory
• protocol first theory


  - cell-like compartment capable of basic metabolic functions arose spontaneously (protocells)

  - initially lacked genetic material, but they would have been able to grow, and able to divide into daughter 'cells'

  - eventually, protocells acquired genetic material (mostly RNA first) as they evolved
• gene first theory


  - spontaneous development of a nucleic acid (most likely RNA) that had the ability to replicate itself

  - evolution by natural selection resulted in genetic variants that could have developed a cell membrane and basic metabolic processes
• metabolism first theory

  - life originated as a system of chemical reactions capable of sustaining itself

  - the system would eventually evolve to form cells and genetic material

  - favoured by many scientists as most life processes essential for the existence of cells require energy released by metabolic reactions
• why was the theory of spontaneous generation believed until the 17th century

  until 17th century, believed that living organisms arose from non-living matter, due to:

  - lack of technology, microscopes not used extensively

  - observations made, Aristotle observing insects form from dew

  - idea supporting cultural and religious beliefs of the time
• what experiment provided evidence for the origin of carbon compounds

  Miller Urey experiment
• The Miller-Urey Experiment

  - recreated conditions thought to have existed on prebiotic earth

  - boiled water to produce vapour, recreating the early primordial soup evaporating in the high temps on earth

  - mixed the vapour with a mixture of gases (incl. methane, hydrogen, ammonia) to recreate the reducing atmosphere (no oxygen)

  - add electrical discharge to the gases to stimulate lighting, an energy source for the reaction

  - cools the mixture, represent the condensation of water in the atmosphere

  - analysed the condensed mixture after a week, found traces of simple organic molecules, incl. amino acids
• significance of the Miller-Urey experiment

  - clearly showed for the 1st time that biomolecules can form under ancient earth-like conditions

  - took what was once speculation (life from chemistry) and transformed a portion into legitimate science
• evaluation of the Miller-Urey experiment

  - methane availability

  - at the time, it was believed that the early atmosphere had high methane levels

  - now it is believed that levels may have been in low supply
• evaluation of the Miller-Urey experiment

  - energy source

  - used electrical discharge instead of UV light

  - for organic molecule synthesis, carbon dioxide, nitrogen and water require nuclear and UV radiation along with electrical discharges
• evaluation of the Miller-Urey experiment

  - water presence

  - in watery environments, amino acids tend to remain as monomers rather than joining to make proteins

  - water is needed to form the 'primordial soup', this contradicts the idea that complex molecules could have formed in the environment
• evaluation of the Miller-Urey experiment

  - nucleotides

  - were unable to generate nucleotides

  - these have since been chemically synthesised using a different approach
• membrane bound compartments

  - necessary to allow the internal chemistry to differ from that outside the compartment

  - likely that membranes of first cells were made of fatty acids due to their ampithatic nature

  - place a few lipids in water = monolayer

  - add more lipids = bilayer

  - bilayers spontaneously form microspheres/small vesicles

  - these could form early cells' membranes
• what was theorised about membrane bound compartments

  - fatty acids could have combined with glycerides to form triglycerides as membranes evolved

  - triglycerides were phosphorylated to from phospholipids
• membrane bound compartments and eukaryotes

  - eukaryotic cells evolved to contain multiple internal compartments, allowing further division of activity within cells
• RNA

  presumed 1st genetic material
• properties of RNA providence evidence that it could be the 1st genetic material

  - self replicate

  - catalyst

  - spontaneous assembly
• RNA self replicates

  - can have acted as genetic material
• RNA can act as a catalyst


  - can have acted as enzymes

  - modern cells have ribozymes in ribosomes

  - these catalyse peptide bond formation in protein synthesis
• RNA can assemble spontaneously


  from nucleotides
• as life evolved

  DNA = genetic material

  Proteins (enzymes) = biological catalysts of chemical reactions
• RNA world hypothesis

  - chains of RNA may have been the 1st things to replicate and evolve
• evidence that RNA may have been around before DNA

  - ribose can be formed from methanal, one of the main products from the `Miller-Urey experiment

  - deoxyribose in DNA is produced from ribose in an enzyme catalysed reaction

  - ribozymes can join amino acids together to form proteins from an RNA template