subject guide notes

A2.1.1—Conditions on early Earth and the pre-biotic formation of carbon compounds

surface of Earth was covered in a single, global ocean with no solid land masses

early Earth underwent bombardment from comets & asteroids

  • provided Earth with & other compounds

  • these incidents & volcanic eruptions, released methane & ammonia

    • these 2 gases formed majority of Earth’s early atmosphere, along with H2O & C02

early Earth’s atmosphere didnt hv free oxygen = no ozone layer

high concentration of CO2 & methane = high temps

Earth’s inner core would’ve been very hot & a liquid

  • increased motion in liquid core = smaller protective magnetic field = Earth would’ve been vulnerable to high levels of cosmic & solar radiation

high proportion of reducing gases in Earth’s atmosphere, means that electrons were donated to other molecules, meaning chemical reactions could occur

  • these reactions formed complex carbon compounds, which eventually joined together to form building blocks of cells

  • spontaneous formation of carbon compounds

A2.1.2—Cells as the smallest units of self-sustaining life

smallest unit of life that can exist on their own

viruses considered non-living as they need a host to reproduce & aren’t formed by cells

A2.1.3—Challenge of explaining the spontaneous origin of cells

cells are very complex & can currently only be produced by division of pre-existing cells

catalysis, self-replication, self-assembly & emergence of compartmentalization were necessary requirements for emergence of first cells

A2.1.4—Evidence for the origin of carbon compounds

Miller-Urey experiment, which stimulated Earth’s pre-biotic atmosphere to show that spontaneous formation of organic molecules did occur

they set up a closed system, which contained

  • water (simulates ocean)

  • gas inlet - allowed to add reducing gases (ammonia, methane & hydrogen)

  • electrical sparks - simulate the electrical storms present on pre-biotic Earth

water was vaporised & ran through the gases & electrical sparks

cooling jacket used to condense water

condensed water formed on side of apparatus & collected at bottom

  • represented the primordial soup

after a week, they found the primordial soup contained basic organic monomers (such as amino acids)

*important to note the experiment did not prove that carbon compounds originated in this way, simply that it could hv happened this way

A2.1.5—Spontaneous formation of vesicles by coalescence of fatty acids into spherical bilayers

membrane-bound compartments would’ve formed when fatty acids spontaneously merged to form spherical bilayer

physical separation means the chemical environment within vesicle, would’ve been different than the chemical environment in the external environment

  • allows cells to maintain & control specific set of conditions for proper cell functioning

A2.1.6—RNA as a presumed first genetic material

scientists believe that RNA was the first building block of the first cell-like structure

  • functioned as a catalyst & genetic material

can check kognity for evidence & suggestions of RNA first hypothesis

ribozymes in ribosomes are used to catalyse peptide bond formation during protein synthesis

A2.1.7—Evidence for a last universal common ancestor

LUCA is thought to have been a single-celled autotrophic, anaerobic, microbe with likely an RNA genome

other life forms likely evolved, but likely became extinct because LUCA or its descendants, outcompeted these life forms

evidence

  • genetic code is universal

    • same codons code for same amino acids in all organisms

A2.1.8—Approaches used to estimate dates of the first living cells and the last universal common ancestor

scientists used to think there were 3 domains of life that split from LUCA

  • bacteria

  • eukaryotes

  • archaea

evidence from 1984 - led scientists to believe there are 2 domains of life

  • eukaryotes originated from endosymbiosis

A2.1.9—Evidence for the evolution of the last universal common ancestor in the vicinity of hydrothermal vents

scientists focused on conserved genes that didn’t undergo horizontal gene transfer

  • found 355 genes that are presumed to have been present in LUCA

also studied stromatolites, in order to understand organisms that existed in the past & types of environments they lived in

found evidence that LUCA existed in alkaline hydrothermal vents

  • would hv been rich in various materials, which LUCA used as energy source

  • high temps meant that LUCA had energy to form complex organic molecules