A2.1 origin of cells

describe the conditions of early earth

• more greenhouse gases

  • CO2

  • CH4

• less oxygen

• lots of UV

• lots of lightning

• higher temperature

what did the conditions in early earth allow for

• spontaneous reactions that created lots of complex carbon compounds

• no life present on earth due to extreme conditions

why was there more methane back then

• more volcanic activity

• meteorite bombardment

why was there more carbon dioxide back then

emission from volcano

why was there higher temperature back then

• more greenhouse gas traps more heat in atmosphere

• not enough oxygen / ozone, allowing solar UV to penetrate more easily

why was there more lightning

• high humidity and moisture levels → lots of heavy clouds and rainfalls created

• higher temperature and UV penetration → triggers chemical reactions

where did carbon compounds form on earth

• hot springs

• hydrothermal vents in oceans

• atmosphere

how did the environment of earth chance after organisms formed and evolved

• increased oxygen concentration → ozone layer → protection against UV

• reduced carbon dioxide levels

2 important characteristics of living organisms

• metabolism

  • self-sustaining, uses energy to keep themselves in highly ordered state

• reproduction

  • passing on the ability to maintain highly ordered state of offspring

how do we know cells are living

• they use energy to maintain themselves at a highly ordered state

• cells can live outside of the organisms

• cells can produce more cells

necessary requirements for the evolution of the first cells

• catalysis: give control over which chemical reactions occur

• self assembly: carbon compounds must assemble to form polymers

• compartmentalisation: membrane must develop to enclose cell contents

• self-replication of molecules: as a basis for inheritance and persistence of successful variants

challenges in explaining the spontaneous origins of cells

• theory says that cells can arise from non-living matter BUT most evidence says that is false

• some theorise cells arise from hydrothermal vents BUT there is little evidence for large hydrocarbon (important for cell formation) being concentrated in hydrothermal vents

• unlikely to be able to find well-preserved fossils of the first cells

• exact conditions of pre-biotic earth is uncertain

explain why the theory of spontaneous generation (living organisms can form from non-living matter) is wrong

1. Louis Pasteur’s experiment discovered every cell is formed by division of pre-existing cell → cell theory

2. if you accept humans have evolved from ancestral species, you can reach the earliest cells when tracing the origins of your cells

3. unless cells came from other places, they must have developed from non-living materials. BUT cells’ complex structure makes it unlikely for them to arise from non-living material naturally.

describe the Miller-Urey experiment

1. add CH4, H2, NH3 in flask (representing pre-biotic atmosphere)

2. boil H2O in another flask to add water vapour

3. add electrical spark (to stimulate lightning)

4. condense the produces produced, return to flask of boiling water

conclusion of Miller-Urey experiment

carbon compounds can form spontaneously before life evolved AS LONG AS the simulation by miller and urey are accurate enough

weaknesses of the experiment

• composition of atmosphere might not be the same as the experiment

• UV could be the source of energy for reaction rather than lightning

• glass apparatus used could have catalysed reactions

• elements from rocks on earth could have catalysed reactions

• amino acids did not assemble polymers or anything resembling primitive life

vesicles

small droplets of fluid enclosed in a membrane, probably played a part in early evolution of cells

why do vesicles form by themselves naturally

phospholipid molecules automatically form bilayer when placed in water, forming spherical structures

RNA

• stores genetic information

• self-replicating

• act as catalyst

  • help form peptide bond in polypeptides

virus

• uses RNA

  • supports theory that RNA is used before DNA

• high mutation rate (for the ones that use RNA)

things that support the existence of LUCA

• universal genetic code in all species → suggests that all species inherit genetic code from common ancestor

• same enzymes that synthesise DNA and RNA in all organisms

• similar ribosome structures in all organisms

whats LUCA

• last universal common ancestor

• other life forms probably existed but when extinct, so all species evolved from LUCA

how to estimate when life first started

• carbon dating on stromatolites (fossils coming from cyanobacteria) → 3.7 billion years ago

• comparing conserved sequences of species, if mutation rate is stable then the number of mutations indicates the time since divergence → estimate when life first started

both approaches support the theory that LUCA developed in proximity to hydrothermal vents

why did people theorise that LUCA originated near hydrothermal vents

• they gush out hot water with inorganic chemicals with alkaline environments, suitable for life

• the fluid gushed out contains high concentration of H2, CH4, NH3, sulfides → steady supply of energy for cells to assemble carbon compounds