A2.1 Origin of Cells Flashcards
What was Earth like 4.5 B years ago?
No atmosphere so asteroids pummeled Earth, increasing temperature.
What was Earth like 4 B years ago?
Number of impacts from asteroid decreased and atmosphere began to form.
Formed of:
- Water vapour
- High CO2 and Methane levels (greenhouse gases; trapped infrared radiation coming from earth surface, preventing escape, increasing surface temperature: greenhouse effect)
- Ammonia
- Hydrogen sulphide
- Hydrogen
X oxygen - This caused there to be no ozone layer: allowed UV rays to penetrate atmosphere = high surface temperature.
Earth at that time was not able to support life, but instrumental in origin of biological compounds that made life possible.
Point of Miller and Urey’s experiment.
Experimented to simulate early atmosphere conditions.
Testing the ‘Primordial Soup’ hypothesis
Test to see if organic molecules (contain carbon) can be formed from abiotic (non-living) organisms.
Describe how the primordial soup hypothesis (an aqueous mix of simple organic molecules) explains the origin of carbon compounds on Earth.
The primordial soup hypothesis states that billions of years ago, there were much higher levels of UV light on Earth which catalysed reactions in the gases of the early atmosphere. The UV radiation broke down inorganic molecules into smaller, highly reactive components. The smaller, highly reactive components reacted to form larger carbon compounds.
What were the conditions set in the Miller-Urey experiment?
- Low O2 levels
- High radiation
- Spark discharges
- Gases (water vapour, methane, hydrogen, ammonia)
- Condenser to cool and drop liquid like rain
- Boiling water
What was the result of the experiment?
After leaving the experiment running for a week: long chains of hydrocarbons + 15 of the 20 amino acids found.
Describe how this experiment showed that the origin of life was possible on early Earth.
Miller and Urey conducted experiments which showed that the compounds present in the early atmosphere (methane, ammonia and hydrogen) could be used to make the organic carbon compounds necessary for life. They did this by heating up water and passing the steam through ammonia, methane and hydrogen and by using electricity to simulate the lightning. They found that these conditions created amino acids and other organic carbon compounds.
What were issues other scientists had with the experiment?
- No scientific evidence of the components of early atmosphere.
- Carbon compounds theorised to come from meteorites.
- If early gases came from volcano, ammonia and methane cannot be present.
Possible theories for origin of life
- Hydrothermal vents
- Water cycle (miller-urey)
Explain Hydrothermal vents
Formed due to cracks in Earth’s crust. This exposes sea water to mantel: mineral rich hot water rising in plumes of black smoke.
Explain the theory for hydrothermal vents and evidence for it. (4)
First observed in 1977: found never discovered before organisms near the vents (e.g. metre long tube worm).
(Disproved theory that seabed is lifeless.)
Evidence:
- Some of world’s oldest fossils found near thermal vents.
- Organisms nearby have similar genetic sequence: suggests common ancestor.
- Mineral rich environment provided necessary material for most chemical reactions.
Define reducing environment.
No oxygen present but hydrogen is.
Define Cell theory
Cell theory describes the properties of cells and their role as fundamental units of life.
What are the three main principles of cell theory
All living things are composed of one or more cells.
Cells are the smallest units of life.
Cells only arise from pre-existing cells.
Which experiment provided evidence for cell theory?
Pasteur’s experiment conducted in 1880s.
Explain Pasteur’s experiment.
Boiled nutrient-rich chicken broth in three swan-neck flasks:
- First flask was not broken, remained clear.
- Second flask: neck was broken, became cloudy as microbes from air carried into flask grew and multiplied.
- Third flask: flask was tilted so broth was exposed to microbes in curve of swan neck.
What are the 7 functions of life that all living organisms carry out?
- Metabolism
- Growth
- Reproduction
- Sensitivity
- Homeostasis
- Nutrition
- Excretion
Explain the formation of a vesicle in early Earth.
Hydrocarbon chains (found in the mixture from Miller-Urey experiment) one of first complex hydrocarbon formed.
One of these carbon compounds called fatty acids.
Fatty acids have a hydrophilic head and hydrophobic tail. Makes them amphipathic.
When many of them placed in water, they started to form cell-sized double layer vesicle. Tail face inwards and head outwards. Regulates passage of substances. Lipid bilayer.
Often referred to as liposomes.
Formed to minimise the interactions between hydrophobic regions and water. Creates a spherical structure called a vesicle.
Significance of formation of vesicles for evolution of cells.
Likely that they were the early precursors for protocells.
Lipid bilayer structure led to development of membrane.
Plasma membranes separate internal cellular environment to external surroundings: allowed for controlled interactions with environment.
Allowed compartmentalisation of different cellular functions.
Protection of genetic material from external environment.
Explain why RNA is believed to be the first genetic material.
For early life to evolve following must emerge:
- a system capable of replicating itself
- ability to catalyse chemical reactions.
In modern cells: DNA and enzymes.
In early cells, these aren’t present so RNA believed to do both.
RNA can store genetic material as well as have enzymatic properties. Hypothesised that as life evolved, enzyme took role of catalysing and DNA took role of genetic storage molecule.
Evidence for this in properties of RNA:
- RNA can assemble spontaneously from nucleotides.
- RNA can replicate itself.
- RNA can control rate of chemical reactions.
Evidence that RNA came before DNA.
Ribose can be formed from methanal: main product of Miller-Urey experiment.
Deoxyribose produced from ribose.
Evidence for last universal common ancestor (LUCA)
All cells share same genetic code.
Over 300 genes common to all cells.
All cells use DNA or RNA as genetic information.
Process of replication of DNA and creation of protein universal to all organisms.
All living organisms have similar transport mechanisms in and out of the cell.
Use of ATP as a common energy currency.
How are fossils dated?
Absolute dating: most accurate way.
Relative dating.
Explain Absolute dating.
Fossils contain traces of unstable radioactive isotopes.
These elements accumulate whilst organism is alive. They decay over time to become more stable.
Time taken for half a sample to decay is called half-life.
Explain Relative dating.
Uses layers of the Earth and fossil index to approximate age of fossils.
Why are viruses not regarded as living organisms?
Cannot carry out metabolic processes.
Lack cellular structures and organelles.
Depend on host cells to replicate/reproduce.
Cannot respond to stimuli or adapt to environmental changes on their own.
