a2.1 origins of cells Flashcards
how was earth formed?
earth formed around 4.5 billion years ago when gravity pulled in swirling gas and dust. the earth was compressed by gravity, causing an increase in temperature which caused the proto-earth to become a hot molten mass
what was the atmosphere like on early earth?
‘reducing atmosphere’ with higher proportion of reactive gases such as ammonia and hydrogen. there was very little oxygen
what was the temperature like on early earth?
it was significantly warmer than today due to heat from asteroid collisions and the formation of the planet
what was the UV radiation like on early earth?
no ozone layer to block radiation from the sun so intense UV radiation reached the surface
what was volcanic activity like on early earth?
volcanic eruptions released gases (H2O and CO2) into the atmosphere
how did carbon compounds form?
various carbon compounds may have formed in hot springs and hydrothermal vents. compounds may have formed in water droplets in the atmosphere, creating an organic aerosol haze. these compounds were then deposited in the ocean by rainfall.
in such a reducing atmosphere, energy from volcanic eruptions and lightning can catalyse the creation of certain basic small molecules of life, like amino acids. once formed, small organic molecules may have been able to spontaneously form polymers on early earth
what does cell theory say?
- cells are the smallest unit of life
- cells come from other/pre-existing cells
- all living things are composed of cells
how was spontaneous generation disproved?
louis pasteur heated broth in a swan neck flask to kill any microbes. the swan neck flask allowed airflow but prevented microbes from entering. no microbial growth occurred in the broth, which meant spontaneous generation didn’t happen.
why is it difficult to test hypotheses about the origins of cells?
it is difficult to test hypotheses about the origins of cells because it happened billions of years ago. well preserved fossils are rare, the methods used to estimate the dates of the first living cells have ranges of uncertainty
what is the evidence for the formation of organic carbon compounds?
in 1929, haldane described the pre-biotic ocean as a ‘hot dilute soup’ in which a variety of carbon compounds formed. he assumed that the atmosphere contained water vapour, carbon dioxide and ammonia. he claimed that when UV light acts on this mixture, a vast variety of organic substances are made, including sugars and amino acids
what did miller & urey do?
miller & urey tested haldane’s hypothesis about the origin of carbon compounds in the 1950s. they passed steam through a mixture of methane, hydrogen and ammonia. the mixture was thought to be representative of the early earth. electrical discharges were used to simulate lightning. analysis showed a variety of carbon compounds had been produced, including more than 20 amino acids. this showed that it was possible for carbon compounds to form spontaneously on earth before life had evolved.
what are vesicles?
vesicles are small droplets of fluid enclosed in a membrane. they are very common structures inside cells and some cells produce extracellular ones called exosomes. vesicles probably played a part in the early evolution of cells because of their non-polar membrane core (phospholipids). the movement of polar molecules into and out of them would be difficult, so they could have developed their own internal chemistry
why is RNA considered the first genetic material?
to replicate DNA and pass on genes, enzymes are needed but to make enzymes, genes are needed. at an earlier phase in evolution, RNA may have been the genetic material. it can store info in the same way as DNA but it is self-replicating and can act as a catalyst. some viruses (which are non living) use RNA which supports the theory that RNA could have been used before DNA evolved.
what is the evidence for having a last universal common ancestor?
organisms store information in a genetic code made up codons (a sequence of 3 bases). each codon has a specific meaning - an amino acid, a ‘start’ signal or a ‘stop’ signal. different species could use different codes but they don’t - the code is universal. the obvious explanation is that they inherited it from a common ancestor.
similar structures can evolve in organisms that do not have a recent common ancestor (like fish and whales) which is called convergent evolution. however, this is not thought to be the reason for the universal code. the most recent common ancestor to have existed is called LUCA.
what is a stromatolite?
a stromatolite is formed when mats of cyanobacteria in shallow sea water trap sediments and secrete calcium carbonate, slowly building rocky mounds over thousands of years
