A2.1 Origins of Cells (HL) & fossil stuff

what term is used to describe the early oceans?

primordial soup

• was used to describe these early oceans, which were rich in simple carbon-based compounds

scientists think this primordial soup was the origin of the first primitive life forms, which gave rise to the variety of species existing since

when Earth was first formed, what were the conditions?

• Earth’s surface was covered with a single global ocean

• no solid land masses

• underwent bombardment from comets & asteroids

what did the bombardments from comets & asteroids, provide Earth with?

the bombardments provided/brought water & other compounds to Earth

what did these incidents, along with volcanic eruptions, result in the release of?

the incidents & volcanic eruptions resulted in the release of ammonia & methane

what did Earth’s  early atmosphere consist of?

the majority of Earth’s early atmosphere consisted of ammonia & methane, along with water vapour & high concentrations of carbon dioxide 

what else did the high concentrations of CO₂ & methane “create”/”form?”

high temps & UV light penetration

did Earth’s early atmosphere hv an ozone layer?

no, it didn’t hv an ozone layer cuz there was no presence of free oxygen

what was the condition of Earth’s inner core?

it would’ve been very hot & liquid

what was the consequence of Earth’s inner core?

since it was very hot & liquid, there was increased motion in the core. this meant that there was a smaller protective magnetic field around the Earth, meaning the planet would’ve been vulnerable to higher levels of cosmic & solar radiation

what would the conditions on Earth allowed the formation of?

would’ve allowed the formation of a variety carbon-based compounds to form spontaneously by chemical processes that do not occur now

these carbon-based compounds would’ve been the building blocks for organic molecules

what’re reducing gases

gases that donate electrons to other molecules, reducing other gases while being oxidised themself

what was the importance of reducing gases on forming carbon compounds?

high proportion of reducing gases = early Earth had a reducing atmosphere, meaning there were reducing gases in the atmosphere. so, electrons were donated to other molecules, enabling chemical rxns to occur. rxns formed complex carbon compounds, which eventually joined together to form building blocks of cell

SO - high proportion of reducing gases = donated electrons to other molecules = enables chemical rxns to happen = formation of complex carbon compounds 

what would some of these complex compounds, hv developed the ability to do?

would’ve developed the ability to self-replicate, eventually becoming packaged into membranes

this would’ve allowed the molecules to exist in different conditions to external environment, leading to formation of cells

why are cells considered to be self-sustaining?

cuz they contain all the necessary components to carry out all 8 processes of life 

cells are also the smallest unit of self-sustaining life

what’re some reasons for something to be considered non-living?

if the organism is self-sustaining, it’s likely living

*add more

why are viruses considered to be non-living?

viruses resemble living organisms cuz they hv genetic material & can reproduce

BUT they’re considered non-living cuz they need a host cell in order to reproduce

• also cannot complete the 8 processes of life

• they’re also not formed by cells

what’s spontaneous generation?

the belief that living things originated from non-living materials

what did scientists believe were the necessary steps for the evolution of the first cells

1. Simple organic molecules such as amino acids and hydrocarbon chains were formed.

2. Chemical reactions were accelerated in the process of 

   catalysis

3. Larger organic molecules including RNA and phospholipids were assembled from smaller molecules.

4. Some of these molecules, including RNA were able to self-replicate.

5. Formation of a membrane-bound compartment (the cell surface membrane) allowed the internal chemistry of the cell to become different from that outside the compartment.

what was the main challenge in explaining the spontaneous origin of cells?

cells are very complex structures that, currently, can only be produced by division of pre-existing cells

catalysis, self-replication of molecules, self-assembly & emergence of compartmentalization were necessary requirements/ingredients for evolution of the first cells

what was an important step in the formation of the first cell?

the formation of a membrane-bound compartment (which were vesicles??)

what would hv caused the formation of a membrane-bound compartment

would hv occurred when fatty acids spontaneously merged to form spherical bilayer (double layer of lipid molecules that encloses a space)

what does this physical separation mean?

it means that the interior of vesicle would’ve been able to provide chemical environment with a different chemistry to external environment

in other words, the membrane-bound compartment allowed for the internal chemistry to be different from the chemistry outside the compartment

why was this separation of inner chemistry & outer chemistry, important?

it was an important step in origins of cells - the separation allowed the cell to control & maintain a specific set of conditions for proper cell functioning

what is the ‘RNA first’ hypothesis

it posits that RNA was the building block of the first cell-like structure, functioning as genetic material & a catalyst

what did the hypothesis suggest?

1. RNA was formed from inorganic sources.

2. RNA was able to replicate using 

   ribozymes

   .

3. RNA was able to catalyse protein synthesis.

4. Membrane compartmentalisation occurred.

5. Inside the cell, RNA was able to produce both protein and DNA.

6. DNA took over as the main genetic material because it is more stable.

7. Proteins took over as the catalytic form (enzymes) because they are more capable of variability.

what’re ribozymes?

RNA molecules capable of acting as catalysts and increasing the rate of chemical reactions.

what was some evidence that supported this hypothesis?

1. RNA can self-duplicate - meaning short RNA sequences can duplicate other RNA molecules

2. RNA has some catalytic activity - meaning it may have initially acted as genetic material & enzymes of earliest cells

3. Ribozymes in ribosome are used to catalyse peptide bond formation during protein synthesis 

what is the Last Common Ancestor (LUCA)?

is one of the life forms on Earth that’s hypothesized to hv given rise to all of the species existing since & today

what was LUCA thought to be?

LUCA was thought to be a single-celled autotrophic (makes its own food) microbe with likely an RNA genome (but there’s arguments that LUCA had DNA genome)

what do scientists think is the relationship between LUCA & extinction of other life forms?

scientists think that LUCA, or its descendants, outcompeted other life forms, leading to the extinction of these other life forms

what is the reason for the universality of LUCA?

the genetic code is universal - uses the same codons to code for some amino acids in all organisms

• is also shared by all living organisms & viruses

so, the reason for this lies in the common ancestry of all living organisms from one LUCA

over time, what’s happened to the genetic code?

it’s been conserved, meaning some codons code for the same amino acids in all organisms 

• any changes to genetic info will likely be highly detrimental to organism

what did scientists used to think were the 3 domains from LUCA?

scientists used to think that there were 3 domains of life that had split from LUCA: bacteria, eukaryotes, & archaea 

but evidence from 1984 led to them believing that there were 2 domains of life & eukaryotes originate from endosymbiosis 

what’re some evidence for the evolution of LUCA that scientists hv found? - dont rlly understand this

• after they discovered that both bacteria & archaea arose directly from LUCA, scientists began searching for genes that were common to both these domains

  • believed that they would hv also been present in LUCA

  • found 11,000 common genes

• BUT scientists realised this approach was confounded by a process called horizontal gene transfer

  • where genes are transferred across species or domains

• so scientists reduced or focused their investigation to include solely ‘ancient’ conserved genes present in bacteria & archaea

  • they found 355 genes that’re assumed to hv been present in LUCA

they also studied stromatolites, which are fossils found in rocks that’re thought to hv been formed by layered communities of microorganisms

what’re conserved genes

they’re genes that’re in a wide variety of organisms & don’t rlly change much over a period of time

conserved genes generally hv important basic functions & are inherited from a common ancestor

what do scientists believe that LUCA originated from?

they believe that LUCA originated from hydrothermal vents

what’re fossils?

preserved remains or traces of any organism from the remote past

2 different types

1. Direct - body fossils, such as bones, teeth, shells, leaves, etc

2. Indirect - trace fossils such as footprints, teeth marks, & burrows

what’s the fossil record?

is the sum of all discovered & undiscovered fossils & their relative placement in rock

• also allows for determining age of fossil

  • the layer of rock in which a fossil is found can be dated & can be used to deduce the age of the fossil

what’re some causes of gaps in the fossil record?

• special circumstances are needed for fossilization to occur

  • these circumstances don’t happen = no fossils

• only the hard parts of an organism are preserved

• fossils can be damaged, so only fragments can be discovered