Cell Theory And The Nature Of Life Flashcards
What are the three tenets of the cell theory?
- all organisms are composed of one or more cells
- Cells are the basic unit of life
- all cells arise from pre-existing cells (Rudolf Virchow)
What are the ramifications of all cells arising from pre-existing cells?
There is an unbroken lineage all the way back to LUCA
The central questions of cell theory are
How and why the first cell evolved
Describe the ‘molecular soup’ origin of life hypothesis
- Early-Earth oceans contained simple organic molecules
- Biological precursors (amino acids, sugars, nucleobases) formed
- Condensation polymerisation
- Chance autocatalytic polymer formation
- Rapid domination of molecular soup (exponential explosion of the replicator)
- Random changes improve replication speed and are selected for (evolution)
How is it hypothesised that Early-Earth simple organic molecules came about
By the action of UV or lightning on atmospheric gases
RNA world advantages:
- DNA cannot replicate itself
- Ribozymes - potential for the first replicator
- Ribozyme catalysis of RNA synthesis
What does DNA need to replicate?
A suite of protein catalysts - ancillary molecules that have not yet evolved
Is RNA a self-replicating molecule
No, both ribozyme and RNA are needed
Limitations of RNA World
- Organic concentration in soup is in ppm range - too dilute
- Chance if two nucleotides meeting is small, and this chance is logarithmically smaller for polymerisation
- Too slow (replication dependent on wet-dry cycles)
- RNA highly unstable in water
- Clay-silicate galleries
- High conc water makes hydrolysis more likely
Cells
Self-sustaining, self-replicating entity of a certain size, bounded by a lipid membrane, containing a genetic system and a metabolism (energy and environmental resources allow biosynthesis for daughter fell production)
Cells do not form
De novo
Describe the environment of the molecular soup
- volcanic land masses, global ocean
- hostile: 50-80 degrees, no oxygen in atmosphere, lots of CO2 and H2 and N2
Explain clay-silicate boundaries
- nucleotides, nucleobases, phosphate and sugars can bind to the surface of clay-silicate minerals and be concentrated in the ‘galleries’ between flat layers of lakes and pool shorelines
- repeat drying and rehydration cycles of the flay minerals in hydrothermal fields
What is the purist form of the RNA World concept?
RNA came before all other molecular components of the cell
Phospholipids are too complex to have formed
Pre-biotically
What, if not phospholipids, would have been present in the primordial soup?
Simpler amphiphiles
Amphiphiles can
Self-assemble into micelles and vesicles
What is the size of a vesicle determined by
The type of amphiphile, and the conditions
What is the size of a cell determined by
The interaction of biophysics and the environment
Proto-cell synthesis experiments:
- when a solution of nucleotides and lipids were heated and concentrated until total dehydration, micelles fuse to form multilamellar films
- after several dehydration/rehydration cycles, RNA molecules form (25-100ru)
- at the end of the reaction, RNA molecules are encapsulated in vesicles, which self-assemble after rehydration
Describe multilamellar films
Nucleotide molecules are encapsulated between the lamellae
ru
Repeating units
Problem with proto-cell synthesis experiments for RNA World hypothesis
Proto-cell membrane has no functional advantage for RNA synthesis and replication
The molecular soup is
Promiscuous
The RNA World hypothesis defines the expectation of
Prebiotic RNA
Vesicle formation is
Spontaneous, because it is a biostable structure
Why is water the universal solvent?
Because of its dipolarity, it can dissolve anything charged
Conditions required for life to begin as a spontaneous metabolism
1) high temperature (40-70 degrees)
2) high pressure
3) reduced redox-active metals such as Fe and Ni
Alkaline smokers as the crucible of life
- serpentinization
How do alkaline smokers form?
Exposure of mantle rocks to seawater
Serpentinization
- hot alkaline hydrothermal fluid percolates through the chimney
- reaction occurs between mantle rocks and seawater
- forms new minerals
- generates heat, H2, OH-
Minerals formed in serpentinization
- greigite (Fe3S4)
- mackinawite (Fe(Ni)S)
- magnetite (Fe3O4)
- awaruite (Ni3Fe)
Proto-metabolism is possible spontaneously under the right conditions
Based on reaction of hydrothermal H2 and CO2 dissolved in the ocean
Difference between black and white smokers
Black smokers - highly acidic
White smokers - highly alkaline
Modern methanogen metabolism
1) flavin electron bifurcation to generate a proton gradient
2) Fe(Ni)S exploits proton gradient to capture electrons from H2
3) transfers to FeS cluster of ferredoxin
4) ferredoxin catalyses CO2 reduction to organic hydrocarbons
Which is the salient enzyme that Fe(Ni)S contain?
Hydrogenase
Membrane-localised FeS crystals provide the mechanism for primitive heredity and evolution
1) amino acids would bind to FeS crystals, stopping their growth
2) FeS/amino acid composite partitioned cell membrane due to hydrophobic amino acids
3) FeS/amino acid composite catalyses CO2 reduction by exploiting geological proton gradient
4) produces more amino and fatty acids
5) fatty acid production leads to growth of cell membrane and division
6) daughter cell inherits membrane-associated catalyst
7) selection driven by daughter cells with best catalysts; fastest growth
How do amino acids stop FeS crystal growth
Preventing cell rupture
What is the biophysical response to increased fatty acid production and growth of cell membrane?
Spontaneous division at a certain threshold - cell replication
Life is based on cells because of
The biophysics of amphiphile self-assembly into membranes
What might have generated an evolvable entity from the pre-biotic world?
Combination of membranes and spontaneous proto-metabolism
Proto-metabolism reactions are
Possible, but extremely difficult
Flavin electron bifurcation is
Way too complicated to have occurred spontaneously
Why were Early-Earth oceans acidic?
Dissolved CO2
What is it about alkaline smokers that allows proton gradient formation?
Porous structure
Which pathway is difficult to envisage spontaneously
Pentose phosphate
What is the key to evolution of a protometabolic entity
The catalyst
Enzymes are
Full of iron sulfurs, which form growing crystals, endangering cell rupture
Describe the chimney of alkaline smokers
porous, metal-saturated mineral structure
