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Biomolecules & the Origin of cellsSBI4U Biology
From Molecules to Cells?Organic biomolecules are the
substances that enabled life to exist
But, if life arose from these biomolecules, how did these complex molecules arise?
Their origins are uncertain, but may lie in an unlikely mix of much simpler organic & inorganic molecules…
Oparin’s TheoryLife could have arise spontaneously on our
planet, but on a very different Earth than we know it:
Seas rich with simple organic compounds
Reducing atmosphere: low in oxygen, conducive to redox reactions
Gases in such an atmosphere would reduce any compounds they came in contact with
These compounds would gain electrons & the atmosphere would slowly be oxidized.
Oparin’s Theory: ConditionsAtmosphere Composition:
NH3
H2O
CO, CO2
N2
H2
CH4
Energy Sources: Volcanic heat (volcanic
out-gassing would release many of the gases of the atmosphere)
Lightning (electrical energy)
Ultraviolet (UV) Radiation (without O2, O3 in the atmosphere, most UV would easily reach the Earth’s surface)
Miller-Urey ExperimentSet up an airtight apparatus with a mixture of
gases as proposed by Oparin
Gases constantly heated & cooled, and circulated past an electric charge
Within hours: HCN formed (very reactive)
Within days: Amino acids & simple polypeptides
Within a week: Lipids, simple sugars, nitrogen compounds that could be basis of DNA…
Within a month: Nucleosides – Sugar + Base dimers that could form DNA, with PO4
Miller-Urey Experiment
Miller-Urey Experiment
Some of the resultant molecules formed within hours & days of starting.
Fate of the first biomolecules?No living organisms to consume them…
No free O2 to oxidize them….
They accumulate to very large amounts in Earth’s oceans, to levels impossible today.
Collisions between molecules in the oceans can create new compounds, but in oceans that’s very hit & miss.
Need a way to concentrate them…
Fate of the first biomolecules?TIDAL POOLS: shallow, temporary, easily
heated by the sun – as water evaporates, concentration increases… add some heat, & reactions possible.
ADSORPTION: clay & other minerals allow molecules to stick to their surfaces; over time, molecules accumulate to a concentration that allows them to react.
ICE: trapped molecules do accumulate to higher concentrations, but reaction rates are very slow.
Fate of the first biomolecules? In any of the scenarios, concentration & energy
matter
If conditions are right, the smaller molecules (monomers) react and form larger polymers: proteins, fatty acids, nucleic acids…
Some of the polymers are capable of catalyzing other reactions, so even more molecules form
Some are self-replicating (RNA)… an early form of continuity?
Coacervates Under the right set of temperatures & pH conditions, some of
these early molecules form solid aggregates = coacervates
About the size of a bacterium
Core of lipids & carbohydrates
Outer shell of amino acids, short polypeptides & water
Coacervates clearly show organization, & can grow by absorbing more molecules…. Are the alive?
Coacervates: the Life Test Form spontaneously, briefly…. But they do show organization.
They have a simple membrane-like coating
Grow by accumulating more biomolecules, which they incorporate into the correct layers (a form of metabolism or homeostasis?)
As they grow, projections can form & break off… reproduction?
They show some of the features of living things: organization, growth,homeostasis, reproduction… but the reproduction is sporadic, unpredictable, and with no real genetic continuity
Coacervates do not display heredity
Coacervates are not a living organism… they are prebionts.
Other Prebionts: Coacervates, Protein microspheres, Liposomes, RNA & DNA…
Cell Membranes & early cellsPhospholipids would have formed readily in
early Earth’s oceans
We know that they spontaneously form bilayers in water, as well as solid micelles & hollow liposomes
If a liposome forms & it traps the right combination of chemicals: some sugars, some lipids, some RNA, some DNA…. That’s a cell!
The first cells would have been Prokaryotes
They would have given rise to ancient bacteria.
Cell Membranes & early cellsProtocells may have formed spontaneously… they gave rise
to 3 domains or cell lineages
Origins of OrganellesAutogenous / Invagination:
Cell membrane is fluid: It moves, bends, ‘flows’
If it moves inward, it creates a pit where materials can accumulate; pinching this section of membrane off creates a vacuole or nucleus
Further inward folding + growth of the cell membrane creates canals, vesicles… the future endoplasmic reticulum (ER), Golgi apparatus, etc.
Symbiosis:
Not all early prokaryotes were the same: they varied in size & metabolism
Symbiosis of a sugar-eating, aerobic prokaryote inside a larger cell = mitochondria
Symbiosis of a photosynthetic prokaryote (cyanobacterium) inside a larger cell = chloroplast
Invagination Theory
Symbiont Theory
Origins of Organelles:
Getting to Multicellular Life forms:Simplest prokaryotes = 3.5 – 3.1 Billion years ago (bya)
First eukaryotes = 1.5 bya
By 600 mya, Colonies of eukaryotic cells form – multicellularity begins
With multicellularity comes specialization: tissues, organs, systems… complex life!