Download Origins - Stosich Science

Biomolecules & the
Origin of cells
SBI4U 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 Theory
 Life 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: Conditions
Atmosphere Composition:
 NH3
Energy Sources:
 H 2O
 Volcanic heat (volcanic
out-gassing would release
many of the gases of the
atmosphere)
 CO, CO2
 Lightning (electrical energy)
 N2
 H2
 CH4
 Ultraviolet (UV) Radiation
(without O2, O3 in the
atmosphere, most UV would
easily reach the Earth’s
surface)
Miller-Urey Experiment
 Set 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 cells
 Phospholipids 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 cells
 Protocells may have formed spontaneously…
they gave rise to 3 domains or cell lineages
Origins of Organelles
Autogenous / 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!