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!