Biomolecules & the Origin of cells

1. Biomolecules & the Origin of cells SBI4U Biology

2. 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…

3. 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.

4. Oparin’s Theory: Conditions Atmosphere Composition: 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) • NH3 • H2O • CO, CO2 • N2 • H2 • CH4

5. 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

6. Miller-Urey Experiment

7. Miller-Urey Experiment Some of the resultant molecules formed within hours & days of starting.

8. 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…

9. 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.

10. 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?

11. 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?

12. 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.

13. Other Prebionts: • Coacervates, Protein microspheres, Liposomes, RNA & DNA…

14. 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.

15. Cell Membranes & early cells • Protocells may have formed spontaneously… they gave rise to 3 domains or cell lineages

16. Origins of Organelles Autogenous / Invagination: 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 • 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.

17. Invagination Theory Symbiont Theory

18. Origins of Organelles:

19. 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!