A2.1 Origin of Cells - Notes

A2.1 Origin of Cells

From the IB Guiding Questions

  • What plausible hypothesis could account for the origin of life?

  • What intermediate stages could there have been between non-living matter and the first living cells?

A2.1.1: Conditions on Early Earth and the Pre-biotic Formation of Carbon Compounds

  • Early Earth conditions:

    • Lack of free oxygen and therefore ozone (O_3).

    • Higher concentrations of carbon dioxide (CO2) and methane (CH4), leading to higher temperatures.

    • Increased ultraviolet light penetration.

  • These conditions may have caused a variety of carbon compounds to form spontaneously through chemical processes that no longer occur.

The Atmosphere of Prebiotic Earth

  • Oparin and Haldane suggested that the prebiotic Earth's atmosphere was significantly different from today's.

  • Evidence suggests it was composed primarily of nitrogen (N2), carbon dioxide (CO2), and water vapor (H2O) with smaller amounts of methane (CH4) and hydrogen (H_2).

  • No oxygen gas (O2) or ozone (O3) was present, as oxygen was first released by photosynthesis.

  • The lack of an ozone layer resulted in high levels of ultraviolet light.

  • High carbon dioxide concentrations resulted in higher temperatures due to the greenhouse effect.

  • Energy from UV light, higher temperatures, lightning, and volcanic activity allowed the spontaneous generation of organic compounds.

  • Over time, these organic molecules became more complex, eventually leading to self-replicating structures and the first cells.

Notes on Compounds

  • Organic compounds: Carbon-containing molecules excluding oxides and carbonates.

  • Inorganic compounds: All other compounds that are not organic.

A2.1.2: Cells as the Smallest Units of Self-Sustaining Life

  • Discuss the differences between living and non-living things.

  • Include reasons why viruses are considered non-living.

Cell Theory

  • Cells are the smallest units of self-sustaining life.

  • The Cell Theory States:

    1. Cells are the basic unit of life.

    2. All organisms are composed of at least one cell.

    3. All cells come from pre-existing cells.

Characteristics Shared by All Living Organisms

  • Have at least one cell.

  • Are capable of carrying out metabolism.

  • Maintain homeostasis.

  • Respond to stimuli.

  • Are capable of reproduction.

  • Grow and develop.

  • Contain genetic information (DNA).

Viruses are not Alive

  • Living organisms should possess all characteristics of life.

  • Viruses have genetic material (DNA or RNA) but lack most life characteristics.

  • Viruses do not have cells which carry out metabolism and homeostasis.

  • They cannot respond to stimuli, and they do not grow.

  • Viruses are not capable of reproducing themselves; they are replicated by host cells.

A2.1.3: Challenge of Explaining the Spontaneous Origin of Cells

  • Cells are highly complex structures that can currently only be produced by division of pre-existing cells.

  • Necessary requirements for the evolution of the first cells:

    • Catalysis

    • Self-replication of molecules

    • Self-assembly

    • Emergence of compartmentalization

Nature of Science

  • Claims in science, including hypotheses and theories, must be testable.

  • The exact conditions on prebiotic Earth cannot be replicated, and the first protocells did not fossilize.

Origin of Cells Requirements

  • Cells are highly complex structures produced by the division of pre-existing cells.

  • The following were required for the development of the first cells:

    • Catalysis

    • Self-replication of molecules

    • Self-assembly

    • The emergence of compartmentalization

NOS - Scientific Claims Must Be Testable

  • It is impossible to know the exact conditions of prebiotic Earth.

  • Scientists have used evidence to identify components of the prebiotic atmosphere but can't conclusively confirm accuracy.

  • The first protocells left no fossils, so there is no evidence of their actual make-up.

  • Claims in science, including hypotheses and theories, must be testable.

A2.1.4: Evidence for the Origin of Carbon Compounds

  • Evaluate the Miller–Urey experiment.

Miller Urey Experiment

  • Miller and Urey tested Haldane and Oparin’s hypothesis that macromolecules of life could have spontaneously generated on prebiotic Earth.

  • They attempted to simulate conditions on prebiotic Earth in a laboratory setting.

  • The gases methane (CH4), ammonia (NH3), and hydrogen (H2) with water vapor (H2O) represented the prebiotic atmosphere.

  • Water was used to model the oceans.

  • A condenser allowed water vapor to return to the liquid water.

  • Energy was supplied through electrical sparks (simulating lightning) and a heat source to evaporate water.

  • After one week, the water became brownish black.

  • Analysis showed that many complex organic molecules, including amino acids, had been produced.

  • The experiment showed that at least some organic molecules required for life could be spontaneously generated under certain conditions.

Evaluate Miller and Urey’s Experiment

  • Strengths:

    • Modelled prebiotic Earth and its atmosphere.

    • Demonstrated that molecules like amino acids can be generated spontaneously under certain conditions.

    • The design allows it to be replicated by other scientists.

  • Limitations:

    • Debate on the actual atmosphere of prebiotic Earth remains.

    • The experiment did not produce all organic molecules required for life.

    • The simulation could not account for all conditions on prebiotic Earth.

A2.1.5: Spontaneous Formation of Vesicles by Coalescence of Fatty Acids into Spherical Bilayers

  • Formation of a membrane-bound compartment is needed to allow internal chemistry to differ from that outside the compartment.

Cell Membranes

  • Cell metabolism requires the separation of the cytoplasm from the external environment.

  • Cell membranes serve this function in modern cells.

  • Cell membranes are formed of phospholipid bilayers.

  • Phospholipids are amphipathic, with hydrophilic phosphate heads and hydrophobic fatty acid tails.

Fatty Acids and Vesicles

  • Fatty acids have formed spontaneously in experiments similar to Miller and Urey’s experiment.

  • Fatty acids spontaneously coalesce to form spherical bilayers (vesicles) when mixed with water.

  • It has been hypothesized that the first genetic material was trapped within a phospholipid vesicle, forming a protocell.

A2.1.6: RNA as a Presumed First Genetic Material

  • RNA can be replicated and has some catalytic activity, so it may have acted initially as both the genetic material and the enzymes of the earliest cells.

  • Ribozymes in the ribosome are still used to catalyze peptide bond formation during protein synthesis.

The First Genetic Material - RNA

  • RNA is hypothesized to be the first genetic material because:

    • RNA can store genetic information.

    • RNA is capable of self-replication.

    • RNA can catalyze reactions.

  • Ribozymes (RNA molecules) in the ribosome catalyze the formation of peptide bonds during protein synthesis.

A2.1.7: Evidence for a Last Universal Common Ancestor

  • Include the universal genetic code and shared genes across all organisms.

  • Include the likelihood of other forms of life having evolved but becoming extinct due to competition from the last universal common ancestor (LUCA) and descendants of LUCA.

Last Universal Common Ancestor

  • The last universal common ancestor (LUCA) is the most recent common ancestor for all organisms on Earth.

  • There may have been other forms of life at the same time as LUCA, but these forms of life became extinct as LUCA and its descendents outcompeted them.

Evidence for LUCA

  • There are no fossil remains for the last universal common ancestor (LUCA).

  • However, the fact that all organisms share the same genetic code strongly suggests that all organisms have evolved from LUCA.

  • All living things share a number of genes, which are assumed to have been inherited from LUCA.

  • Cladistics allows scientists to estimate the age of common ancestors by analyzing changes in the genetic code.

A2.1.8: Approaches Used to Estimate Dates of the First Living Cells and the Last Universal Common Ancestor

  • Students should develop an appreciation of the immense length of time over which life has been evolving on Earth.

The Earth is 4.6 Billion Years Old!

  • LUCA appeared approximately 4 billion years ago.

  • Life has continued to evolve for the last 4 billion years, producing the biodiversity currently found on our planet.

Estimating the Age of the First Living Cells

  • The oldest fossils of cells (of cyanobacteria) are approximately 3.5 billion years old.

  • The minerals and rocks around the earliest fossils have been dated using radioactive dating of uranium, which decays at a steady rate.

  • Molecular clocks: Comparing the number of mutations in genes between species allows scientists to estimate when the two species diverged into separate species. Scientists have used this technique to estimate that LUCA existed approximately 4 billion years ago.

A2.1.9: Evidence for the Evolution of the Last Universal Common Ancestor in the Vicinity of Hydrothermal Vents

  • Include fossilized evidence of life from ancient seafloor hydrothermal vent precipitates and evidence of conserved sequences from genomic analysis.

LUCA Evolved Near Hydrothermal Vents

  • Evidence suggests that LUCA evolved in the vicinity of hydrothermal vents.

  • Hydrothermal vents are rich in organic compounds, such as methane CH_4 CO2), and hydrogen (H_2$$).

  • Experiments similar to Miller and Urey’s show that the building blocks of life, such as proteins and nucleotides, can form from these chemicals.

Hydrothermal Vents and LUCA

  • Fossils of cyanobacteria have been discovered from ancient seafloor hydrothermal vent precipitates.

  • An analysis of genes common to all three domains of life has identified genes that are highly conserved.

  • Some of these genes are involved in thermophilic metabolism, suggesting that LUCA may have evolved in a high-temperature environment such as hydrothermal vents.

HL Only Key Terms

  • Pre-Biotic

  • Organic Compounds

  • Spontaneous Generation

  • LUCA

  • Domains

  • Eubacteria

  • Archaea

  • Eukarya

  • Evolution

  • Amphipathic

  • Hydrophilic

  • Hydrophobic

  • Vesicle

  • Ribozyme

  • Hydrothermal Vent

  • Thermophiles

Linking the IB Questions

  • For what reasons is heredity an essential feature of living things?

  • What is needed for structures to be able to evolve by natural selection?