A2.1 Origin of Cells
Higher Level
A2.1.1 – Early Earth Conditions & Carbon Compounds
How was early Earth different, and how did it support pre-biotic chemistry?
Early Earth lacked atmospheric oxygen but had high levels of carbon dioxide and methane. Without an ozone layer, UV radiation was intense, and the planet was warmer than today.
These extreme conditions could have provided the energy required for the spontaneous formation of organic compounds.
The Miller-Urey experiment demonstrated that amino acids could form when water vapor, methane, ammonia, and hydrogen were exposed to electric sparks simulating lightning.
However, the experiment has limitations:
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It assumed early atmospheric conditions that are now disputed.
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It did not produce all organic compounds used by living organisms.
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It did not demonstrate how amino acids could be assembled into complex molecules like proteins.
Higher Level
Higher Level
A2.1.2 – Cells as Units of Life
What defines a living cell, and how do cells compare with viruses?
Cell Theory includes three major points:
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Living organisms are composed of cells.
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Cells are the smallest units of life.
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All cells arise from pre-existing cells.
Similarities between cells and viruses:
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Both are made of organic molecules.
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Both store genetic material in nucleic acids.
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Both can evolve.
Differences between cells and viruses:
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Cells are self-sustaining; viruses cannot function without a host.
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Cells can reproduce independently; viruses require host machinery.
Higher Level
Higher Level
Higher Level
A2.1.3 – Spontaneous Origin of Cells
What steps were required for life to emerge from non-life?
Three essential conditions were needed inside early cells:
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Catalysis – Enzyme-like molecules were required to drive chemical reactions.
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Self-assembly – Organic molecules had to assemble into organized structures.
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Compartmentalization – A membrane was needed to separate the internal environment from the outside.
Challenges to studying the origin of cells:
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The exact early Earth conditions cannot be replicated in the laboratory.
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The first protocells did not leave fossils, making direct evidence scarce.
Higher Level
A2.1.4 – Carbon Compound Formation
Where did life’s building blocks come from?
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The Miller-Urey experiment supported the hypothesis that organic compounds like amino acids could form under prebiotic conditions.
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This provided a basis for the idea that life’s molecular precursors could arise naturally from simpler substances.
Higher Level
A2.1.5 – Formation of Membranes
How could primitive membranes have formed spontaneously?
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Fatty acids are amphipathic, meaning they have both hydrophilic and hydrophobic properties.
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In aqueous environments, they spontaneously form spherical vesicles (bilayers), which resemble modern cell membranes.
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Membranes were essential for early life because they created a protected internal environment, allowing for chemical reactions to occur in a controlled space.
Higher Level
A2.1.6 – RNA as First Genetic Material
Why is RNA considered the first genetic material?
RNA has two key features that support this idea:
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It can store genetic information.
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It can act as a catalyst (ribozyme), unlike DNA.
Higher Level
Higher Level
A2.1.7 – Evidence for LUCA
What evidence suggests all organisms share a common ancestor? All known life shares key features:​
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The same universal genetic code.
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Ribosomes with similar structure and function.
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Shared metabolic pathways like glycolysis and respiration.
These conserved traits point to a single origin: the Last Universal Common Ancestor (LUCA).
Higher Level
A2.1.8 – Dating First Life
How do scientists estimate when life began?
Techniques used to date the origin of life include:
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Radiometric dating of ancient rocks and microfossils.
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Molecular clock analysis based on mutation rates in DNA.
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Phylogenetic trees tracing genetic divergence from LUCA.
These methods are used collectively to estimate the appearance of the first living cells around 3.5 to 4 billion years ago.
Higher Level
A2.1.9 – LUCA and Hydrothermal Vents
LUCA may have evolved near hydrothermal vents, which offered heat, minerals, and stable chemical energy. These conditions could drive the formation of organic molecules. Modern vent-dwelling prokaryotes and fossil evidence in ancient vent rocks support this origin.
