A1.2 Origin of cells HL

(HL) Prebiotic conditions on early Earth

Early Earth lacked free oxygen and therefore ozone, had higher concentrations of carbon dioxide and methane, higher temperatures, and high levels of ultraviolet radiation

(HL) Absence of ozone layer

Without ozone in the stratosphere, ultraviolet radiation reached Earth’s surface, providing energy for chemical reactions

(HL) Effect of UV radiation on early Earth

UV radiation supplied energy that could drive spontaneous chemical reactions forming organic molecules

(HL) Atmospheric composition of early Earth

Dominated by carbon dioxide, methane, water vapor, and nitrogen with little or no oxygen

(HL) Prebiotic formation of carbon compounds

Early Earth conditions allowed carbon compounds to form spontaneously through chemical processes that do not occur today

(HL) Cells as the smallest units of life

Cells are the smallest structures capable of carrying out all functions of life independently

(HL) Characteristics of living organisms

Metabolism, homeostasis, response to stimuli, growth, reproduction, and self-sustaining chemical reactions

(HL) Why viruses are considered non-living

Viruses lack metabolism, cannot reproduce independently, and rely entirely on host cells

(HL) Challenge of explaining spontaneous origin of cells

Cells are highly complex and currently can only arise from pre-existing cells

(HL) Requirement: catalysis

Chemical reactions needed to be accelerated by catalysts to sustain early metabolic pathways

(HL) Requirement: self-replication

Molecules capable of copying themselves were necessary for inheritance and evolution

(HL) Requirement: self-assembly

Molecules needed to spontaneously organize into larger structures

(HL) Requirement: compartmentalization

Separation of internal chemistry from the environment was essential for early cells

(HL) NOS limitation in origin-of-life studies

Exact prebiotic conditions cannot be replicated and protocells did not fossilize, making hypotheses difficult to test

(HL) Miller–Urey experiment

Simulated early Earth conditions using gases and electrical sparks to test spontaneous formation of organic molecules

(HL) Results of Miller–Urey experiment

Amino acids and other organic compounds formed spontaneously

(HL) Evaluation of Miller–Urey experiment

Supports abiotic synthesis of organic molecules but does not explain origin of cells or genetic systems

(HL) Limitation of Miller–Urey experiment

Atmospheric composition used may not fully represent early Earth conditions

(HL) Formation of vesicles

Fatty acids spontaneously coalesce into spherical bilayers in aqueous environments

(HL) Importance of vesicle formation

Membrane-bound compartments allow internal chemistry to differ from the environment

(HL) Fatty acid bilayers

Simple membranes that can form without enzymes

(HL) RNA as first genetic material

RNA can store information, self-replicate, and catalyse reactions

(HL) Ribozymes

RNA molecules with catalytic activity

(HL) Evidence for RNA world

Ribozymes in ribosomes still catalyse peptide bond formation today

(HL) LUCA

The last universal common ancestor from which all current life descended

(HL) Evidence for LUCA

Universal genetic code and shared genes across all organisms

(HL) Universal genetic code

All organisms use the same codons to specify amino acids

(HL) Possibility of extinct early life forms

Other life forms may have evolved but were outcompeted by LUCA and its descendants

(HL) Estimating age of first living cells

Uses geological evidence, isotopes, and molecular data

(HL) Isotope evidence

Ratios of carbon isotopes in ancient rocks indicate biological activity

(HL) Stromatolites

Layered sedimentary structures formed by ancient microbial communities

(HL) Cyanobacteria

Photosynthetic prokaryotes associated with stromatolite formation

(HL) Molecular clock

Estimates evolutionary divergence based on mutation rates in DNA or proteins

(HL) Genomic analysis

Comparison of conserved genes across species to infer evolutionary relationships

(HL) Evidence for hydrothermal vent origin of life

Fossil evidence from ancient vent precipitates and conserved genetic sequences

(HL) Hydrothermal vents

Deep-sea environments rich in minerals and chemical energy

(HL) Advantages of hydrothermal vents

Provide stable energy sources and protected environments for early life

(HL Exam) Discuss why explaining the origin of cells is challenging

Cellular complexity, lack of fossils, and inability to replicate exact early Earth conditions

(HL Exam) Evaluate the evidence for abiotic synthesis of organic molecules

Supported by experiments like Miller–Urey but incomplete for explaining full cellular life

(HL Exam) Explain why RNA is a strong candidate for the first genetic material

RNA can both store information and catalyse reactions

(HL Exam) Explain how compartmentalization contributed to early cell evolution

Membranes allowed controlled internal chemistry and natural selection to act

(HL Exam) Explain the significance of LUCA

Demonstrates shared ancestry and explains universal molecular features of lif