A2.1 Origins of cellsIB Biology HL: Revision notes

The pre-biotic Earth had no free oxygen, and therefore no ozone layer, so ultraviolet light penetrated to the surface. Concentrations of carbon dioxide and methane were higher, giving higher temperatures. These conditions may have caused a variety of carbon compounds to form spontaneously by chemical processes that do not occur now (oxygen and living organisms would break them down today).

Cells are the smallest units of self-sustaining life: they carry out all the functions of life independently (metabolism, growth, reproduction, response, homeostasis, nutrition, excretion). Non-living things may show one feature, such as a crystal growing, but not all of them.

Viruses are considered non-living: they are not cells, have no cytoplasm and few or no enzymes, have no metabolism of their own, and can only reproduce inside a host cell using its ribosomes and energy.

Cells are highly complex and today are produced only by division of pre-existing cells. For the first cells to evolve, four things were needed:

• catalysis (to speed up reactions);
• self-replication of molecules (to carry information);
• self-assembly (structures forming spontaneously);
• compartmentalization (a membrane-bound space).

NOS: scientific claims must be testable. Origin-of-life hypotheses are hard to test because the exact pre-biotic conditions cannot be replicated and the first protocells did not fossilise.

The Miller–Urey experiment passed electric sparks (simulating lightning) through methane, ammonia, hydrogen and water vapour with no oxygen. After a week, amino acids had formed. Strengths: shows carbon compounds can form spontaneously from inorganic substances. Limitations: the early atmosphere was probably mainly carbon dioxide and nitrogen; only monomers formed, not polymers or cells.

Vesicles form spontaneously when fatty acids coalesce into spherical bilayers. A membrane-bound compartment lets internal chemistry become different from outside.

RNA can be replicated (template, complementary base pairing) and has catalytic activity, so it may have acted as both the genetic material and the enzymes of the earliest cells (the RNA world). Evidence: ribozymes in the ribosome still catalyse peptide bond formation during protein synthesis.

Evidence for a last universal common ancestor (LUCA): the universal genetic code and genes shared by all organisms. Other forms of life probably evolved but became extinct due to competition from LUCA and its descendants.

Dating: radiometric dating of rocks containing the oldest fossils (first cells) and the molecular clock applied to conserved sequences (LUCA, about 4 billion years ago). Life has evolved over an immense length of time.

Hydrothermal vents: fossilised evidence of life in ancient seafloor vent precipitates, and conserved sequences from genomic analysis suggesting LUCA was anaerobic, heat-tolerant and used hydrogen, carbon dioxide and iron–sulfur proteins.