1. 1
   In 1952 Stanley Miller and Harold Urey sealed a mixture of methane, ammonia, hydrogen and water vapour, with no oxygen, inside glass apparatus. Water was boiled in one flask, and the gases were circulated through a second flask in which electrical sparks were discharged between two electrodes. The gases were then cooled so that water condensed and collected in a trap before returning to the boiling flask. After one week the water had turned dark brown. Analysis identified several amino acids, including glycine and alanine, that were not present at the start. Current evidence suggests that the early atmosphere was mainly carbon dioxide and nitrogen, with less methane and ammonia than Miller used.

   (a)

   Which condition on the early Earth were the electrical sparks intended to simulate?

   [1 mark]

   • AUltraviolet radiation from the Sun
   • BHeat from volcanoes
   • CLightning
   • DImpacts from asteroids

   (b)

   Why was oxygen excluded from the apparatus?

   [1 mark]

   • AOxygen would make the amino acids formed toxic to living cells
   • BThe early atmosphere contained more oxygen than today, so it had to be added separately later
   • COxygen would have stopped the water from boiling
   • DThe early atmosphere lacked free oxygen, and oxygen would oxidise and break down the carbon compounds formed

   (c)

   Evaluate the Miller–Urey experiment as evidence for the pre-biotic formation of carbon compounds.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student is asked to decide whether three things are living or non-living. Sample 1 is yeast cells in a sugar solution: they take up sugar, release carbon dioxide, grow and divide. Sample 2 is influenza virus particles stored in a freezer: they increase in number only when added to a culture of animal cells. Sample 3 is a copper sulfate crystal suspended in a saturated copper sulfate solution: it slowly grows larger as ions from the solution are added to its surface.

   (a)

   Why are cells, such as the yeast in Sample 1, described as the smallest units of self-sustaining life?

   [1 mark]

   • AThey are the smallest structures that contain nucleic acid
   • BThey are the smallest structures that can carry out all the functions of life independently
   • CThey are the smallest structures that can increase in size
   • DThey are the smallest structures made from carbon compounds

   (b)

   Which observation best shows that the growing crystal in Sample 3 is non-living?

   [1 mark]

   • AIt has no metabolism: ions are added from the solution without any enzyme-catalysed reactions or energy release
   • BIt increases in size over time
   • CIt forms in a solution containing water
   • DIt is too small to be seen without a microscope

   (c)

   Explain why the influenza virus in Sample 2 is considered to be non-living.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Researchers added a fatty acid (decanoic acid) to water at pH 7.5. Above a critical concentration, the molecules spontaneously formed spherical vesicles, each bounded by a bilayer. Adding particles of montmorillonite, a clay mineral common on the early Earth, increased the rate of vesicle formation about 100-fold, and RNA molecules bound to the clay particles became trapped inside the vesicles. Vesicles containing RNA took up fatty acids from neighbouring empty vesicles and grew, while the empty vesicles shrank. In separate experiments, montmorillonite clay catalysed the joining of activated RNA nucleotides into chains of up to about 50 nucleotides.

   (a)

   Explain the significance of the vesicle results for hypotheses about the origin of the first cells.

   [3 marks]

   (b)

   Evaluate the hypothesis that RNA was the first genetic material.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Earth formed about 4.54 billion years ago. Tube- and filament-shaped structures made of iron oxide, similar to those made by modern iron-oxidising bacteria, have been found in rocks from Quebec, Canada, that formed from seafloor hydrothermal vent precipitates between 3.77 and 4.28 billion years ago. Comparison of the genomes of thousands of bacteria and archaea identified 355 gene families, shared across both groups, that are likely to have been present in the last universal common ancestor (LUCA). Many of these genes code for proteins that work without oxygen, that are stable at high temperatures, that use hydrogen gas and carbon dioxide, and that contain iron–sulfur clusters. Molecular clock estimates based on differences in highly conserved sequences place LUCA at around 4.2 billion years ago.

   (a)

   Explain the evidence that all living organisms share a last universal common ancestor, and that it evolved in the vicinity of hydrothermal vents.

   [6 marks]

   (b)

   Discuss the approaches used to estimate the dates of the first living cells and of LUCA, and why hypotheses about the origin of life are difficult to test.

   [6 marks]

   Total for question 4: 12 marks

End of questions