A2.1 Origins of cells. Practice questions with markscheme.

• B: a boundary maintains concentrations, keeps replicators with their products, and creates internal conditions different from outside, a requirement for evolution of metabolism;

• C: natural gradients across mineral pores resemble chemiosmosis, with catalysts and reduced chemicals available, conditions suited to early metabolism;

• A: the hypothesis is testable in principle but the evidence is hard to obtain, because pre-biotic conditions are uncertain and protocells left no fossils; B is wrong because historical hypotheses can generate testable predictions; C confuses a present-day observation with an impossibility; D is contradicted by the Miller–Urey experiment;

• B: LUCA = last universal common ancestor: the most recent population from which all current life descends; not necessarily the first cell;

• B: today's oxidizing atmosphere destroys accumulating organic molecules / existing life would metabolize them; conditions no longer allow the slow accumulation needed;

• A: ribozymes are catalytic RNA molecules; their existence supports the hypothesis that RNA once carried out both information storage and catalysis;

• D: ozone forms from free oxygen, which was absent until photosynthesis released it, so nothing absorbed ultraviolet before it reached the surface; A is wrong because the early Sun was fainter, not brighter; B is wrong because the early atmosphere was rich in carbon dioxide and methane; C describes a process that does not occur;

• Earth's age (~4.5 billion years) is known from radiometric dating (of rocks/meteorites);
• the oldest (accepted) fossils/stromatolites (~3.5 billion years) set a minimum age for life;
• (chemical traces/isotope signatures in older rocks push estimates earlier, with uncertainty), OWTTE;

• a closed, spherical boundary drawn as a bilayer of fatty acids/amphipathic molecules with hydrophilic heads outwards and hydrophobic tails inwards, labelled;
• annotation that the membrane/compartment keeps the internal chemistry different from the surroundings / concentrates molecules inside;
• RNA (or another self-replicating molecule) shown inside, annotated as genetic material that can be copied / self-replication;
• catalysis shown or annotated: RNA (ribozyme) or another catalyst speeding reactions inside the compartment;
• annotation that small molecules (nutrients/nucleotides) enter across the bilayer, OWTTE;
• annotation of self-assembly: the membrane forms spontaneously / grows by adding fatty acids, and division/budding produces new protocells;

Award annotation marks only if linked to a drawn feature. Do not accept a modern cell drawn with a nucleus or organelles.

• (a) [2]
  • the more strongly reducing the atmosphere, the greater the yield of amino acids;
  • yield falls steeply as conditions become less reducing (100 → 18 → 2), OWTTE;
• (b) [1]
  • a control: to show that amino acids form because of the energy input (spark), not from contamination or the gases alone, OWTTE;
• (c) [2]
  • they support the hypothesis: amino acids can form abiotically given a suitable energy source, and do so most readily under reducing conditions;
  • but the relevance depends on the true composition of the early atmosphere, which is uncertain; if it was only weakly reducing, yields would have been low (and other sources such as vents/meteorites may have been more important), OWTTE;
• (d) [2]
  • repeat each mixture many times / run replicates and compare mean yields (to assess reliability);
  • or: standardize other variables (temperature, spark energy, duration) and/or test the atmosphere thought most realistic, OWTTE;