A2.1 Origins of cells (HL). Practice questions with markscheme.

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

• 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 Earth's orbit has not changed significantly; B is wrong because the early atmosphere was rich in carbon dioxide and methane; C describes a process that does not occur;

• a fire shows some features associated with life: it uses a source of energy (fuel and oxygen) / it 'grows' / it 'reproduces' by spreading, OWTTE;
• however, it is not made of cells / has no membrane-bound compartment separating it from its surroundings;
• it has no genetic material / does not pass information (DNA/RNA) to new fires / new fires are not copies made from a template;
• it does not carry out metabolism controlled by enzymes / homeostasis / does not respond to stimuli in a regulated way;
• the smallest unit of self-sustaining life is a cell, so a fire is a chemical process and non-living;

For full marks, at least one point for and one point against. Do not accept 'a fire is not an organism' without a reason.

• broth was boiled/sterilized, killing any cells already present;
• the curved (swan) neck admitted air but trapped dust/microbes (in the bend);
• no growth occurred while the neck was intact, but growth followed when the neck was snapped/broth was tipped into the bend;
• (so) growth requires entry of pre-existing cells, not air/broth alone / cells only come from pre-existing cells, OWTTE;

• (a) [1]
  • 3.5 (billion years);

  Accept any value from 3.4 to 3.6 (billion years) read from the timeline.

• (b) [2]
  • 3.5 billion years ago / no later than the oldest fossils;
  • (because) fossil cells already existed by then, so their common ancestor must be at least as old, OWTTE;

  Accept any value from 3.4 to 3.6 billion years read from the timeline. Accept 3.7 billion years with the biological carbon signature as the reason.

• (c) [1]
  • there is no evidence of liquid water before the oldest zircons / cells require liquid water, OWTTE;

  Accept: the Earth was too hot / molten.

• (d) [3]
  • life existed by 3.5 billion years ago (the oldest fossils), and possibly earlier (3.7 billion years from the carbon signature / up to 4.2 billion years from the estimates for LUCA);
  • the Earth formed 4.6 billion years ago, so life has existed for at least 3.5 / 4.6 = about 76 % / about three quarters of the Earth's history;
  • (so) life arose early, within about 1.1 billion years of the Earth forming, and has been evolving for an immense length of time, OWTTE;

  Accept 74 % to 78 %, or ECF from the value given in (a). Accept about 80 % if 3.7 billion years (the carbon signature) is taken as the start of life.

• (a) [2]
  • vesicles form only in a limited pH range / few or none at pH 6 and pH 10;
  • formation is greatest at (around) pH 8–9 / rises to a peak near pH 8 then falls, OWTTE;
• (b) [2 max]
  • each molecule has a hydrophilic (head) and hydrophobic (tail) region;
  • tails associate away from water / heads face the water, so bilayers form spontaneously (self-assembly);
  • bilayer sheets close into spheres/vesicles (minimizing exposed edges), OWTTE;
• (c) [2]
  • provides compartmentalization: an internal chemistry separated from the surroundings;
  • (so) replicating molecules and their products stay together / concentrations can be maintained (a protocell), OWTTE;
• (d) [1]
  • conditions (pH/temperature/concentrations) of early oceans are uncertain / lab concentrations may be unrealistically high / other molecules present may interfere, OWTTE;