Biology HL

1. 1.

   A hypothetical sealed apparatus circulates water vapour and a gas mixture through an electrical discharge. Organic molecules appear after several days. A matched apparatus without the discharge produces far fewer. Explain the role of the comparison and evaluate the claim that the experiment has created living cells and proved the exact origin of life on Earth.

   [4 marks] · no calculator

   Worked explanation

   1. The paired apparatus isolates the electrical energy input, provided gases, temperature, time and contamination control are matched.
   2. Distinguish building blocks from a self-sustaining cell capable of heredity and evolution.
   3. A laboratory demonstration tests a possible mechanism; it cannot establish that this precise sequence happened on early Earth.

   Common mistake: Finding amino acids is not equivalent to producing a living cell or proving one uniquely correct historical pathway.

   Marking points

   • The no-discharge comparison tests whether the energy input increases organic synthesis under the stated conditions.
   • Organic molecules are not cells with coordinated metabolism, heredity and compartments.
   • The result supports the possibility of abiotic synthesis under some conditions, not creation of life.
   • Uncertainty about actual early-Earth conditions and alternative pathways prevents proof of a unique historical origin.

   Examiner tip: Finding amino acids is not equivalent to producing a living cell or proving one uniquely correct historical pathway.

2. 2.

   Fatty acids in water can assemble into membrane-bounded vesicles. Explain why this is a plausible step towards the origin of cells, and give two additional capabilities needed before a vesicle population could undergo sustained Darwinian evolution.

   [4 marks] · no calculator

   Worked explanation

   1. Use spontaneous assembly to explain a boundary without assuming a modern protein-making system already exists.
   2. Compartmentalization can retain useful molecules but is not itself heredity.
   3. Replication with variation plus differential persistence and reproduction is needed for selection to improve a population over generations.

   Common mistake: A spherical membrane alone does not establish metabolism, genetic inheritance or natural selection.

   Marking points

   • Amphipathic fatty acids can self-assemble with hydrophobic regions shielded from water.
   • A boundary permits internal conditions and concentrations to differ from the surroundings.
   • A heritable information system must be copied with occasional variation.
   • Differences in inherited properties must affect survival/reproduction, supported by energy and material use; vesicles need not already resemble modern cells.

   Examiner tip: A spherical membrane alone does not establish metabolism, genetic inheritance or natural selection.

3. 3.

   Explain why RNA is a candidate for an early hereditary molecule. Then explain how widespread use of the same genetic code supports common ancestry, without implying that the last universal common ancestor was necessarily the first life ever to arise.

   [4 marks] · no calculator

   Worked explanation

   1. An RNA-first hypothesis reduces the initial need for separate DNA storage and protein catalysts, without claiming modern RNA copies itself unaided.
   2. Shared complex molecular conventions are consistent with descent from a common ancestral population.
   3. Distinguish the ancestry of extant organisms from every origin or lineage that may once have existed.

   Common mistake: LUCA means the last universal common ancestor of surviving lineages, not a demonstrated first cell.

   Marking points

   • RNA base sequences can store information and template complementary sequences.
   • Some RNA molecules catalyse reactions, potentially combining informational and catalytic roles.
   • A widely shared genetic code and conserved genes support inheritance from a common ancestor.
   • Earlier lineages may have existed and become extinct; the last shared ancestor of surviving lineages need not be the first organism.

   Examiner tip: LUCA means the last universal common ancestor of surviving lineages, not a demonstrated first cell.

4. 4.

   In a hypothetical experiment, fatty acids form vesicles in water. A fluorescent solute is initially trapped inside some vesicles but gradually appears outside. Explain how this result supports compartment formation but does not demonstrate a fully living cell. Suggest a control for leakage from the vesicles.

   [4 marks] · no calculator

   Worked explanation

   1. A boundary can concentrate reactants even if some small molecules cross it; perfect impermeability is not required for a compartment.
   2. The observed fluorescence concerns containment and permeability, not proof of a self-maintaining organism.
   3. Remove unencapsulated solute before the trial and use a matched background/free-solute comparison so an outside signal is not automatically attributed to leakage.

   Common mistake: A spherical lipid boundary is not by itself evidence of a living cell, and external fluorescence may include solute never trapped initially.

   Marking points

   • The vesicle boundary separates an internal solution from its surroundings.
   • Solute escape indicates that the boundary is not completely impermeable under these conditions.
   • Compartment formation alone does not demonstrate coordinated metabolism, replication and heredity.
   • Compare matched vesicles without the solute or an appropriate free-solute control, with equal time and detection conditions, to check background or unencapsulated solute.

   Examiner tip: A spherical lipid boundary is not by itself evidence of a living cell, and external fluorescence may include solute never trapped initially.

5. 5.

   A hypothetical RNA molecule catalyses a reaction in a laboratory but cannot copy itself under the tested conditions. Explain why catalytic RNA is relevant to an RNA-world hypothesis and why this result alone cannot establish how the first self-replicating system arose.

   [3 marks] · no calculator

   Worked explanation

   1. Identify the two distinct requirements: retaining information and accelerating useful chemistry.
   2. The experiment tests a catalytic role but does not demonstrate a complete replication cycle.
   3. Historical plausibility also requires compatible environmental conditions and a source of components; a laboratory capability is supporting evidence, not proof of history.

   Common mistake: Do not describe every RNA molecule as self-replicating or treat one catalytic reaction as proof that life began by a single known route.

   Marking points

   • RNA can carry sequence information and some RNA molecules can catalyse reactions.
   • These properties make RNA a candidate for combining informational and catalytic roles in an early system.
   • Catalysis of one reaction without demonstrated copying does not establish self-replication, plausible prebiotic supply or the actual historical pathway.

   Examiner tip: Do not describe every RNA molecule as self-replicating or treat one catalytic reaction as proof that life began by a single known route.

Original, unofficial practice questions from SubjectScout — not official past-paper questions or complete syllabus coverage. Added English teaching explanations are AI-authored; independent teacher review is pending.