Question 1
Paper 1A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanWhich feature of the early Earth allowed high levels of ultraviolet light to reach the surface?
- High concentrations of methane in the atmosphere
- The absence of free oxygen, so no ozone layer could form
- High temperatures caused by the greenhouse effect
- Frequent volcanic eruptions releasing water vapour
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Answer: B [1]
Question 2
Paper 1A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanWhich observation supports the hypothesis that RNA was the first genetic material?
- DNA is chemically more stable than RNA.
- Ribosomal RNA catalyses the formation of peptide bonds.
- RNA contains the base uracil instead of thymine.
- mRNA carries the genetic code from the nucleus to the ribosomes.
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Answer: B [1]
Question 3
Paper 1A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanWhich observation is the strongest evidence that all living organisms share a last universal common ancestor?
- All organisms need liquid water.
- Almost all organisms use the same genetic code.
- Fossil microorganisms occur in rocks 3.5 billion years old.
- Some organisms live near hydrothermal vents.
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Answer: B [1]
Question 4
Paper 1B style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanStudents repeated a spark-discharge experiment based on the Miller–Urey method, using two gas mixtures with and without electric sparks. Each flask was run for 7 days and the liquid was then analysed by chromatography. The data are for practice and are not from a real study. (a) Identify the flask that produced the greatest variety of amino acids. [1] (b) Calculate how many times greater the total yield of amino acids was in flask 1 than in flask 3. [1] (c) State the purpose of flask 2. [1] (d) Many scientists now think the early atmosphere contained mainly CO₂, N₂ and water vapour. Evaluate the extent to which these data support the hypothesis that amino acids formed spontaneously on the early Earth. [3]
| Flask | Gas mixture | Sparks | Number of different amino acids detected | Total yield of amino acids / mg |
|---|---|---|---|---|
| 1 | CH₄, NH₃, H₂, H₂O | yes | 11 | 24.0 |
| 2 | CH₄, NH₃, H₂, H₂O | no | 0 | 0.0 |
| 3 | CO₂, N₂, H₂O | yes | 3 | 0.6 |
| 4 | CO₂, N₂, H₂O | no | 0 | 0.0 |
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- (a) flask 1 / CH₄, NH₃, H₂ and H₂O with sparks [1]
- (b) 24.0 ÷ 0.6 = 40 (times) [1]
- (c) control / shows that the energy from sparks is needed for amino acids to form / shows the amino acids are not contaminants OWTTE [1]
- (d) amino acids formed from inorganic gases when energy was supplied, so spontaneous formation is possible [1]
- (d) but with the mixture closest to the likely early atmosphere (flask 3) only 3 types / 0.6 mg formed / yield 40 times lower [1]
- (d) so the mixture in flask 1 may not represent early Earth and support is limited [1]
- (d) only monomers were tested / no polymers or cells formed [1]
- (d) no repeats shown / one trial per condition, so reliability is unknown [1]
- (d) sparks may not represent other early energy sources such as ultraviolet light OWTTE [1]
- max 3 for (d)
Question 5
Paper 2A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanOutline the reasons that viruses are considered to be non-living. [3]
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- not made of cells / have no cytoplasm / no plasma membrane of their own [1]
- no metabolism of their own / do not respire / produce no ATP [1]
- have few or no enzymes [1]
- cannot reproduce independently / only reproduce inside a host cell [1]
- rely on host ribosomes for protein synthesis [1]
- do not grow / no homeostasis / no excretion [1]
- max 3
Question 6
Paper 2A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanExplain how vesicles could have formed spontaneously on the early Earth and why this was a necessary step in the origin of cells. [3]
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- fatty acids are amphipathic / have a hydrophilic head and a hydrophobic tail [1]
- in water they coalesce into a bilayer with the tails inside, away from water [1]
- the bilayer closes into a sphere / vesicle without enzymes or an energy input [1]
- this forms a membrane-bound compartment [1]
- so the internal chemistry can become different from that outside [1]
- molecules such as RNA are concentrated inside, so they react more often [1]
- max 3
Question 7
Paper 2A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels mean(a) Outline two approaches used to estimate the date of the last universal common ancestor (LUCA) or of the first cells. [2] (b) Describe the evidence that LUCA evolved in the vicinity of hydrothermal vents. [2]
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- (a) molecular clock: number of base differences in conserved genes, assuming a roughly constant rate of mutation [1]
- (a) molecular clock calibrated using dated fossils [1]
- (a) radiometric dating of rocks containing the oldest fossils / microfossils / stromatolites [1]
- (a) carbon isotope ratios: rocks enriched in carbon-12 indicate biological activity [1]
- max 2 for (a)
- (b) fossilised microbe-like filaments / tubes found in ancient sea-floor hydrothermal vent precipitates [1]
- (b) genomic analysis of conserved sequences / genes traced back to LUCA [1]
- (b) these genes suit vent conditions: anaerobic / high temperature / use of H₂ and CO₂ / iron–sulfur proteins [1]
- max 2 for (b)
Question 8
Paper 2B style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanExplain the requirements that had to be met for the first cells to arise from non-living matter, and why hypotheses about the origin of cells are difficult to test. [7]
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- cells today only arise by division of pre-existing cells, so the first cells must have arisen another way [1]
- carbon compounds formed spontaneously on the early Earth [1]
- catalysis was needed to speed up reactions [1]
- early catalysts could have been RNA / ribozymes / mineral surfaces [1]
- self-replication of molecules was needed so that information could be inherited [1]
- RNA can be replicated by complementary base pairing [1]
- RNA could act as both the genetic material and an enzyme [1]
- self-assembly of molecules into larger structures / polymers / membranes [1]
- compartmentalisation: a membrane enclosed the molecules [1]
- fatty acids form vesicles spontaneously [1]
- allowing the internal chemistry to differ from the outside [1]
- scientific hypotheses must be testable [1]
- the exact conditions on pre-biotic Earth cannot be replicated [1]
- the first protocells did not fossilise, so there is no direct evidence [1]
- max 7