IB Biology A2.1: Origins of cells worked solutions

Free molecular oxygen was scarce. Consequently, there was little ozone screening to absorb incoming UV, allowing greater UV penetration. Award one mark for oxygen/ozone scarcity and one for the linked radiation consequence. Greenhouse warming is a separate process.

The condenser removes heat so water vapour condenses. This returns liquid to the circulating system. A heater supplies energy to vaporise water; electrodes provide the discharge; an analytical detector measures products. Award the mark for C.

Amino-acid production establishes a chemical product. Sustained self-maintenance and hereditary reproduction would address properties of an organised living system. A stronger discharge, more product or a different chromatographic spot would remain chemical observations. Award the mark for B.

An RNA nucleotide sequence can carry information that is copied, providing a route to inheritance. Some RNA sequences catalyse reactions, providing a route to organised chemistry without requiring a pre-existing protein enzyme system. Award one mark for each function; structural traits alone do not answer the question.

Shared homologous genes are consistent with inheritance from a common ancestral lineage of surviving cellular organisms. Award one mark for that inference. Award one for a valid limit, such as inability to exclude extinct independent origins or to identify the first cell. Modern genomes need not be identical.

Heads face the aqueous solution on both the inner and outer surfaces, earning one mark. Tails face one another within the bilayer, away from water, earning one mark. A drawing with tails filling the entire lumen would describe the wrong structure.

Information copying or inheritance is absent, earning one mark. For a heritable advantageous variant to spread through differential reproduction, its information must be passed to descendants, earning the second. Growth of the boundary alone does not establish such inheritance.

Name a plausible resource, such as chemical energy, mineral surfaces or a chemical gradient, for one mark. Name a different evidential observation, such as credible coeval biological microstructures in ancient vent deposits, for one mark. A warm modern vent alone does not identify an ancestral habitat.

(a) The filter is the intended difference between X and Y. It blocks entry of cells while allowing air exchange. Award one mark for the matched comparison and one for distinguishing cells from air. (b) Z demonstrates that the filtered system and medium can support growth when viable cells are introduced; award one mark. Growth is consistent with entry of existing microbes, with no observed spontaneous cellular origin in X; award one mark. Do not extend the conclusion to all prebiotic environments.

(a) At 20 degrees C, K exceeds J (10 versus 6 mg). At 40 degrees C, both yield 14 mg. At 60 degrees C, J exceeds K (19 versus 12 mg). Award one mark per paired comparison, including the reversal. (b) Without replicate numbers and variation, the means do not establish statistical significance; award one mark. These are supplied synthetic measurements, not living-cell counts.

(a) Ammonia is the missing classic starting gas; award one mark. Electrical discharges model lightning, earning one mark. Cooling condenses vapour and permits liquid circulation/collection, earning one mark. (b) The products support abiotic chemical synthesis, but no self-maintaining, reproducing cellular system was shown; award one mark.

(a) The result shows catalytic activity by RNA, providing a way to promote chemistry without a protein enzyme. Award one mark for catalysis and one for its relevance to a pre-protein system. (b) RNA would also need to carry and transmit sequence information; award one mark. The experiment establishes one possible function under its conditions, without observing ancient organisms or their genomes; award one mark.

(a) Related versions are homologous, consistent with inheritance from an ancestral cellular lineage; award two marks for connecting the shared pattern to inherited common origin. (b) Changes at important functional sites may reduce successful reproduction, so selection tends to preserve them; award one mark. LUCA is the last ancestor shared by surviving cellular life and can have earlier ancestors; award one mark.

(a) A closed fatty-acid bilayer separates an aqueous lumen from the exterior, earning one mark. Retention keeps RNA local instead of letting it disperse, earning one mark. (b) Incoming precursors can react near the retained RNA so concentrations and reaction products differ from the exterior, earning one mark. A missing coupled function could be hereditary copying, sustained energy coupling or reproduction of the organised system; award one mark for an identified function.

For the first chain, state scarcity of free molecular oxygen and hence ozone protection (one mark), leading to greater UV penetration (one mark). For the second, state greater abundance of carbon dioxide and methane (one mark), linked through retention of outgoing infrared radiation to warmer conditions (one mark). UV and heat can supply conditions relevant to chemical reactions; do not confuse their mechanisms.

(a) The youngest permitted age is 3.72 minus 0.06 = 3.66 billion years; award one mark. Life must be at least as old as a genuine coeval biological trace, but may have originated earlier than its oldest preserved record; award two marks for the bound and the preservation limit. (b) The genomic result reconstructs ancestral timing using rates and calibrations, rather than dating this organism directly; award one mark.

(a) Before heating, M exceeds N by 24 minus 8 = 16 mg. After heating, N exceeds M by 12 minus 10 = 2 mg. Award two marks for the paired differences and one for the change in ranking. (b) The data show treatment-dependent chemical yields, not a maintained hereditary system. Copying and transmission of information have not been tested. Award one mark for each linked limitation. (c) Repeat matched M/N treatments with independent replicates and report variability, while holding starting quantities, heating exposure and measurement methods constant; award one mark for a relevant controlled improvement.

(a) The membrane retains catalysts and copied RNA, while admitting reactants. Local reaction products can therefore remain associated with the compartment instead of dispersing. Award two marks for retention plus its chemical consequence. (b) S may make fewer functional copies despite its faster total copying, and rapid use of limited nutrients may leave fewer resources for maintenance. Award two marks for two relevant explanations; the information does not rank the variants conclusively. (c) Compare numbers of viable descendant compartments over generations and test whether they inherit the RNA variant. Award one mark for differential reproduction and one for inheritance of the difference.

(a) Modern gradients show a plausible resource, ancient traces potentially show occupancy, and ancestral proteins are an inference about past biology from descendants. Award one mark for each distinction. (b) Demonstrate that the traces are biological rather than abiotic mineral forms and that they are original to the ancient deposit rather than later contamination. Award one mark per check. (c) The observations support compatibility with an ancient vent habitat but do not identify a unique location or show where the first cell arose; award one mark.

(a) With entry of pre-existing cells prevented, a matched treatment comparison links detectable amino acids to the chemical treatment, supporting abiotic synthesis under these conditions. Award two marks. (b) Growth after inoculation shows the medium and incubation support viable cells and that the detection procedure can register growth, earning two marks. (c) The study has not demonstrated cellular self-maintenance or reproduction without inoculation, and failure in these conditions cannot exclude other ancient chemical pathways. Award two marks for the distinct limits.

(a) Amphipathic molecules organise with heads toward aqueous lumen and exterior, and tails away from water in the membrane interior. Award two marks for the orientation and closed bilayer. (b) Substrate entry supplies reactant while retention keeps useful catalysts/products local, allowing concentrations and reactions to differ from the surroundings. Award two marks. (c) Any two relevant requirements earn one each: sustained energy supply/coupling, replacement of components, hereditary copying or reproduction of the maintained system. Catalysis by itself does not establish life.

(a) RNA stores information in sequence and some RNA catalyses reactions; award two marks for these complementary functions. (b) H/L relative surviving offspring = 3/2 = 1.5. Because the functional difference is inherited and affects successful reproduction, H can increase in frequency, assuming the supplied difference persists. Award one mark for the ratio and one for the evolutionary explanation. (c) No ancient organism, date or universal shared ancestry has been measured, so this illustrates a mechanism of evolution rather than identifying LUCA; award two marks for the distinction.

(a) Repeated layered structures are compatible with community growth, while isotope enrichment can result from biological selection between isotopes. Award one mark per distinct line; non-biological alternatives still need checking. (b) A producer using chemical energy to fix carbon could supply organic matter to consumers without sunlight. Award one mark for chemical-energy-supported production and one for transfer of organic matter/energy to other organisms. (c) The evidence does not identify photosynthetic machinery or a light-energy source, nor establish that the organisms were ancestral to all surviving life. Award one mark per unsupported claim.