Previous IB Exam Essay Questions: Unit 10

Use these model essay questions and responses to prepare for essay questions on your in-class tests, as well as the IB Examination, Paper 3. The questions below have appeared on IB HL Examinations over the past several years. The answers following the questions are the markscheme ideal responses used to evaluate student examination responses.

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1. Outline the experiments of Miller and Urey into the origin of organic compounds on Earth. 6 marks

• apparatus contructed in which chemicals could circulate
• gases of pre-biotic Earth`s atmosphere mixed inside
• methane (as a carbon source)
• ammonia water vapor and hydrogen
• sparks/ electric discharge (to simulate lightning)
• high temperature/ boiling water
• no free oxygen
• amino acids spontaneously formed
• example of other organic compound formed

2. Outline the methods used by Miller and Urey in their experiments into the origin of organic compounds on Earth. 2 marks

• conditions on pre-biotic Earth were recreated in their apparatus
• mixture of any three of methane, ammonia, water vapor, and hydrogen
• water boiled and recondensed to simulate high temperature
• electrical discharge / sparks to simulate lightning

3. Outline the conclusion that Miller and Urey drew from their experiments. 2 marks

• organic compounds / amino acids were formed from inorganic compounds
• organic compounds could have existed on pre-biotic Earth
• life might have arisen from non-living material

4. Discuss the possible role of RNA in the origin of life on Earth. 5 marks

• short polymers of RNA have been produced abiotically
• from dilute solution of RNA nucleotide monomers in test tube experiments
• thus, RNA is capable of self-assembly
• when short RNA polytmers are added to a test-tube containing RNA nucleotide monomers
• copies are made from the template following base-pairing rules
• thus, RNA is capable of self-replication
• if zinc is added, RNA polymers up to 40 nucleotides are copied with less than 1% error
• RNA molecules are important catalysts in modern cells
• ribozymes remove introns from RNA
• ribozymes help catalyze the synthesis of new RNA
• thus, RNA is autocatalytic

5. Answer the following questions concerning endosymbiotic theory. 5 marks

a. Identify which one of the five kingdoms of living organisms was the first to appear on Earth. 1 mark

• Prokaryotes / Monera / Bacteria

b. State one organelle, apart from chloroplasts, which is part of the endosymbiotic theory. 1 mark

• mitochondrion

c. State three kingdoms in which this organelle is found. 1 mark

• any three of: fungi, protista, animals, plants

d. State two characteristics of mitochondria which support the idea that they were once independent prokaryotes. 2 marks

• mitochondria have their own DNA and ribosomes / conduct their own protein synthesis
• mitochondrial DNA is circular
• mitochondrial DNA lacks histones
• mitochondrial DNA lacks introns
• mitochondria replicate independently of the host cell nucleus
• mitochondrial size is similar to prokaryotes
• mitochondrial ribosomes similar to prokaryoties in size / antibiotic sensitivity

6. Discuss the definition of the term species. 8 marks

• a species is a group of organisms
• a species shares a common gene pool
• showing similar morphology/characteristics
• capable of interbreeding
• and producing fertile offspring
• but dissimilar organisms sometimes interbreed
• mule formed by crossing horse and donkey/other example of interspecific hybridisation
• interspecific hybrids are sometimes fertile
• sometimes organisms that are very similar will not interbreed
• Drosophila pseudoobscura and D. persimilis/other example of sibling species
• reference to the problem of defining fossil species
• reference to the problem of species that only reproduce asexually
• reference to the problem of isolated populations gradually diverging

7. Discuss how isolation can lead to speciation. 7 marks

(Award 1 mark for each answer below up to 6 marks; Award 1 mark for some mention/'weighing up' of arguments/likelihood of isolation leading to speciation. 7 marks maximum.)

• for speciation to occur populations of a species must beome isolated
• isolation can be geographical
• isolation of populations living together is less likely
• isolation may be ecological
• e.g. by different flowering times/pollinators/temporal/behavioral/mechanical
• new traits by chance mutations
• natural selection/selection pressures will alter population by favoring certain traits
• speciation can occur by divergent evolution
• e.g. by occypying different microhabitats/other example
• isolation prevents interbreeding
• gene pools become separated
• isolated populations can diverge by adapting to different conditions
• over a long period of time populations may diverge considerably
• with sufficient divergence, populations can no longer interbreed
• speciation has occurred when populations cannot interbreed

8. Discuss how different types of isolation contribute to speciation. 7 marks

• any type of isolation can contribute if it prevents interbreeding/prevents gene flow
• geographical isolation is (probably) the commonest method
• physical barrier/long distance prevents interbreeding
• e.g. islands/mountain ranges/other barrier
• ecological isolation is harder to achieve
• e.g. populations occupy different habitats in the same area
• e.g. flower at different times/have different mating season
• other example of ecological isolation
• natural selection must act differently on the two populations
• populations must diverge sufficiently enough for interbreeding to be impossible
• behavioral isolation
• structural isolation
• a small gene pool so any mutations show more quickly

9. Discuss the theory that evolution occurs by punctuated equilibrium. 3 marks

• long periods where there was no (apparent) change/stasis
• short periods of rapid evolution
• periods of mass extinctions leading to opportunities/caused by environmental disruption/rapid environmental change in short periods
• supported by lack of fossils showing gradual changes
• an example of such environmental disruption (meteors, earthquakes, volcanoes, etc.)
• alternative theory is gradualism
• punctuated equilibrium is based on fossil evidence rather than biochemical evidence

10. Compare evolution by punctuated equilibrium with evolution by gradualism. 3 marks

• punctuated equilibrium involves faster mutation rates
• punctuated equlibrium involves more powerful natural selection
• punctuated equilibrium implies that the environment undergoes sudden changes
• punctuated equibrium involves discontinuous evolution
• gradualism implies continuous evolution
• punctuated equilibrium involves faster evolution rates (when it occurs)

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