IB Biology SL Theme A: Unity and diversity questions

Exam-style multiple choice and written questions with full markschemes. Check multiple choice answers straight away, or type, photograph or draw a written answer and get it marked.

Type

Multiple choice Written

Difficulty

Easy Medium Hard

Question 1

Paper 2Medium16 marks

Figure 1 represents a water molecule. The positions of the electron pairs are shown.

A class investigated respiration in yeast cells with different water contents. The cultures were supplied with equal glucose and oxygen and kept at 25 °C. Table 1 gives the mean rate of carbon dioxide production.

Water content of yeast cells / %	Mean rate of carbon dioxide production / μmol g⁻¹ dry yeast min⁻¹
20	0.4
35	1.2
50	2.4
65	3.6
80	4.0

(a)

In Figure 1, identify the type of bond represented by each solid line and state what is shared in this bond.

[2]

(b)

Using Figure 1, explain how water molecules can form hydrogen bonds with one another.

[3]

(c)

Draw two water molecules, showing the partial charges and one hydrogen bond between them. Use solid lines for covalent bonds and a dotted line for a hydrogen bond.

[3]

(d)

At 65% water content, calculate the amount of carbon dioxide produced in 30 min by 0.50 g of dry yeast. Assume that the rate remains constant.

[2]

(e)

Describe the relationship between water content and the rate of carbon dioxide production shown in Table 1.

[2]

(f)

Evaluate the conclusion that these results demonstrate that water is a medium for life and that the first cells originated in water. Use at least one value from Table 1 and your biological knowledge.

[4]

Question 2

Multiple choiceHard1 mark

In a plant, aqueous phloem sap carries substance P to cells, where a cytoplasmic enzyme uses P as a substrate. Molecule L is found in the hydrophobic interior of the cell-surface membrane. Which explanation links water’s solvent properties to these roles?

Show answer and explanation

Answer: A. Hydrophilic substances dissolve in water, supporting transport and availability in aqueous environments; hydrophobic substances are insoluble and can associate with a membrane’s hydrophobic interior. P dissolves in aqueous phloem sap and is present in aqueous cytoplasm, where enzymes catalyse reactions. L’s insolubility is consistent with its membrane location.

B: Treats dissolution as preventing enzyme–substrate interaction; enzymes catalyse reactions in aqueous solution. C: Reverses the effect of solubility on transport: dissolution enables P to be carried in water-based phloem sap. D: Reverses the hydrophilic and hydrophobic roles; hydrophobic P would not dissolve in aqueous sap, and hydrophilic L is not suited to the membrane interior.

Remember: water-soluble molecules can be transported in aqueous fluids and participate in metabolism in aqueous solution.

Question 3

Paper 2Medium16 marks

A short region of a gene is transcribed in a cell. The DNA template strand is shown in the direction in which it is read. Spaces separate groups of three bases.

DNA template: 3′–TAC GGA CTT ACG–5′

(a)

Compare DNA and RNA with respect to their number of strands, nitrogenous bases and pentose sugar. Include the structural difference between the sugars.

[5]

(b)

Determine the mRNA sequence produced, writing it in the 5′ to 3′ direction.

[4]

(c)

During translation, the second codon of the mRNA is read at a ribosome. For parts (i) and (ii), use the codon 5′–CCU–3′, even if your answer to part (b) differs.

(i)

Write the tRNA anticodon that pairs with this codon, in the 3′ to 5′ direction.

[1]

(ii)

Explain how this pairing contributes to the formation of a polypeptide.

[2]

(d)

Before a cell divides, its DNA is copied. Explain how complementary base pairing enables this copying.

[4]

Question 4

Paper 2Medium16 marks

Figure 1 presents diploid chromosome numbers for six organisms. Humans and chimpanzees are the named examples in the IB Biology guide. Plant P, Plant Q, Animal R and Animal S are hypothetical species; no prior knowledge of their chromosome numbers is required. For each of these four hypothetical species, the plotted count was obtained from one diploid cell.

(a)

Calculate the difference between the diploid chromosome numbers of chimpanzees and humans.

[2]

(b)

State the highest and lowest diploid chromosome numbers shown in Figure 1, and calculate the range of the values.

[3]

(c)

Explain why diploid cells have an even number of chromosomes.

[2]

(d)

Plant Q has a diploid chromosome number of 28. Determine the number of chromosomes in a gamete produced by meiosis and in the zygote formed when two such gametes fuse. Explain how the chromosome number changes between these stages.

[3]

(e)

A student proposes that an animal species always has a higher diploid chromosome number than a plant species. Evaluate this proposal using the data for Plant P, Plant Q, Animal R and Animal S in Figure 1.

[4]

(f)

Suggest an improvement to the sampling method that would make the chromosome number recorded for each hypothetical species more reliable.

[2]

Question 5

Multiple choiceHard1 mark

In a beetle population, 20% of adults have dark shells before the substrate darkens. After the change, 70% of the same adults have dark shells. Adults of both shades contribute equally to an offspring cohort, of which 20% have dark shells when raised on the original substrate. Which interpretation applies the definition of evolution to shell shade?

Show answer and explanation

Answer: C. Evolution is a change in the heritable characteristics of a population, not a change acquired by individuals during their lives. The proportion of dark-shelled adults rises from 20% to 70%, but their offspring, raised on the original substrate, have the initial 20% frequency. Thus, no inherited shift in shell shade is demonstrated. A: treats a change in adult appearance as an evolutionary change in heritable population characteristics. B: applies Lamarckian inheritance, but the offspring do not retain the increase in dark shells. D: confuses some offspring having a characteristic with the frequency of that characteristic changing across generations. Remember: evolution requires a change in heritable population characteristics across generations.

Question 6

Multiple choiceMedium1 mark

In one case study, mixed dipterocarp forest in Southeast Asia is cleared for oil-palm plantations. In another, a local wetland is drained for housing as population growth increases demand for land. Which option identifies a direct cause in the forest case and an indirect human driver in the wetland case?

Show answer and explanation

Answer: D. A direct cause is the immediate land-use change; an indirect driver is an underlying human pressure that leads to that change. Clearing the mixed dipterocarp forest for plantations is direct. Population growth raises demand for land, indirectly driving wetland loss through drainage for housing. A: identifies the indirect demand for palm oil rather than the direct forest clearance. B: gives the immediate drainage action for the wetland, not its indirect driver. C: gives an indirect forest pressure and the direct wetland action, reversing both requested categories. Remember: distinguish the land conversion itself from the human pressure that motivates it.

Question 7

Paper 2Hard16 marks

Two case studies of ecosystem loss were investigated: mixed dipterocarp forest in Indonesian Borneo, Southeast Asia, and the wetlands of the Florida Everglades, USA. Field teams mapped the areas of these ecosystems in selected landscape regions in 1990 and 2020. The loss categories in Figure 1 are non-overlapping estimates of areas no longer mapped as the named ecosystem by 2020. All areas are rounded to the nearest 100 km².

In Borneo, roads were opened for timber extraction. Market demand for timber and palm oil encouraged extraction and plantation development. Some areas were burned to clear land.

In the Everglades, canals were built to drain land and alter water supply. Farming and urban areas expanded, increasing demand for land and water.

(a)

Calculate the percentage of the 1990 ecosystem area that had been lost by 2020 in each case study.

[2]

(b)

Compare the largest category of ecosystem loss in each case study, using Figure 1.

[2]

(c)

Explain a causal chain linking an indirect human pressure to the loss of mixed dipterocarp forest in Indonesian Borneo. Use information from the stem and Figure 1.

[3]

(d)

Explain how drainage or canalization can cause loss of Everglades wetland even where the land is not mapped as farmland or urban land.

[2]

(e)

An adviser claims that preventing direct conversion of the remaining ecosystems to plantations, farms, or urban and transport land would be sufficient to prevent further ecosystem loss in both regions. Evaluate this claim using Figure 1 and the information in the stem.

[3]

(f)

Suggest one action for each case study and explain how each action could address a cause of ecosystem loss that would not necessarily be stopped by drawing a boundary around the remaining ecosystem.

[4]

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