Theme A: Unity and diversity

Link structure to function at every level, from base pairing to cell organelles. In classification and evolution questions, ask what evidence each claim rests on.

A lesson for this theme

Multiple choice, Paper 1A style

Question 1

Which statement explains why water molecules are attracted to one another?

Question 2

In a tall tree, evaporation from the leaves pulls water up the xylem as a continuous column under tension. Which property of water stops the column from breaking?

Question 3

Glucose and sodium chloride travel dissolved in blood plasma, but cholesterol travels inside lipoprotein particles. What explains the difference?

Question 4

The graph shows the temperature of the air and of the water in a pond over one day. Which statement about the data and its explanation is correct?

Question 5 · HL

Which pair of factors best explains why Earth has kept liquid water at its surface for billions of years?

Question 6 · HL

Table of four planets. Planet W: mean surface temperature −120 °C, thin atmosphere, water detected as ice. Planet X: 14 °C, dense atmosphere, water vapour in the atmosphere. Planet Y: 430 °C, dense atmosphere, water vapour in the atmosphere. Planet Z: 20 °C, no atmosphere, ice in permanently shaded craters.
Planet	Mean surface temperature / °C	Atmosphere	Water detected
W	−120	thin	ice
X	14	dense	vapour in atmosphere
Y	430	dense	vapour in atmosphere
Z	20	none	ice in shaded craters

Astronomers searching for life as we know it compare four planets, shown in the table. Which planet is the best candidate?

Question 7

Which three components make up one nucleotide of DNA?

Question 8

The Venn diagram compares the components of DNA and RNA. Which component belongs in region Y?

Question 9

In a sample of double-stranded DNA, 22% of the bases are thymine. What percentage of the bases are guanine?

Question 10

Which sequence is complementary to the DNA base sequence AGCTTA, pairing base by base?

Question 11

How many different base sequences are possible for a single strand of DNA six nucleotides long?

Question 12

A jellyfish gene for green fluorescent protein is inserted into a bacterium, and the bacterium makes the same protein. Which feature of genetic material explains this?

Question 13 · HL

How are adjacent nucleotides linked within one strand of DNA?

Question 14 · HL

Table of radioactivity after blending and centrifuging. Phages labelled with ³⁵S: 20% in the pellet of bacteria, 80% in the liquid. Phages labelled with ³²P: 70% in the pellet, 30% in the liquid.
Label in phages	In pellet (bacteria) / %	In liquid / %
³⁵S	20	80
³²P	70	30

In the Hershey-Chase experiment, phages labelled with radioactive sulfur or phosphorus infected bacteria. The mixture was blended to knock off phage coats and centrifuged, so the bacteria formed a pellet. The table shows typical results. Which conclusion do they support?

Question 15 · HL

Table of base composition in percent. Sample P: A 30.4, T 29.6, G 19.8, C 20.2. Sample Q: A 24.8, T 25.2, G 25.1, C 24.9. Sample R: A 24.0, T 31.2, G 23.3, C 21.5. Sample S: A 17.2, T 16.8, G 33.1, C 32.9.
Sample	A / %	T / %	G / %	C / %
P	30.4	29.6	19.8	20.2
Q	24.8	25.2	25.1	24.9
R	24.0	31.2	23.3	21.5
S	17.2	16.8	33.1	32.9

The table shows the base composition of DNA from four sources. Which sample is most likely from a virus with single-stranded DNA?

Question 16 · HL

Summary table. Starting materials: methane, ammonia, hydrogen and water vapour, with no oxygen. Energy source: electric sparks. Duration: about one week. Products detected: several amino acids, including glycine and alanine.
Feature	Miller-Urey experiment
Starting materials	methane, ammonia, hydrogen, water vapour
Oxygen	absent
Energy source	electric sparks
Duration	about one week
Products detected	amino acids, including glycine and alanine

The table summarises the Miller-Urey experiment. What did it show?

Question 17 · HL

Why is RNA, rather than DNA or protein, thought to have been the first genetic material?

Question 18 · HL

What is the main evidence that all living organisms descend from a single last universal common ancestor (LUCA)?

Question 19

In an electron micrograph, a mitochondrion is 36 mm long at a magnification of ×12 000. What is its actual length?

Question 20

The diagram shows a bacterial cell. Which labelled structure is the site of protein synthesis?

Question 21

Table comparing four cells. Cell I: cellulose wall, chloroplasts present, nucleus present, stores starch. Cell II: chitin wall, chloroplasts absent, nucleus present, stores glycogen. Cell III: peptidoglycan wall, chloroplasts absent, nucleus absent, no stored carbohydrate seen. Cell IV: no wall, chloroplasts absent, nucleus present, stores glycogen.
Cell	Cell wall	Chloroplasts	Nucleus	Carbohydrate store
I	cellulose	present	present	starch
II	chitin	absent	present	glycogen
III	peptidoglycan	absent	absent	none seen
IV	none	absent	present	glycogen

The table compares four cells. Which description fits cell II?

Question 22

The diagram shows a cell from a leaf. A cell from the root of the same plant is examined. Which labelled structure would the root cell most likely lack?

Question 23

Which pair of mature cells both lack a nucleus?

Question 24

A skeletal muscle fibre is a single unit of cytoplasm containing hundreds of nuclei. How does a fibre come to have so many nuclei?

Question 25

A researcher wants to see where a particular protein moves inside a living cell over an hour. Which technique is most suitable?

Question 26 · HL

Which observation supports the hypothesis that mitochondria evolved from bacteria taken in by an ancestral cell?

Question 27 · HL

A nerve cell and a muscle cell from the same person have very different structures. What best explains this?

Question 28 · HL

Which feature is shared by all viruses?

Question 29 · HL

Phage lambda infects a bacterium and enters the lysogenic cycle. What happens to the phage DNA?

Question 30 · HL

Why is a new influenza vaccine produced most years?

Question 31

Which is the correct way to write the scientific name of the grey wolf?

Question 32

Table of five species. Homo sapiens: 46 chromosomes, genome 3.1 × 10⁹ base pairs. Pan troglodytes: 48, 3.1 × 10⁹. Canis familiaris: 78, 2.4 × 10⁹. Oryza sativa: 24, 0.43 × 10⁹. Paris japonica: 40, 149 × 10⁹.
Species	Diploid chromosome number	Genome size / 10⁹ base pairs
Homo sapiens	46	3.1
Pan troglodytes	48	3.1
Canis familiaris	78	2.4
Oryza sativa	24	0.43
Paris japonica	40	149

The table shows the diploid chromosome number and the genome size of five species. Which conclusion do the data support?

Question 33

Summary of a karyogram. Autosomes 1 to 20: two copies each. Chromosome 21: three copies. Chromosome 22: two copies. X chromosomes: two. Y chromosomes: none. Total: 47 chromosomes.
Chromosomes	Copies
1 to 20	2 of each
21	3
22	2
X	2
Y	0
Total	47

The table summarises a karyogram prepared from a foetal cell. What does it show?

Question 34

Two populations of frogs look alike. Where their ranges meet they mate, but all the hybrid offspring are infertile. How does the biological species concept classify them?

Question 35 · HL

Why is the biological species concept difficult to apply to bacteria?

Question 36 · HL

Which description defines a clade?

Question 37 · HL

Table of base differences in a gene. A and B: 12. A and C: 30. B and C: 30.
Species compared	Base differences
A and B	12
A and C	30
B and C	30

The table shows the number of base differences in a gene between three species. Differences build up between two lineages at 2 per million years. Estimate how long ago the lineages leading to species A and C diverged, in millions of years.

Question 38 · HL

Many species once placed in the figwort family were moved to other families after their DNA was sequenced. What was the reason?

Question 39

The flipper of a whale, the wing of a bat and the arm of a human contain the same set of bones, differing in shape and size. What does this provide evidence for?

Question 40

Plant breeders sowed only the seeds of the maize plants with the highest kernel oil content each generation. The graph shows the mean oil content. What was the mean increase in oil content per generation over the 100 generations, in percentage points?

Question 41 · HL

A river changes course and divides a population of flightless beetles into two. After many generations, beetles from the two sides no longer interbreed when brought together. Which term describes this?

Question 42 · HL

Table of chromosome numbers in body cells. Parent species, diploid: 14. New tetraploid plant: 28.
Plant	Chromosomes in a body cell
Parent species (diploid)	14
Tetraploid	28

A tetraploid plant arises from a diploid species when meiosis fails, as shown in the table. The tetraploid is crossed with a diploid plant of the parent species. What are the offspring?

Question 43

A seed bank stores seeds collected from 200 different wild populations of one species of wheat relative. Which level of biodiversity does this mainly conserve?

Question 44

Which is an example of in situ conservation?

Question 45

The Lord Howe Island stick insect disappeared from its island soon after black rats came ashore from a shipwreck in 1918. Which cause of biodiversity loss does this illustrate?

Question 46

Volunteers counted a farmland bird species in the same survey squares every ten years. The bar chart shows the mean count per square. What was the percentage decrease from 1990 to 2020?

Question 47

Table comparing two surveys. Survey 1: 5 000 volunteers, each counts wherever and whenever they choose, no training, records any butterflies seen. Survey 2: 800 volunteers, each walks the same fixed route weekly in set weather conditions, all trained, records all butterflies within 5 m of the route.
Feature	Survey 1	Survey 2
Volunteers	5 000	800
Where and when	any place, any time	fixed route, weekly, set weather
Training	none	all trained
What is recorded	any butterflies seen	all within 5 m of the route

Two citizen-science surveys estimate trends in butterfly numbers, as summarised in the table. Which statement about the evidence is best supported?

Structured questions, Paper 1B and Paper 2 style

Capillary rise in glass tubesPaper 1B style · SL · 6 marks

Students dipped clean glass tubes of four internal diameters into water at 20 °C and measured how far the water rose above the outside level. Each tube was tested three times. The table shows the results.

Table of capillary rise. Internal diameter 0.5 mm: trials 58, 60, 59 mm, mean 59.0 mm. 1.0 mm: 30, 29, 31, mean 30.0. 1.5 mm: 20, 19, 21, mean 20.0. 2.0 mm: 15, 14, 16, mean 15.0.
Internal diameter / mm	Trial 1 / mm	Trial 2 / mm	Trial 3 / mm	Mean height / mm
0.5	58	60	59	59.0
1.0	30	29	31	30.0
1.5	20	19	21	20.0
2.0	15	14	16	15.0

(a) Calculate the percentage decrease in mean height from the 0.5 mm tube to the 2.0 mm tube, in %. [2]

(b) Identify the independent variable. [1]

(c) For every tube, mean height × diameter is close to 30 mm². Assuming this holds, predict the mean height for a tube of diameter 0.75 mm, in mm. [2]

(d) Which explanation of the trend is best? [1]

Bees on field marginsPaper 1B style · SL · 6 marks

Volunteers walked a 100 m route along ten field margins on calm, sunny days in July: five margins sown with wildflowers and five of grass only. They counted bees seen within 2 m of the route. The table gives the counts on the wildflower margins, the standard deviation for each type and the mean for the grass margins.

Table of bee counts per 100 m route. Wildflower margins: 14, 22, 17, 24, 15; mean not given, standard deviation 4.4. Grass margins: individual counts not given, mean 6.2, standard deviation 2.3.
Margin type	Counts on each margin	Mean	Standard deviation
Wildflower	14, 22, 17, 24, 15	to calculate	4.4
Grass only	not given	6.2	2.3

(a) Calculate the mean count for the five wildflower margins, in bees per 100 m. [2]

bees per 100 m

(b) The wildflower mean is 18.4 bees per 100 m. Calculate how many times more bees were counted on wildflower margins than on grass margins, using the means. [2]

(d) Which was a controlled variable? [1]

An electron micrograph of a pancreas cellPaper 2 style · SL · 6 marks

An electron micrograph of a cell from the pancreas has a scale bar labelled 2 µm that is 25 mm long. On the micrograph, the nucleus is 140 mm across.

(a) Calculate the magnification of the micrograph. [2]

(b) The magnification is ×12 500. Calculate the actual diameter of the nucleus, in µm. [2]

µm

(d) Why was an electron microscope used rather than a light microscope? [1]

Base composition of wheat DNAPaper 2 style · HL · 6 marks

Double-stranded DNA was extracted from wheat. In this DNA, 27% of the bases are adenine.

(a) Calculate the percentage of bases that are guanine, in %. [2]

(b) One DNA molecule from this sample is 1.2 × 10⁶ base pairs long. Guanine makes up 23% of its bases. Calculate the number of guanine bases in the molecule. [2]

(d) Chargaff's finding that A = T and G = C in DNA from many species refuted which earlier idea? [1]

Calibrating a molecular clockPaper 2 style · HL · 6 marks

The table shows the number of base differences in a 1 200-base gene between four species of mammal. Fossils show that the lineage leading to species S split from the lineage leading to P, Q and R 33 million years ago.

Table of base differences in a 1 200-base gene. P and Q: 18. P and R: 42. Q and R: 42. P and S: 66. Q and S: 66. R and S: 66.
Species compared	Base differences
P and Q	18
P and R	42
Q and R	42
P and S	66
Q and S	66
R and S	66

(a) Calculate the rate at which base differences between S and P built up, counting the changes on both lineages together, in differences per million years. [2]

differences per million years

(b) Using a rate of 2.0 differences per million years, estimate how long ago the lineages leading to P and Q diverged, in million years. [2]

million years

(d) Which assumption does this method rely on? [1]

A new species by polyploidyPaper 2 style · HL · 6 marks

Plant species A has a diploid number of 18 and species B a diploid number of 14. Pollen from B fertilises an ovule of A and a hybrid grows, but it sets no seed. Years later, a shoot on the hybrid doubles its chromosome number and produces a fertile plant.

(a) Calculate the number of chromosomes in a body cell of the hybrid. [2]

(b) Why is the hybrid sterile? [1]

(d) Why is the fertile plant regarded as a new species? [1]

How many phages does one cell release?Paper 2 style · HL · 4 marks

A culture of bacteria was infected with a phage, and free phages were removed after 2 minutes. Samples were then taken to count infective units: each infected cell counts as one unit until it bursts, and each free phage counts as one unit. The graph shows the results.

(a) Read from the graph the time at which the number of infective units starts to rise, in min. [1]

min

(b) Calculate the mean number of phages released by one infected cell. [2]