IGCSE Biology Paper 4 (Theory / Extended)

Topic 1: Characteristics and Classification of Living Organisms -- Challenge Exam 2

Duration1 hour 15 minutes

Total Marks80

Questions7

Time Remaining75:00

Syllabus0610

Instructions

Answer all questions in the spaces provided.
Show all working for calculations.
Use appropriate scientific terminology.
Your answers will be automatically graded when you submit.

Question Navigation

This paper covers the whole of Topic 1. Like a real Cambridge paper, the seven questions range across every sub-topic — characteristics of living organisms, classification and dichotomous keys, and the features of the five kingdoms — and they are deliberately mixed rather than grouped. All three Topic 1 papers do; they differ in the angle they come at it from, not in what they cover.

Question 1 -- The Resurrection Plant

Total: 12 marks

The resurrection plant of southern Africa can lose almost all the water from its tissues, curl into a brown ball and remain in that state for years. Within a day of rainfall it uncurls and turns green. A researcher seals equal masses of dried plants and of watered plants in flasks fitted with carbon dioxide sensors, in the dark, at 25 °C. Her results are shown.

condition of plant	carbon dioxide produced in 24 hours / arbitrary units per gram
dried, brown	0.4
watered for 3 hours	11.6
watered for 24 hours	38.2
dried plants that had first been boiled and cooled	0.0

(a) [3]

Using the results, explain what is happening in the dried, brown plants.

Model Answer -- 1(a)

the dried plants are still respiring, since carbon dioxide is being produced [1]

respiration is the chemical reactions in cells that break down nutrient molecules and release energy for metabolism [1]

the rate is very low (0.4 compared with 38.2 units), so the plants are dormant rather than dead [1]

⚠ If you missed marks here: The comparison of numbers is what earns the third mark: quote 0.4 against 38.2 rather than writing “a bit of carbon dioxide”. The commonest error is to treat a small value as zero and conclude that the dried plants are dead, which the boiled control in the last row directly contradicts.

1 mark -- dried plants are respiring, shown by carbon dioxide production 1 mark -- respiration defined as reactions in cells releasing energy from nutrient molecules 1 mark -- low rate compared with watered plants, so dormant rather than dead

(b) [3]

Explain the purpose of the last row of results, and state what conclusion can be drawn from it.

Model Answer -- 1(b)

the boiled plants act as a control [1]

boiling kills the plant cells, so no living cells remain to respire [1]

because they produce no carbon dioxide at all, the 0.4 units from the dried plants must come from living cells and not from non-living chemical change in the dried material [1]

⚠ If you missed marks here: A control is only worth marks if you say what it eliminates. Here it removes the possibility that the small carbon dioxide reading is produced by chemical breakdown of dead plant material, which is exactly the objection a sceptical reader would raise.

1 mark -- identified as a control 1 mark -- boiling kills the cells so none can respire 1 mark -- the zero reading shows the 0.4 units come from living cells

The researcher now wishes to find out whether the dried plants grow while they are dormant. Describe how she should do this, and explain why measuring the mass of the same plants before and after would not be valid.

Model Answer -- 1(c)

harvest samples of dried plants at the start and again after several months [1]

dry each sample in an oven and weigh repeatedly until the mass is constant, then compare the mean dry mass per plant [1]

growth requires a permanent increase in size and dry mass, so only dry mass is valid evidence [1]

measuring the same plants is invalid because their mass changes enormously as they take up and lose water, and because drying to constant mass destroys the sample [1]

⚠ If you missed marks here: Two ideas are being tested at once: the correct measurement (dry mass, to constant mass) and the reason the obvious alternative fails. With a plant that can lose almost all its water, fresh mass is worse than useless — it could halve overnight without any change in the material the plant is made of.

1 mark -- take samples at the start and after an interval 1 mark -- dry in an oven to constant mass and compare mean dry mass 1 mark -- quotes the definition of growth including dry mass 1 mark -- explains that fresh mass varies with water content and that drying is destructive

(d) [2]

Name one substance excreted by these plants when they are fully watered and in the dark, and state the process that produces it.

Model Answer -- 1(d)

carbon dioxide [1]

produced by respiration in the cells, so it is a waste product of metabolism and its removal is excretion [1]

⚠ If you missed marks here: The words “in the dark” are there for a reason: in the light the plant would also excrete oxygen from photosynthesis, so an answer of oxygen would be wrong under these conditions. Water vapour is not accepted, because that water was absorbed rather than produced by the plant.

1 mark -- carbon dioxide named 1 mark -- produced by respiration, identified as a waste product of metabolism

Question 2 -- Two Populations of Butterfly

Total: 12 marks

Two populations of a swallowtail butterfly live on either side of a mountain range in the Western Ghats. Population A has broad yellow bands on its wings; population B has narrow white bands. They are currently recorded as papilio Polytes and Papilio Demoleus in an old field guide.

In captivity, males of A mate with females of B and produce healthy adult butterflies, but those butterflies lay eggs that never hatch.

pair compared	percentage of DNA bases identical in a 900-base region
A and B	94
A and C	76
A and D	98
A and E	61

Species C, D and E are three other butterflies from the same region.

(a) [3]

Decide whether populations A and B belong to the same species. Justify your answer using the breeding evidence.

Model Answer -- 2(a)

they belong to different species [1]

because a species is a group of organisms that can reproduce to produce fertile offspring [1]

the hybrid butterflies lay eggs that never hatch, so the offspring are sterile, not fertile [1]

⚠ If you missed marks here: The high DNA similarity (94%) is deliberately placed in the same question to tempt you into answering “same species”. Similarity of DNA shows relatedness; only the fertility of the offspring decides the species question. Lions and tigers make the same point.

1 mark -- different species stated 1 mark -- definition quoted including the word fertile 1 mark -- hybrid offspring identified as sterile from the evidence given

(b) [2]

The two names in the field guide are both written incorrectly. Rewrite them correctly and state the rule you have applied.

Model Answer -- 2(b)

Papilio polytes and Papilio demoleus [1]

the genus takes a capital letter and the species does not, and both words are underlined when handwritten or italicised in print [1]

⚠ If you missed marks here: One mark for the corrected names and one for the rule — giving the names without the rule caps you at one. Both errors in the question are the standard ones: a lower case genus and a capitalised species.

1 mark -- both names rewritten correctly with capitals and underlining 1 mark -- rule stated: capital genus, lower case species, underlined or italic

Place C, D and E in order of relatedness to A, most closely related first, and explain what the percentages indicate.

Model Answer -- 2(c)

order: D (98%), C (76%), E (61%) [1]

the table shows the percentage of bases that are identical, so a higher value means a closer relationship [1]

differences in base sequences accumulate over long periods of time [1]

so a high percentage of identical bases indicates that the two species separated from a common ancestor recently [1]

⚠ If you missed marks here: This table is a similarity column, so high is close — the opposite of a table counting differences. Read the heading before ranking. The reasoning marks need the ideas of accumulation over time and a common ancestor; “they are more alike” simply restates the data.

1 mark -- correct order D, C, E 1 mark -- recognises the column shows similarity so higher is closer 1 mark -- states that differences accumulate over time 1 mark -- links high similarity to a recent common ancestor

(d) [3]

Species C and species E look almost identical, with the same wing pattern and the same size, yet the DNA data place them far apart. Suggest an explanation, and explain what this shows about the use of appearance in classification.

Model Answer -- 2(d)

the two species may live in the same habitat, where the same wing pattern is advantageous — for example as camouflage or as a warning to predators [1]

so similar appearances can arise in species that are not closely related [1]

appearance is therefore unreliable evidence of relatedness, and DNA base sequences are used instead because they reflect evolutionary relationships [1]

⚠ If you missed marks here: The reasoning here is identical to the dolphin-and-shark case and to the cactus-and-euphorbia case, and examiners set it in as many disguises as they can find. A full answer names a reason why the same appearance is advantageous, states that unrelated species can therefore converge, and then draws the conclusion about evidence.

1 mark -- suggests a reason why the same appearance is advantageous in both species 1 mark -- states that unrelated species can come to look alike 1 mark -- concludes that DNA is better evidence because classification reflects evolutionary relationships

Question 3 -- A Key for Five Leaves

Total: 12 marks

Five leaves are collected from a school garden.

leaf	description
1	single blade; veins running side by side; edge smooth
2	single blade; veins branching into a network; edge smooth
3	single blade; veins branching into a network; edge toothed
4	divided into five separate leaflets; veins branching into a network; leaflet edges toothed
5	divided into five separate leaflets; veins branching into a network; leaflet edges smooth

A gardener has written this key for the same leaves:
“1a leaf big → go to 2; 1b leaf small → go to 3. 2a leaf from a tree → leaf 4; 2b leaf from a shrub → leaf 5. 3a leaf pretty → leaf 1; 3b leaf plain → go to 4. 4a leaf toothed → leaf 3; 4b leaf not toothed → leaf 2.”

(a) [4]

Construct a dichotomous key that would identify all five leaves.

Model Answer -- 3(a)

step 1 divides the leaves into those with a single blade and those divided into separate leaflets, for example: 1a leaf a single blade → go to 2; 1b leaf divided into separate leaflets → go to 4 [1]

the single-blade group is separated by vein pattern, for example: 2a veins running side by side → leaf 1; 2b veins branching into a network → go to 3 [1]

a further step separates leaves 2 and 3, for example: 3a edge smooth → leaf 2; 3b edge toothed → leaf 3 [1]

the divided group is separated by leaflet edge, for example: 4a leaflet edges smooth → leaf 5; 4b leaflet edges toothed → leaf 4; every step offers two visible choices and every leaf reaches one endpoint only [1]

⚠ If you missed marks here: Choosing the first step well matters: single blade against divided gives a three-and-two split, whereas starting with the leaf edge produces an awkward split and a longer key. The final mark is a quality mark, earned only if every leaf can be traced to exactly one endpoint using features shown in the descriptions.

1 mark -- first step separates single blades from divided leaves 1 mark -- single-blade group separated by vein pattern 1 mark -- leaves 2 and 3 separated by leaf edge 1 mark -- divided leaves separated by leaflet edge, key fully dichotomous and complete

(b) [3]

Identify three faults in the gardener’s key, giving a reason in each case.

Model Answer -- 3(b)

step 1 uses “big” and “small”, a vague comparison with no measurement, so different users would disagree [1]

step 2 asks whether the leaf came from a tree or a shrub, which is not visible in the leaf itself [1]

step 3 uses “pretty” and “plain”, which are personal opinions rather than structural features [1]

⚠ If you missed marks here: Each mark requires the fault and the reason; naming the step alone earns nothing. Notice that the gardener’s key would in fact separate the five leaves if the user already knew the answers — which is precisely why a key must be judged by whether a stranger could use it, not by whether it reaches the right names.

1 mark -- step 1 fault: vague comparison with no measurement 1 mark -- step 2 fault: needs information not visible in the specimen 1 mark -- step 3 fault: subjective judgement rather than a structural feature

Rewrite step 1 of the gardener’s key so that it would work in the field, and explain why your version is an improvement.

Model Answer -- 3(c)

a suitable rewrite, for example: 1a leaf more than three times as long as it is wide → go to 2; 1b leaf less than three times as long as it is wide → go to 3 [1]

the two choices are exact opposites, so every leaf fits one and only one branch [1]

the comparison is between two parts of the same leaf, so it gives the same answer whatever the size of the leaf or the scale of a photograph [1]

⚠ If you missed marks here: Comparing one dimension of a specimen with another dimension of the same specimen is the professional way of avoiding “large” and “small”. An absolute measurement in centimetres would gain the first mark but not the third, because it fails on photographs and on young leaves.

1 mark -- rewritten step uses a measurable or comparative structural feature 1 mark -- two branches are genuine opposites so every leaf fits exactly one 1 mark -- comparison within the same specimen works at any scale

(d) [2]

Explain why the first step of a key should divide the specimens into two groups of similar size wherever possible.

Model Answer -- 3(d)

each step of a key halves the number of possibilities, so an even split makes the identification as quick as possible [1]

a very uneven split leaves most specimens still to be separated, so more steps are needed and there are more opportunities for error [1]

⚠ If you missed marks here: This part asks about efficiency rather than validity: an uneven key is not wrong, only slower. The marks are for the halving idea and its consequence, so an answer that simply says “it is neater” will not score.

1 mark -- each step halves the possibilities so an even split is quickest 1 mark -- an uneven split needs more steps and creates more chances of error

Question 4 -- Organisms of a Hot Spring

Total: 12 marks

Water and mud from a hot spring in Ladakh are examined. Five things are described.

	description
F	single-celled; no nucleus; cell wall present; photosynthesises; DNA in a single circular loop
G	single-celled; nucleus present; no cell wall; moves using a long whip-like thread; engulfs smaller organisms
H	multicellular; branching threads; cell wall present, not cellulose; no chloroplasts; grows on a dead insect at the water’s edge
I	multicellular; roots, stems and leaves; cell walls of cellulose; chloroplasts present; produces spores on the underside of its leaves
J	not a cell; protein coat around genetic material; multiplies only inside F

(a) [4]

Place each of F, G, H and I in the correct kingdom.

Model Answer -- 4(a)

F — prokaryote [1]

G — protoctist [1]

H — fungus [1]

I — plant [1]

⚠ If you missed marks here: F is the difficult one: it photosynthesises, which pulls candidates towards the plant or protoctist kingdoms, but it has no nucleus and that single feature settles the matter. Always ask the nucleus question first and let it override everything else.

1 mark -- F prokaryote 1 mark -- G protoctist 1 mark -- H fungus 1 mark -- I plant

(b) [3]

Explain why H is not classified in the same kingdom as I, using features from the descriptions.

Model Answer -- 4(b)

H has no chloroplasts, so it cannot photosynthesise, whereas I has chloroplasts [1]

the cell wall of H is not made of cellulose, whereas the wall of I is cellulose [1]

H obtains nutrients from the dead insect by saprophytic nutrition, releasing enzymes and absorbing the products [1]

⚠ If you missed marks here: Both organisms are multicellular with cell walls and neither moves, so those features cannot be used. The three that work are chloroplasts, the composition of the wall, and the method of nutrition — and each must be stated for H, ideally contrasted with I.

1 mark -- no chloroplasts in H, chloroplasts in I 1 mark -- cell wall of H not cellulose, wall of I is cellulose 1 mark -- H feeds saprophytically on dead material

State three features of F that are typical of its kingdom.

Model Answer -- 4(c)

no nucleus — the DNA is a single circular loop free in the cytoplasm [1]

single-celled [1]

a cell wall that is not made of cellulose (plasmids and the absence of mitochondria are also accepted) [1]

⚠ If you missed marks here: The nucleus mark is the essential one, and it must be phrased as the DNA being loose in the cytoplasm rather than simply “the DNA is different”. Answering “it photosynthesises” earns nothing here, because that is not typical of the kingdom as a whole.

1 mark -- no nucleus, DNA in a circular loop in the cytoplasm 1 mark -- single-celled 1 mark -- cell wall present but not made of cellulose, or plasmids present

(d) [2]

A researcher suggests treating the spring water with an antibiotic to remove J. Explain why this would not work.

Model Answer -- 4(d)

J is a virus, which is not a cell and has no cell wall, cytoplasm or metabolism of its own [1]

antibiotics act on structures and processes found in bacterial cells, so there is nothing in a virus for the antibiotic to act on [1]

⚠ If you missed marks here: The answer must contrast what a bacterium has with what a virus lacks. “Antibiotics only work on bacteria” restates the question without explaining anything and is not credited. Note the extra subtlety here: the antibiotic might well kill F, the virus’s host, but that is a different mechanism.

1 mark -- J identified as a virus, not a cell, with no cell structures or metabolism 1 mark -- antibiotics act on bacterial cell structures which a virus does not possess

Question 5 -- Three Unusual Vertebrates

Total: 10 marks

Three vertebrates are described.

	description
P	covered in coarse hair and spines; lays leathery-shelled eggs; the young that hatch feed on milk released onto the mother’s fur; body temperature constant
Q	covered in feathers; wings too small for flight; swims underwater to catch fish; lays hard-shelled eggs that are incubated; body temperature constant
R	skin moist and without scales; young hatch in water with feathery external gills; adult uses lungs and moist skin; body temperature varies with the surroundings

(a) [3]

State the vertebrate group to which each of P, Q and R belongs.

Model Answer -- 5(a)

P — mammal [1]

Q — bird [1]

R — amphibian [1]

⚠ If you missed marks here: Each animal is described in a way designed to mislead: P lays eggs, Q cannot fly and lives largely in water, and R changes its method of gas exchange during its life. In every case one feature is decisive — hair and milk, feathers, and moist scale-free skin with aquatic gilled young.

1 mark -- P mammal 1 mark -- Q bird 1 mark -- R amphibian

(b) [4]

A student argues that P cannot be a mammal because it lays eggs. Evaluate this argument, using features from the description.

Model Answer -- 5(b)

the argument is incorrect [1]

P has hair, which is found only in mammals [1]

P feeds its young on milk, produced by mammary glands, which is also unique to mammals [1]

laying eggs is not a feature that excludes mammals, because fish, amphibians, reptiles and birds all lay eggs and a small number of mammals do too [1]

⚠ If you missed marks here: The final mark is the one that separates candidates: it requires you to say why the student’s evidence is worthless rather than simply to give better evidence. Egg-laying is shared by four of the five vertebrate groups, so it can never be used to exclude an animal from any group.

1 mark -- argument rejected 1 mark -- hair given as a feature unique to mammals 1 mark -- milk from mammary glands given 1 mark -- explains that egg-laying is shared by other groups so does not exclude a mammal

Explain why “lives in water” and “keeps a constant body temperature” are both poor features to use when identifying which group a vertebrate belongs to.

Model Answer -- 5(c)

living in water is a habitat, not a structural feature, and vertebrates from every group live in water — fish, amphibians, aquatic reptiles, penguins and whales [1]

a constant body temperature is shared by both birds and mammals, so it cannot separate those two groups [1]

a good feature must be visible in the specimen and possessed by one group only, such as feathers or hair and mammary glands [1]

⚠ If you missed marks here: Two of the three marks are for naming specific examples that break each feature, so a general statement such as “it is not specific enough” will not do. The third mark asks you to state the standard a good feature has to meet, which is the principle behind every key you will ever construct.

1 mark -- living in water is a habitat shared by members of every group, with an example 1 mark -- constant body temperature is shared by birds and mammals 1 mark -- states that a good feature is visible and unique to one group, with an example

Question 6 -- Classifying Animals from a Cave

Total: 12 marks

Four animals are collected from a cave system in Meghalaya.

animal	legs	body	antennae	other
K	4 pairs	two parts	none	produces silk; eyes reduced
L	3 pairs	three parts	1 pair, very long	no wings; pale, no pigment
M	7 pairs	two regions	2 pairs	chalky exoskeleton; lives in cave pools
N	1 pair on each of 40 similar segments	head plus many similar segments	1 pair	flattened body

(a) [4]

Name the arthropod group of K, L, M and N.

Model Answer -- 6(a)

K — arachnid [1]

L — insect [1]

M — crustacean [1]

N — myriapod [1]

⚠ If you missed marks here: Work down the legs column first, then use the antennae for the two animals with many legs. The distractors built into this table are the wingless insect (many candidates believe every insect has wings) and the aquatic crustacean living in a cave, which looks nothing like a crab.

1 mark -- K arachnid 1 mark -- L insect 1 mark -- M crustacean 1 mark -- N myriapod

(b) [3]

Explain why the number of pairs of legs alone cannot be used to distinguish M from N, and state which feature in the table does distinguish them.

Model Answer -- 6(b)

both M and N have more than four pairs of legs, so the leg count places them in the same half of any key [1]

the number of legs also varies within these groups, so no fixed number could be quoted [1]

the distinguishing feature is the number of pairs of antennae: two pairs in M and one pair in N [1]

⚠ If you missed marks here: This is the point at which a leg-based key runs out of power, and knowing it is the difference between a key that works and one that stalls. Answers claiming that N simply has “more legs” are refused, because a crustacean such as a crab can have a similar number to a short myriapod.

1 mark -- both have more than four pairs of legs so the count does not separate them 1 mark -- leg number varies within these groups so no fixed number can be used 1 mark -- antennae number given as the distinguishing feature

Animal L is pale and has no pigment, and its eyes are very small. A student concludes that L should therefore be placed in a new group of its own. Evaluate this conclusion.

Model Answer -- 6(c)

the conclusion is incorrect: L has three pairs of legs, three body parts and one pair of antennae, so it is an insect [1]

classification is based on structural features, and features such as pale colour and reduced eyes are found in cave animals of many different groups [1]

similar conditions produce similar features in unrelated organisms, so those features are not evidence of a separate group [1]

⚠ If you missed marks here: Every animal in this table lives in a cave and several are pale with reduced eyes, which is the clue the question is offering you. Features that arise from a shared environment rather than shared ancestry are exactly the features modern classification tries to see past.

1 mark -- conclusion rejected, L placed in the insects using its structural features 1 mark -- states that pale colour and reduced eyes occur in cave animals of many groups 1 mark -- explains that shared conditions produce similar features in unrelated organisms

(d) [2]

Give two features that all four of these animals share and that place them in the arthropods.

Model Answer -- 6(d)

an exoskeleton [1]

a segmented body with jointed legs and no backbone [1]

⚠ If you missed marks here: This part deliberately follows the group-level questions to check that you can switch levels. Offering “jointed legs and one pair of antennae” would fail, because antennae number is a group feature and animal K has none at all.

1 mark -- exoskeleton 1 mark -- segmented body with jointed legs and no backbone

Question 7 -- Testing a Response in Woodlice

Total: 10 marks

A student investigates how woodlice respond to humidity. She uses a choice chamber divided into two halves: one half contains a drying agent and the other contains damp cotton wool. She places 20 woodlice in the centre, covers the chamber, and records how many are in each half every two minutes.

time / minutes	0	2	4	6	8	10
number in the damp half	10	13	16	18	18	19
number in the dry half	10	7	4	2	2	1

(a) [3]

Name the characteristic of living organisms being investigated, and use the data to describe the response of the woodlice.

Model Answer -- 7(a)

the characteristic is sensitivity, the ability to detect and respond to changes in the environment [1]

the woodlice move from the dry half to the damp half [1]

the number in the damp half rises from 10 to 19 out of 20 within ten minutes, with most of the change occurring in the first six minutes [1]

⚠ If you missed marks here: The third mark is for using figures from the table rather than describing the shape of the change in words. Note also that the movement itself demonstrates a second characteristic, movement, but the question asks what is being investigated, which is the response.

1 mark -- sensitivity named with its definition 1 mark -- describes movement from the dry half to the damp half 1 mark -- quotes figures from the table to support the description

(b) [4]

Identify two variables the student should have kept the same, and describe two further improvements that would make her conclusion more reliable.

Model Answer -- 7(b)

keep the temperature the same in both halves of the chamber [1]

keep the light intensity the same in both halves, for example by covering the whole chamber, since woodlice also respond to light [1]

repeat the investigation with further groups of woodlice and calculate a mean [1]

use a larger number of woodlice, or leave the woodlice to settle before starting timing, so that the starting distribution is not itself a response to being handled [1]

⚠ If you missed marks here: Light is the variable most often forgotten, and it matters because woodlice respond to it strongly — if one half were darker, the result could be caused by light rather than by humidity. For the improvement marks, “repeat it” alone is too vague; say what would be repeated and that a mean would be calculated.

1 mark -- temperature kept the same in both halves 1 mark -- light intensity kept the same in both halves, with a reason 1 mark -- repeat with further groups and calculate a mean 1 mark -- use more woodlice or allow a settling period before timing

The student concludes: “Woodlice prefer damp conditions because damp places contain more food.” Evaluate this conclusion.

Model Answer -- 7(c)

the observation that the woodlice move to the damp half is supported by the data [1]

the explanation about food is not supported, because no food was present in either half of the chamber and none was measured [1]

to test the food explanation she would need a separate investigation, for example offering food in one half with humidity kept the same in both halves [1]

⚠ If you missed marks here: This is a classic evaluation split: the observation is sound and the explanation is invented. Marks are lost by rejecting the whole conclusion or by accepting the whole conclusion; the skill being tested is separating what the data show from what the student has assumed. Any sensible test of the food idea earns the final mark, provided it changes only the food and controls humidity.

1 mark -- recognises the movement to the damp half is supported by the data 1 mark -- explains that the food explanation is unsupported because no food was present or measured 1 mark -- suggests a suitable further investigation to test the food explanation

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