IGCSE Biology Paper 4 (Theory / Extended)

Topic 1: Characteristics and Classification of Living Organisms -- Challenge Exam 3

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 -- Surviving the Dry Season

Total: 12 marks

An African lungfish survives the dry season by burrowing into mud and secreting a mucus cocoon around itself. It remains in the cocoon for up to four years, breathing air through a hole in the mud and using no food at all. Researchers measured several quantities in fish that were active in water and in fish that had been in the mud for six months.

	active in water	in the mud for 6 months
oxygen used / cm³ per kg per hour	62	4
mass of urea in the body / g per kg	0.3	7.9
body mass / g	410	288

(a) [3]

Use the data to explain whether the lungfish in the mud is alive.

Model Answer -- 1(a)

the fish is alive: it is still using oxygen, so respiration is continuing [1]

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

the rate is greatly reduced (4 compared with 62 units), so the fish is dormant, and a low rate is not the absence of the characteristic [1]

⚠ If you missed marks here: Quote both figures rather than saying “less oxygen”. The misconception being tested is that a very low rate means no life at all, which is the same error made about dry seeds and hibernating mammals — and the oxygen figure in the table settles it directly.

1 mark -- alive, shown by continued oxygen use indicating respiration 1 mark -- respiration defined as reactions in cells releasing energy 1 mark -- compares the two rates and concludes the fish is dormant, not dead

(b) [3]

The mass of urea in the body rises from 0.3 to 7.9 g per kg during the six months in the mud. Explain what urea is, why it is normally removed, and suggest why it accumulates in the buried fish.

Model Answer -- 1(b)

urea is a waste product of metabolism, produced from the breakdown of excess amino acids or of the fish’s own protein [1]

its removal from the body is excretion, one of the seven characteristics of living organisms [1]

the buried fish is surrounded by mud and has no water flowing over it, so the urea cannot be released into the surroundings and builds up inside the body [1]

⚠ If you missed marks here: Three separate ideas: what urea is, what removing it is called, and why removal has stopped. Candidates who write “urea is waste” without the words of metabolism usually lose the first mark, because that phrase is what distinguishes excretion from egestion.

1 mark -- urea identified as a waste product of metabolism from breakdown of protein or amino acids 1 mark -- removal identified as excretion 1 mark -- explains that the buried fish cannot release urea into the surroundings

Body mass falls from 410 g to 288 g during the six months. A student says this shows the fish has “grown backwards”. Explain why this description is unsatisfactory, and describe how the researchers could find out whether the fish has lost dry mass.

Model Answer -- 1(c)

growth is defined as a permanent increase in size and dry mass, so a loss of mass is simply not growth, and the phrase “grown backwards” has no defined meaning [1]

the fall in body mass may be partly loss of water, which is not a loss of the material the fish is built from [1]

to measure dry mass they would need to dry samples of tissue in an oven until the mass is constant and compare the mean dry mass per gram of tissue [1]

this measurement is destructive, so different individuals must be sampled before and after, all kept under the same conditions [1]

⚠ If you missed marks here: The strongest answers separate two questions: is this growth (no), and has real material been lost (probably yes, since the fish has used its own stores in respiration for six months, but body mass alone cannot show it). The destructive-measurement point is worth a mark in almost every dry-mass question on this syllabus.

1 mark -- growth requires a permanent increase, so loss of mass is not growth 1 mark -- part of the mass lost may be water rather than body material 1 mark -- dry samples of tissue in an oven to constant mass and compare 1 mark -- measurement is destructive so different individuals must be sampled

(d) [2]

State two characteristics of living organisms, other than respiration and excretion, that the buried fish continues to show, giving evidence or a reason for each.

Model Answer -- 1(d)

sensitivity — it detects the return of water and responds by emerging from the cocoon [1]

nutrition is not occurring, but movement continues, since it breathes air through a hole in the mud, which requires movement of part of the organism [1]

⚠ If you missed marks here: The question deliberately excludes the two easy answers, so you must look at the description for evidence rather than reciting the list. Nutrition is the one characteristic genuinely paused here, which is why the stem says the fish uses no food at all — noticing that is worth more than guessing.

1 mark -- sensitivity with evidence, such as responding to the return of water 1 mark -- movement with evidence, such as breathing movements through the hole in the mud

Question 2 -- Fish on a Market Stall

Total: 12 marks

Fillets of fish sold in a market are often difficult to identify because the head, skin and fins have been removed. Inspectors take samples from four stalls and compare a 650-base region of DNA from each sample with a reference sequence from the protected species Thunnus maccoyii.

sample	number of bases differing from the reference	label on the stall
1	3	“yellowfin tuna”
2	188	“southern bluefin tuna”
3	7	“swordfish”
4	402	“mackerel”

(a) [3]

Explain why DNA evidence can identify the species of a fish fillet when its appearance cannot.

Model Answer -- 2(a)

the visible features used in classification — scales, fins, body shape — have been removed, so appearance cannot be used at all [1]

DNA is present in every cell of the fillet and its base sequence is unchanged by cutting or storing the fish [1]

members of the same species have almost identical base sequences, while different species differ measurably, so the sample can be matched against a reference sequence [1]

⚠ If you missed marks here: All three marks are about evidence rather than technique — no knowledge of how DNA is sequenced is required or expected. The third mark needs the comparison with a reference to be spelled out; “you can test the DNA” on its own explains nothing.

1 mark -- visible features have been removed so appearance cannot be used 1 mark -- DNA base sequence present in every cell and unchanged by processing 1 mark -- same species have almost identical sequences, so the sample can be matched to a reference

(b) [4]

Use the data to state which samples are most likely to be the protected species, and explain your reasoning.

Model Answer -- 2(b)

samples 1 and 3, with 3 and 7 differing bases [1]

the table gives the number of bases that differ, so the smallest values indicate the closest match [1]

differences of only 3 and 7 bases in 650 are of the size expected between individuals of the same species [1]

samples 2 and 4, differing at 188 and 402 bases, are clearly other species, even though sample 2 is labelled as the protected one [1]

⚠ If you missed marks here: Two traps operate together. First, the column counts differences, so small is close. Second, the labels contradict the data, and the mark scheme follows the DNA, not the stall. Answers that trust the label for sample 2 lose the final mark even if the ranking is right.

1 mark -- samples 1 and 3 identified 1 mark -- recognises the column shows differences so the smallest values match best 1 mark -- notes that 3 and 7 differences are of the size expected within a species 1 mark -- identifies samples 2 and 4 as other species despite the labelling

Explain what is meant by a species, and explain why two fish with almost identical DNA base sequences could still belong to different species.

Model Answer -- 2(c)

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

DNA similarity shows how closely related two organisms are, but the species definition depends on whether their offspring are fertile [1]

two very closely related species may have very similar sequences and still produce sterile offspring, as lions and tigers do [1]

⚠ If you missed marks here: This part checks that you have not quietly replaced the species definition with “identical DNA”. DNA evidence is used to identify and to rank relatedness; it does not define what a species is. A named example is the cleanest way to earn the third mark.

1 mark -- species defined including the word fertile 1 mark -- distinguishes DNA similarity from the fertility test 1 mark -- gives an example of closely related species with sterile hybrids

(d) [2]

The reference species is written on one document as thunnus Maccoyii. Rewrite it correctly and state which part is the genus.

Model Answer -- 2(d)

Thunnus maccoyii, with a capital T, a lower case m, and both words underlined or in italics [1]

Thunnus is the genus [1]

⚠ If you missed marks here: Both errors here are the standard pair: a lower case genus and a capitalised species. Note that the second mark is for identifying the genus, so a corrected name alone caps the answer at one mark.

1 mark -- name rewritten correctly with capital genus, lower case species, underlined 1 mark -- Thunnus identified as the genus

Question 3 -- Freshwater Invertebrates

Total: 12 marks

A student surveying a stream in the Nilgiris records five invertebrates.

animal	description
V	body of many similar segments; no legs; no shell; no antennae
W	single coiled shell; muscular foot; two tentacles on the head
X	exoskeleton; three pairs of legs; three body parts; one pair of antennae; three tail filaments
Y	exoskeleton; five pairs of legs; two pairs of antennae; hard chalky covering
Z	exoskeleton; four pairs of legs; two body parts; no antennae

She is given this key to use:
“1a exoskeleton and jointed legs present → go to 2; 1b exoskeleton and jointed legs absent → go to 4. 2a three pairs of legs → X; 2b more than three pairs of legs → go to 3. 3a two pairs of antennae → Y; 3b no antennae → Z. 4a shell present → W; 4b shell absent → V.”

(a) [4]

Write down the route through the key that identifies animal Y, and the route that identifies animal V. For each animal, state the feature that decided the final step.

Model Answer -- 3(a)

Y: 1a, 2b, 3a [1]

the final step for Y was decided by the presence of two pairs of antennae [1]

V: 1b, 4b [1]

the final step for V was decided by the absence of a shell [1]

⚠ If you missed marks here: Writing the route down as you go is what makes an error recoverable, and questions like this one award marks for the route itself. The commonest slip is to take the first branch of each numbered step without reading it, which sends Y to Z and V to W.

1 mark -- route for Y given as 1a, 2b, 3a 1 mark -- two pairs of antennae given as the deciding feature for Y 1 mark -- route for V given as 1b, 4b 1 mark -- absence of a shell given as the deciding feature for V

(b) [4]

Construct a different dichotomous key for the same five animals, starting with a feature other than the presence of an exoskeleton.

Model Answer -- 3(b)

a valid first step using a different feature, for example: 1a antennae present → go to 2; 1b antennae absent → go to 3 [1]

the animals with antennae separated correctly, for example: 2a one pair of antennae → X; 2b two pairs of antennae → Y [1]

the animals without antennae separated, for example: 3a jointed legs present → Z; 3b jointed legs absent → go to 4 [1]

the final pair separated, for example: 4a shell present → W; 4b shell absent, body of many similar segments → V; all steps dichotomous and using visible features [1]

⚠ If you missed marks here: There is more than one correct key, and examiners mark by tracing yours. The instruction to avoid the exoskeleton feature is there to test whether you can find another even split rather than reproduce the key you were just given — read that kind of instruction carefully, because ignoring it costs every mark in the part.

1 mark -- valid first step using a feature other than the exoskeleton 1 mark -- second step separates two of the animals correctly 1 mark -- third step separates a further animal correctly 1 mark -- final step completes the key, all steps dichotomous and using visible features

Explain why the step “2a three pairs of legs → X; 2b more than three pairs of legs → go to 3” is better written than “2a few legs → X; 2b many legs → go to 3”.

Model Answer -- 3(c)

it gives a countable number, so every user reaches the same decision, whereas “few” and “many” are vague and different users would disagree [1]

the two branches are exact opposites (three pairs, or more than three pairs), so every animal fits one branch and only one [1]

⚠ If you missed marks here: Two separate points are needed: precision and mutual exclusivity. A step can be perfectly precise and still faulty if its branches overlap, and it can be neatly opposite and still faulty if the wording is vague.

1 mark -- countable number so all users reach the same decision 1 mark -- branches are exact opposites so every animal fits exactly one

(d) [2]

Animal W is not an arthropod. State two features from its description that show this.

Model Answer -- 3(d)

it has no exoskeleton — it has a shell instead, which is not part of a segmented body wall [1]

it has no jointed legs, moving instead on a muscular foot, and its body is not segmented [1]

⚠ If you missed marks here: The features must be the ones that define the arthropods, quoted as absent. Saying “it is a mollusc” names a group without giving a feature, and saying “it has a shell” is not enough on its own, because a hard outer covering is exactly what tempts people to call a snail an arthropod in the first place.

1 mark -- no exoskeleton; the shell is not an exoskeleton 1 mark -- no jointed legs and body not segmented

Question 4 -- Comparing the Five Kingdoms

Total: 12 marks

A textbook contains this incomplete table.

kingdom	nucleus	cell wall	chloroplasts	nutrition
animal	present	absent	absent	feeds on organic material made by other organisms
plant	present	cellulose	present	photosynthesis
fungus	present	not cellulose	?	?
protoctist	?	varies	varies	varies
prokaryote	?	present, not cellulose	usually absent	varies

Until the middle of the twentieth century, fungi were classified as members of the plant kingdom.

(a) [4]

Complete the four cells marked with a question mark.

Model Answer -- 4(a)

fungus, chloroplasts: absent [1]

fungus, nutrition: saprophytic or parasitic — enzymes are released onto food and the products absorbed [1]

protoctist, nucleus: present [1]

prokaryote, nucleus: absent, the DNA being a circular loop free in the cytoplasm [1]

⚠ If you missed marks here: The two nucleus cells are the ones that matter most, because the nucleus is the first question in any kingdom decision. For the fungus nutrition cell, the word saprophytic or parasitic is expected; “it eats dead things” is too informal to be credited.

1 mark -- fungus chloroplasts absent 1 mark -- fungus nutrition saprophytic or parasitic 1 mark -- protoctist nucleus present 1 mark -- prokaryote nucleus absent with DNA free in the cytoplasm

(b) [4]

Suggest why fungi were once classified as plants, and explain what evidence shows that they should not be.

Model Answer -- 4(b)

fungi are multicellular, have cell walls and do not move from place to place, so they resemble plants superficially [1]

they also grow in soil and were once thought to obtain nutrients from it as plants do [1]

however they have no chloroplasts and cannot photosynthesise, so their method of nutrition is completely different [1]

and their cell wall is not made of cellulose; comparison of DNA base sequences also shows they are not closely related to plants [1]

⚠ If you missed marks here: The question has two halves and both must be answered, which candidates often forget when the first half is easy. The historical half rewards you for recognising that classification by appearance is exactly what modern classification tries to move beyond — the same point as the dolphin and the shark.

1 mark -- fungi resemble plants in being multicellular, walled and not moving 1 mark -- a second superficial resemblance, such as growing in soil 1 mark -- no chloroplasts so they cannot photosynthesise 1 mark -- cell wall not cellulose, or DNA evidence shows they are not closely related to plants

The table has no row for viruses. State the two features of a virus and explain why no row is possible.

Model Answer -- 4(c)

a protein coat surrounding genetic material [1]

every column in the table describes part of a cell, and a virus is not a cell, so none of the columns can be completed for it [1]

⚠ If you missed marks here: This is a neat way of asking the “why no kingdom” question, and the answer is the same: the classification system is built on cell structure. Attempting to fill in the row — for example writing “nucleus absent” — misses the point, since a virus has no cytoplasm for a nucleus to be absent from.

1 mark -- protein coat and genetic material given as the two features 1 mark -- explains that the columns describe cell structures and a virus is not a cell

(d) [2]

An organism is found to be single-celled with a nucleus, chloroplasts and a cell wall. Explain why it is classified as a protoctist rather than as a plant.

Model Answer -- 4(d)

members of the plant kingdom are multicellular, and this organism is a single cell [1]

the protoctist kingdom contains mostly single-celled organisms with a nucleus, some of which have chloroplasts and cell walls and some of which do not [1]

⚠ If you missed marks here: The presence of chloroplasts is the distractor, and it works because “chloroplasts means plant” is one of the first things everyone learns. The number of cells is the deciding feature here, which is why the kingdom questions must be asked in a fixed order.

1 mark -- plants are multicellular whereas this organism is single-celled 1 mark -- protoctists are mostly single-celled with a nucleus and varied other features

Question 5 -- Wings, Fins and Groups

Total: 10 marks

A flying fish can glide above the surface of the sea for up to 200 metres using enlarged fins. A flying squirrel glides between trees using a flap of skin. A bat flies using wings of skin stretched between long finger bones. A student writes: “All three of these animals fly, so they should be classified together with the birds.”

The flying fish has wet scales, gills throughout life, and lays soft eggs with no shell in the sea. The flying squirrel and the bat both have fur and feed their young on milk.

(a) [3]

Name the vertebrate group of the flying fish, the flying squirrel and the bat, giving one feature for each.

Model Answer -- 5(a)

the flying fish is a fish — wet scales and gills used throughout life [1]

the flying squirrel is a mammal — fur and milk from mammary glands [1]

the bat is a mammal — fur and milk from mammary glands [1]

⚠ If you missed marks here: Each mark needs the group and a feature, and the feature must be one that actually identifies the group. “It has a backbone” places all three as vertebrates and separates nothing, and “it flies” is the very thing the question is warning you about.

1 mark -- flying fish identified as a fish with a supporting feature 1 mark -- flying squirrel identified as a mammal with a supporting feature 1 mark -- bat identified as a mammal with a supporting feature

(b) [4]

Evaluate the student’s statement.

Model Answer -- 5(b)

the statement is incorrect [1]

classification places organisms in groups on the basis of shared features of structure, not on what they can do [1]

the ability to move through the air has arisen separately in animals of different groups, using completely different structures — enlarged fins, a flap of skin and skin stretched over finger bones, none of which is a feather [1]

modern classification aims to reflect evolutionary relationships, and these animals do not share a recent common ancestor with birds [1]

⚠ If you missed marks here: The strongest answers make the structural point explicitly: three different flying surfaces, three different origins. Simply asserting that the animals are in different groups repeats the previous part; the marks here are for explaining why a shared ability is not evidence of a shared group.

1 mark -- statement rejected 1 mark -- classification uses shared structural features rather than abilities 1 mark -- notes that the flying structures are different in each animal 1 mark -- refers to evolutionary relationships and the absence of a recent common ancestor

Give one feature that birds have and no other vertebrate group has, one feature that mammals have and no other group has, and one feature that birds and mammals share.

Model Answer -- 5(c)

unique to birds: feathers [1]

unique to mammals: mammary glands producing milk, or hair or fur [1]

shared by both: a body temperature kept constant and independent of the surroundings (also acceptable: lungs, internal fertilisation) [1]

⚠ If you missed marks here: The third mark is a check on the second: candidates who offer “warm-blooded” as the unique mammal feature immediately contradict themselves when asked for a shared feature. Only feathers, and hair with mammary glands, are truly exclusive.

1 mark -- feathers given as unique to birds 1 mark -- hair or mammary glands given as unique to mammals 1 mark -- a genuinely shared feature given, such as a constant body temperature

Question 6 -- The Plant Kingdom

Total: 12 marks

Four plants are described.

plant	leaves	reproduction	other
A	fronds, divided, with brown patches beneath	no flowers	roots, stems and leaves present
B	long and narrow, veins side by side	flowers with six petals; seeds inside a dry fruit	fibrous roots
C	broad, veins branching from a midrib	flowers with five petals; seeds inside a fleshy fruit	a single thick main root
D	long and narrow, veins side by side	flowers with three petals	grows from a bulb

(a) [4]

Classify each of A, B, C and D as fully as the description allows.

Model Answer -- 6(a)

A — a fern [1]

B — a flowering plant, and a monocotyledon [1]

C — a flowering plant, and a dicotyledon [1]

D — a flowering plant, and a monocotyledon [1]

⚠ If you missed marks here: The instruction “as fully as the description allows” means the answer “flowering plant” alone is incomplete for B, C and D, since the vein pattern and flower parts identify the group within the flowering plants. Answers that stop at the wider group are the commonest way of losing three marks here.

1 mark -- A identified as a fern 1 mark -- B identified as a monocotyledon 1 mark -- C identified as a dicotyledon 1 mark -- D identified as a monocotyledon

(b) [3]

State two features from the table that distinguish monocotyledons from dicotyledons, and state the feature that the two group names actually refer to.

Model Answer -- 6(b)

leaf veins: parallel in monocotyledons, a branching network in dicotyledons [1]

flower parts: multiples of three in monocotyledons, multiples of four or five in dicotyledons [1]

the names refer to the number of cotyledons (seed leaves) in the seed: one in a monocotyledon, two in a dicotyledon [1]

⚠ If you missed marks here: Each difference must be written for both groups, not just for one, which is where comparison marks are usually lost. The final point is the reminder that these groups are named after a feature you almost never see and identified by features you always can.

1 mark -- vein pattern compared for both groups 1 mark -- flower part number compared for both groups 1 mark -- names refer to the number of cotyledons in the seed

Explain why plant A is placed in the plant kingdom although it has no flowers or seeds, and explain why a mushroom growing beside it is not.

Model Answer -- 6(c)

plant A has chloroplasts and feeds by photosynthesis, and it has roots, stems and leaves [1]

its cell walls are made of cellulose [1]

a mushroom has no chloroplasts and cannot photosynthesise, and its cell wall is not made of cellulose, so it belongs to the fungus kingdom [1]

⚠ If you missed marks here: Flowers and seeds are features of one group within the plant kingdom, not of the kingdom itself — recognising that is the point of the first half. The second half is the standard fungus correction, and it must be structural rather than a statement that the mushroom “is a fungus”.

1 mark -- plant A has chloroplasts and photosynthesises, with roots, stems and leaves 1 mark -- cell walls made of cellulose 1 mark -- mushroom has no chloroplasts and a wall that is not cellulose, so it is a fungus

(d) [2]

Compare how plant A and plant C reproduce.

Model Answer -- 6(d)

plant A reproduces by spores, produced in the brown patches on the underside of the fronds, and has no flowers [1]

whereas plant C reproduces by seeds, produced inside flowers and contained within a fruit [1]

⚠ If you missed marks here: A comparison needs both sides, ideally in one linked sentence. Naming the brown patches as the source of the spores shows you have used the table rather than recalled a general fact, which is what the examiner is looking for in an applied question.

1 mark -- plant A reproduces by spores produced on the underside of the fronds 1 mark -- plant C reproduces by seeds produced in flowers and contained in fruits

Question 7 -- A Tick in the Grass

Total: 10 marks

A student finds a small animal on a blade of grass and draws it. Her drawing shows a rounded body in one piece with no obvious division, eight legs, no wings and no antennae. She labels the drawing “an insect, because it is a small crawling animal with legs”.

Three other arthropods are collected at the same time: a grasshopper (three pairs of legs, three body parts, one pair of antennae, two pairs of wings), a millipede (two pairs of legs on each of many similar segments, one pair of antennae) and a sandhopper (seven pairs of legs, two pairs of antennae, chalky exoskeleton).

(a) [4]

Name the arthropod group of the student’s animal, the grasshopper, the millipede and the sandhopper.

Model Answer -- 7(a)

the student’s animal — arachnid [1]

grasshopper — insect [1]

millipede — myriapod [1]

sandhopper — crustacean [1]

⚠ If you missed marks here: Four pairs of legs with no antennae places the student’s animal in the arachnids even though its body looks like a single piece. Two pairs of antennae, not the leg count, is what makes the sandhopper a crustacean rather than a myriapod.

1 mark -- student's animal identified as an arachnid 1 mark -- grasshopper identified as an insect 1 mark -- millipede identified as a myriapod 1 mark -- sandhopper identified as a crustacean

(b) [3]

Evaluate the student’s label, and state the features she should have used.

Model Answer -- 7(b)

the label is incorrect: the animal has four pairs of legs and no antennae, so it is not an insect [1]

an insect has three pairs of legs, a body in three parts and one pair of antennae [1]

“a small crawling animal with legs” describes size and behaviour rather than structural features, and would fit every animal collected [1]

⚠ If you missed marks here: The third mark is the one candidates miss: it asks you to say what is wrong with the kind of reason she gave, not only with her conclusion. Size and behaviour are never classification features, and a reason that fits every animal in the sample cannot identify any of them.

1 mark -- label rejected, using the leg count and absence of antennae 1 mark -- correct insect features stated 1 mark -- explains that size and behaviour are not classification features

Explain why the features “exoskeleton”, “segmented body” and “jointed legs” could not be used to tell the four animals apart, and state what those features do tell you.

Model Answer -- 7(c)

all four animals possess all three features [1]

so any key step based on them would send every animal down the same branch and separate nothing [1]

those three features place all four animals in the arthropods — they identify the wider group, not the group within it [1]

⚠ If you missed marks here: This is the two-levels problem in its clearest form, and it is worth reading the wording of every classification question to see which level is being asked about. Answers that simply list the arthropod features without explaining that they are shared will not reach the second mark.

1 mark -- states that all four animals have all three features 1 mark -- explains that a key step using them would not separate the animals 1 mark -- states that the features place the animals in the arthropods

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