IGCSE Biology Paper 4 (Theory / Extended)

Topic 2: Organisation of the Organism -- 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 2. Like a real Cambridge paper, the seven questions range across every sub-topic — cell structure and organisation, and the size of specimens — and they are deliberately mixed rather than grouped. All three Topic 2 papers do; they differ in the angle they come at it from, not in what they cover.

Question 1 -- Two Slides, One Microscope

Total: 12 marks

A student in a school laboratory prepares two slides. Slide 1 holds a single layer of cells scraped from the inside of her cheek and stained with methylene blue. Slide 2 holds a single leaf taken from a piece of pondweed. Under the microscope, the cells on slide 1 appear as rounded shapes with a stained round body inside each one. The cells on slide 2 appear as neat rectangular boxes packed with small green ovals, each box containing one large clear central region.

(a) [4]

Identify the type of cell on each slide and give two pieces of evidence from the observations for each identification.

Model Answer -- 1(a)

Slide 1 shows animal cells [1]

because they have a rounded, irregular outline with no rigid boundary and no large central space [1]

Slide 2 shows plant cells [1]

because they have a straight-edged rectangular outline (a cell wall) and contain chloroplasts and a large central vacuole [1]

⚠ If you missed marks here: The evidence must come from what was actually observed, not from general knowledge. Writing “because cheek cells are animal cells” is circular and earns nothing. Note also that the neat rectangular shape is itself evidence of a cell wall — only walled cells hold a fixed geometric outline.

1 mark -- slide 1 identified as animal cells 1 mark -- evidence of rounded outline with no wall or large vacuole 1 mark -- slide 2 identified as plant cells 1 mark -- evidence of rectangular outline with chloroplasts and large vacuole

(b) [4]

Name the small green ovals seen on slide 2 and state their function. Name the large clear central region and state two functions of it.

Model Answer -- 1(b)

The green ovals are chloroplasts [1]

They contain chlorophyll, which absorbs light energy for photosynthesis [1]

The large clear central region is the large permanent vacuole, filled with cell sap [1]

It stores dissolved substances, and its pressure pushing outwards on the cell wall supports the cell [1]

⚠ If you missed marks here: “Chloroplasts make food” is only half an answer — two marking points hide in this function, the pigment (chlorophyll) and the process (photosynthesis). For the vacuole, “it stores water” is weak; name cell sap and give the support function, because the second mark is for a second distinct role.

1 mark -- green ovals identified as chloroplasts 1 mark -- contain chlorophyll which absorbs light for photosynthesis 1 mark -- central region identified as the large permanent vacuole containing cell sap 1 mark -- stores dissolved substances and supports the cell by pressing on the wall

The student writes that the cheek cells have no cell membrane because she cannot see one. Explain why she is wrong. Then name two structures that are present in both the cheek cells and the pondweed cells, giving the function of each.

Model Answer -- 1(c)

She is wrong because every cell has a cell membrane; it is extremely thin and cannot be seen with a light microscope [1]

Without a membrane the cell would have no boundary and could not control what enters and leaves [1]

Both contain a nucleus, which holds the genetic material and controls the activities of the cell [1]

Both contain cytoplasm, where most of the chemical reactions of the cell take place (accept ribosomes for protein synthesis, or mitochondria for aerobic respiration) [1]

⚠ If you missed marks here: “Not visible” and “not present” are different claims, and examiners set this question to see whether you know the difference. In the second half, read the word both carefully: naming a chloroplast or a cell wall here scores nothing because cheek cells have neither.

1 mark -- every cell has a membrane but it is too thin to see with a light microscope 1 mark -- without a membrane the cell could not control what enters and leaves 1 mark -- shared structure named with its function 1 mark -- second shared structure named with its function

Question 2 -- The Drawing and the Cell

Total: 12 marks

A student views an animal cell under a microscope and makes a labelled drawing of it. Her finished drawing is 105 mm wide. Using a stage micrometer she then measures the real cell and finds it is 0.021 mm wide.

(a) [3]

Calculate the magnification of her drawing. Show your working and give the answer in the correct form.

Model Answer -- 2(a)

magnification = image size ÷ actual size [1]

105 ÷ 0.021 [1]

= ×5000, with no units [1]

⚠ If you missed marks here: Both lengths are already in millimetres here, so no conversion is needed — but you should check that before dividing, because in most questions it is not true. Writing “5000 mm” loses the final mark: a magnification is a ratio of two lengths and the units cancel.

1 mark -- formula magnification = image divided by actual stated 1 mark -- substitution 105 divided by 0.021 shown 1 mark -- answer 5000 with no unit

(b) [3]

Express the actual width of the cell in micrometres, and state whether this is a typical size for an animal cell, giving a reason.

Model Answer -- 2(b)

0.021 mm × 1000 = 21 µm [1]

This is typical of an animal cell [1]

because animal cells are usually around 20 µm across, whereas plant cells are nearer 100 µm and bacteria are only 1 to 5 µm [1]

⚠ If you missed marks here: The direction test protects the first mark: a micrometre is a smaller unit than a millimetre, so the number must get bigger. For the comment, a bare “yes it is typical” earns one mark at most — quote a comparison size so the examiner can see the judgement is informed.

1 mark -- conversion to 21 micrometres 1 mark -- stated that this is a typical animal cell size 1 mark -- reason given by comparison with typical cell sizes

The student is asked to redraw the same cell at a magnification of ×2500. Calculate how wide her new drawing will be, showing which rearrangement of the formula you used.

Model Answer -- 2(c)

image size = actual size × magnification [1]

0.021 × 2500 [1]

= 52.5 mm [1]

⚠ If you missed marks here: This is the one rearrangement that multiplies, because image size sits alone at the top of the formula triangle. A quick check confirms it: the new magnification is half the old one, so the new drawing should be half as wide as 105 mm, and 52.5 mm is exactly that.

1 mark -- rearrangement image = actual multiplied by magnification 1 mark -- substitution 0.021 multiplied by 2500 1 mark -- answer 52.5 mm

(d) [3]

A second student writes the magnification of the original drawing as “5000 µm” and then states that the actual cell must therefore be 5000 µm wide. Explain both of the errors he has made.

Model Answer -- 2(d)

A magnification has no unit, because it is one length divided by another and the units cancel [1]

He has confused the magnification with an actual size; the magnification is simply how many times larger the image is [1]

An actual width of 5000 µm is 5 mm, which would make the cell visible without a microscope and is therefore impossible [1]

⚠ If you missed marks here: The examiner wants both errors named separately, so answer in two clear sentences. The plausibility argument is worth its own mark and is the fastest way to catch this mistake in your own work: any “cell” you can see with the naked eye is a wrong answer.

1 mark -- magnification has no unit since units cancel 1 mark -- magnification confused with an actual size 1 mark -- 5000 micrometres is 5 mm which would be visible without a microscope

Question 3 -- Adapted for One Job

Total: 12 marks

Three specialised cells are shown to a class: a palisade mesophyll cell from a leaf, a neurone from a mammal, and an egg cell. The teacher asks the class to explain how the shape and contents of each cell fit the job it does.

(a) [4]

Explain two ways in which a palisade mesophyll cell is adapted for photosynthesis.

Model Answer -- 3(a)

It contains many chloroplasts [1]

so more light energy is absorbed by chlorophyll for photosynthesis [1]

It is tall and column-shaped and lies just below the upper surface of the leaf [1]

so a large number of these cells fit into the layer that receives the most light [1]

⚠ If you missed marks here: Two adaptations means two feature-and-consequence pairs, so a list of features alone will collect only half the marks. Avoid “it makes food” as the consequence — state that light energy is absorbed for photosynthesis, which is what the chloroplasts actually do.

1 mark -- contains many chloroplasts 1 mark -- so more light energy is absorbed for photosynthesis 1 mark -- tall column shape positioned near the upper leaf surface 1 mark -- so many cells fit into the layer receiving most light

(b) [4]

Describe the shape of a neurone and explain how it is adapted to its function. Then explain why a neurone would be a poor choice of cell for absorbing substances from the gut.

Model Answer -- 3(b)

A neurone is extremely long and thin, with branched endings [1]

so it can conduct electrical impulses over long distances between distant parts of the body [1]

Absorption depends on a large surface area in contact with the substances being absorbed [1]

A neurone is adapted for length rather than for surface area in contact with gut contents, and it is not positioned in the gut lining, so it would absorb very little [1]

⚠ If you missed marks here: The final mark is an application mark: you are asked to argue from adaptation to unsuitability. Answers that simply repeat “because it carries impulses” do not address absorption at all. Compare the neurone with a cell that is adapted for absorption, such as the root hair cell, and the contrast writes itself.

1 mark -- neurone described as very long and thin with branched endings 1 mark -- conducts electrical impulses over long distances 1 mark -- absorption requires a large surface area in contact with the substances 1 mark -- neurone is not adapted or positioned for this so absorbs very little

Compare an egg cell with a sperm cell. Give two differences in structure and state the function that each difference makes possible.

Model Answer -- 3(c)

The egg cell is much larger than the sperm cell [1]

because it contains a store of food for the developing embryo, whereas the sperm carries almost none [1]

The sperm cell has a tail, whereas the egg cell does not [1]

so the sperm can swim to the egg for fertilisation, while the egg is moved passively [1]

⚠ If you missed marks here: A comparison must mention both cells in the same statement — use whereas. Two separate descriptions, one of each cell, often lose the comparative marks. Also make sure the difference is structural: “the egg is female and the sperm is male” describes the organism, not the cell.

1 mark -- egg cell is much larger than the sperm cell 1 mark -- because it stores food for the developing embryo 1 mark -- sperm cell has a tail whereas the egg cell does not 1 mark -- so the sperm can swim to the egg for fertilisation

Question 4 -- When the Wall Fails

Total: 12 marks

A microbiologist treats a culture of rod-shaped bacteria with an enzyme that breaks down the bacterial cell wall. She then divides the treated culture in two. Sample X is placed in a solution of the same concentration as the cell contents; sample Y is placed in pure water. After ten minutes, the cells in sample X are still intact but have become spherical, while almost all the cells in sample Y have burst.

(a) [4]

List the six structures that make up a bacterial cell according to the syllabus, and state which of them are not found in an animal cell.

Model Answer -- 4(a)

Cell wall, cell membrane, cytoplasm [1]

ribosomes, circular DNA, plasmids [1]

The cell wall is not found in an animal cell [1]

Circular DNA and plasmids are not found in an animal cell either, since animal DNA is inside a nucleus [1]

⚠ If you missed marks here: The syllabus list is exactly six items, and adding a nucleus, mitochondria or chloroplasts loses marks rather than gaining them. In the second half, be careful with DNA: animal cells certainly have DNA, so the difference is that it is circular and free in the cytoplasm in a bacterium.

1 mark -- cell wall, cell membrane and cytoplasm named 1 mark -- ribosomes, circular DNA and plasmids named 1 mark -- cell wall absent from animal cells 1 mark -- circular DNA and plasmids absent from animal cells

(b) [4]

Explain why the cells in sample X became spherical but survived, while those in sample Y burst. Use the functions of the cell wall and the cell membrane in your answer.

Model Answer -- 4(b)

The cell wall gives the cell its fixed rod shape and support, so without it the cells lose their shape and become spherical [1]

In sample X there is no net movement of water into the cells, so they do not swell [1]

In sample Y water enters the cells from the pure water, so they swell [1]

The cell membrane is thin and flexible and cannot resist the pressure, and with no wall to push back the cells burst [1]

⚠ If you missed marks here: The wall is doing two separate jobs here and both are marked: it holds the shape, and it resists bursting. Answers that say “the membrane broke because it is weak” without mentioning water entering earn nothing, because the cause of the pressure has to be stated.

1 mark -- cell wall gives fixed shape so cells become spherical without it 1 mark -- no net water entry in sample X so no swelling 1 mark -- water enters cells in pure water so they swell 1 mark -- membrane cannot resist the pressure and with no wall the cells burst

A plant cell placed in pure water swells but does not burst. Explain this difference, and suggest why the microbiologist used sample X as well as sample Y.

Model Answer -- 4(c)

A plant cell has a strong, inelastic cellulose cell wall that has not been removed [1]

The wall resists the outward pressure of the swelling contents, so the cell becomes firm instead of bursting [1]

Sample X acts as a control [1]

It shows that the bursting in sample Y was caused by the water entering, not by the enzyme treatment itself [1]

⚠ If you missed marks here: The word control is worth a mark on its own, but only if you then say what it controls for. Answers such as “to compare the results” are too vague. Sample X changes only one factor, so it isolates the effect of the surrounding solution from the effect of removing the wall.

1 mark -- plant cell has a strong inelastic cellulose cell wall 1 mark -- the wall resists the pressure so the cell becomes firm rather than bursting 1 mark -- sample X is a control 1 mark -- it shows bursting was caused by water entry and not by the enzyme

Question 5 -- Levels in a Leaf

Total: 10 marks

A biology class is asked to arrange the following into the correct levels of organisation: a palisade mesophyll cell, a leaf, xylem, a whole oak tree, and the shoot system of the tree.

(a) [4]

Place the five items in order from the smallest level of organisation to the largest, and name the level of organisation that each one represents.

Model Answer -- 5(a)

Order: palisade mesophyll cell, xylem, leaf, shoot system, oak tree [1]

The palisade mesophyll cell is a cell [1]

Xylem is a tissue and the leaf is an organ [1]

The shoot system is an organ system and the oak tree is an organism [1]

⚠ If you missed marks here: A single palisade cell is one cell, but palisade mesophyll as a layer is a tissue — read the wording. The usual slip is placing the leaf before xylem, which reverses the hierarchy: tissues are the building blocks of organs, so tissue always comes first.

1 mark -- correct order from cell to organism 1 mark -- palisade mesophyll cell identified as a cell 1 mark -- xylem identified as a tissue and leaf as an organ 1 mark -- shoot system identified as an organ system and oak tree as an organism

(b) [3]

Explain why a leaf is classified as an organ and not as a tissue, naming two tissues found in it.

Model Answer -- 5(b)

An organ is a structure made of a group of different tissues working together to perform a specific function [1]

A tissue is a group of cells with similar structures only, so a leaf cannot be a tissue [1]

Two tissues found in a leaf: any two of palisade mesophyll, spongy mesophyll, epidermis, xylem [1]

⚠ If you missed marks here: The word different carries the first mark and the word similar carries the second. Naming the tissues is worth its own mark, so do not stop after the definitions — the examiner wants evidence that you can apply the test rather than just quote it.

1 mark -- organ defined as different tissues working together 1 mark -- tissue defined as cells of similar structure so a leaf does not fit 1 mark -- two tissues found in a leaf named

An oak tree grows from an acorn into a tree many metres tall. State where all the new cells come from, and explain why a gardener who removes a branch does not find the tree growing a completely new organ system.

Model Answer -- 5(c)

All new cells are produced by the division of existing cells [1]

The tree has no way of producing cells from anything other than cells that are already present [1]

New growth is added to existing tissues and organs, so the tree replaces or extends structures rather than building a new organ system from nothing [1]

⚠ If you missed marks here: The marking phrase is “division of existing cells” — “the tree makes new cells” and “the cells multiply” are both refused. The second half is a “suggest”-style application: the point is that growth is always an extension of what already exists, never a fresh start.

1 mark -- new cells produced by division of existing cells 1 mark -- cells can only come from cells already present 1 mark -- growth extends existing tissues and organs rather than creating a new organ system

Question 6 -- Six Students, One Cell

Total: 12 marks

Six students each measure the length of the same onion cell on the same photomicrograph, which was printed at a magnification of ×500. Their measurements are shown below.

student	A	B	C	D	E	F
image length / mm	62	60	61	74	59	60

(a) [4]

Identify the anomalous result and suggest one reason for it. Calculate the mean image length, explaining which values you have used.

Model Answer -- 6(a)

The anomalous result is student D, 74 mm [1]

A possible reason: the student measured a different cell, or measured from the outside of the wall at one end and the inside at the other, or misread the ruler [1]

The anomaly is excluded, so the mean uses the five remaining values [1]

Mean = (62 + 60 + 61 + 59 + 60) ÷ 5 = 302 ÷ 5 = 60.4 mm [1]

⚠ If you missed marks here: An anomaly must be identified and excluded, and the exclusion has to be stated explicitly or the third mark is lost. Averaging all six values gives 62.7 mm and quietly corrupts every calculation that follows. Any sensible practical reason is accepted, but “the student made a mistake” is too vague.

1 mark -- student D 74 mm identified as the anomaly 1 mark -- sensible reason suggested for the anomalous reading 1 mark -- anomaly excluded from the mean 1 mark -- mean image length 60.4 mm

(b) [4]

Use the mean image length to calculate the actual length of the onion cell. Give your answer in µm and comment on whether it is a realistic value.

Model Answer -- 6(b)

actual = image ÷ magnification [1]

60.4 ÷ 500 = 0.1208 mm [1]

0.1208 × 1000 = 121 µm (to 3 significant figures) [1]

This is realistic, because plant cells are typically around 100 µm long [1]

⚠ If you missed marks here: Round only at the very end: rounding the mean to 60 mm first gives 120 µm, which is close but shows the habit that costs marks in harder questions. The final mark is for the reality check, which is free once you know that a plant cell is about 100 µm.

1 mark -- rearrangement actual = image divided by magnification 1 mark -- 60.4 divided by 500 = 0.1208 mm 1 mark -- answer 121 micrometres 1 mark -- comment that this is realistic for a plant cell

Explain why the class was asked to take six measurements rather than one. Suggest two improvements to the method that would make the calculated actual length more reliable.

Model Answer -- 6(c)

Repeating allows anomalous results to be identified and reduces the effect of random errors in measuring [1]

A mean of several readings is closer to the true value than any single reading [1]

Improvement: agree a rule for exactly where to measure from and to, for example from the outside of the wall at both ends [1]

Improvement: measure several different cells and take a mean, or use a ruler marked in half millimetres so readings are more precise [1]

⚠ If you missed marks here: “To make it more accurate” without saying how is not enough for either of the first two marks. The strongest improvements attack the source of the variation, and here that is inconsistent measuring points, not the calculator work — so an answer about doing the arithmetic more carefully misses the point.

1 mark -- repeats allow anomalies to be spotted and reduce random error 1 mark -- a mean is closer to the true value than one reading 1 mark -- improvement of agreeing exactly where to measure from and to 1 mark -- second sensible improvement such as measuring more cells or using a finer ruler

Question 7 -- Reading the Bar

Total: 10 marks

An electron micrograph shows a single rod-shaped object. The micrograph carries a scale bar labelled 1 µm that measures 25 mm on the page. The rod-shaped object measures 75 mm long.

(a) [3]

Calculate the magnification of the micrograph, showing the unit conversion clearly.

Model Answer -- 7(a)

1 µm = 0.001 mm [1]

magnification = image ÷ actual = 25 ÷ 0.001 [1]

= ×25 000 [1]

⚠ If you missed marks here: Dividing 25 by 1 gives a magnification of 25, which for an electron micrograph is absurdly low — a hand lens does better. Show the conversion on its own line and the error becomes impossible; it also earns a mark by itself even if the later arithmetic slips.

1 mark -- conversion 1 micrometre to 0.001 mm 1 mark -- substitution 25 divided by 0.001 1 mark -- magnification 25 000

(b) [3]

Calculate the actual length of the rod-shaped object in µm, and suggest what the object is.

Model Answer -- 7(b)

75 ÷ 25 000 = 0.003 mm [1]

0.003 × 1000 = 3 µm [1]

The object is most likely a bacterium (a rod-shaped bacterial cell) [1]

⚠ If you missed marks here: A faster route is available and fully creditable: the object is 75/25 = 3 times the length of the scale bar, so it is 3 µm long. Use it as a check. The identification mark depends on knowing that bacteria are 1 to 5 µm long, which is exactly why those typical sizes are worth memorising.

1 mark -- 75 divided by 25 000 = 0.003 mm 1 mark -- answer 3 micrometres 1 mark -- object identified as a bacterium

Use the scale bar to explain why this object cannot be a whole plant cell. Name two structures you would expect to find inside it, and name one structure it will not contain.

Model Answer -- 7(c)

A plant cell is typically about 100 µm long, and this object is only 3 µm, roughly thirty times too small [1]

Two structures expected inside: any two of cytoplasm, ribosomes, circular DNA, plasmids [1]

(second mark for a correct second structure) [1]

It will not contain a nucleus (accept mitochondria or chloroplasts) [1]

⚠ If you missed marks here: The scale bar is evidence, and the mark is given for the numerical comparison, not for “it looks too small”. When naming structures, stay inside the syllabus list of six for a bacterium — and remember that the absent structure is the nucleus, not the DNA, which is present as a circular molecule.

1 mark -- comparison of 3 micrometres with about 100 micrometres for a plant cell 1 mark -- one correct internal structure named 1 mark -- second correct internal structure named 1 mark -- nucleus named as a structure it will not contain

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