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IGCSE Biology Paper 4 (Theory / Extended)

Topic 21: Biotechnology and Genetic Modification -- Challenge Exam 3
1 hour 15 minutes
80
7
75:00
0610

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This paper covers the whole of Topic 21. Like a real Cambridge paper it ranges across every sub-topic — 21.1 why bacteria are useful, 21.2 biotechnology (yeast, industrial enzymes and fermenters), and 21.3 genetic modification — and it mixes them inside single questions. All three Topic 21 papers do; they differ in the angle they come at it from, not in what they cover.
Question 1 — The Chain, and What Would Break It
Total: 12 marks
Fig. 1.1 shows the process with two steps left blank.
Fig. 1.1 — producing a human protein in bacteriaTwo steps have been left blank.1Cut the gene outa restriction enzyme cutsthe human gene out of human DNA,leaving sticky ends2Cut the plasmid openthe SAME restriction enzyme cutsthe bacterial plasmid, givingcomplementary sticky ends3?you write this step4Put it into bacteriathe recombinant plasmids areinserted into bacteria5?you write this step6Collect the proteinthe human gene is expressed,so the bacteria make thehuman protein
(a) [4]
Fig. 1.1 shows the six-step process by which bacteria are modified to produce a human protein, with two steps left blank. Write out the two missing steps in full.
Model Answer — 1(a)
step 3: DNA ligase joins the human gene into the plasmid [1]
forming a recombinant plasmid [1]
step 5: the bacteria containing the recombinant plasmid multiply [1]
so that very large numbers of cells all carry the human gene [1]
⚠ If you missed marks here: Two of these four marks are single words: ligase and recombinant. Check also that you have not written “the gene is expressed” for step 5 — that is step 6, and using it early loses the multiplication mark and leaves nothing for the final step.
(b) [4]
Explain what would happen at each of steps 2 and 3 if a different restriction enzyme were used on the plasmid from the one used on the human DNA.
Model Answer — 1(b)
at step 2 the plasmid would still be cut open, and sticky ends would still be formed [1]
but they would be a different shape, so they would not be complementary to those on the human gene [1]
at step 3 the gene and the plasmid could not pair together, because the overhang on one would not fit the gap on the other [1]
so DNA ligase would have nothing to seal and no recombinant plasmid would form [1]
⚠ If you missed marks here: The first mark is a test of whether you have read carefully: the plasmid still gets cut, so an answer that says nothing happens at step 2 has overstated the problem. The failure is at step 3, and the reason is that the ends are the wrong shape, not that no ends exist.
(c) [4]
The modified bacteria are grown in a fermenter for two days before the protein is collected. Explain why this stage is necessary, referring to what happens to the plasmid as the bacteria divide.
Model Answer — 1(c)
one bacterium makes only a very small amount of the protein [1]
each time a bacterium divides, the recombinant plasmid is copied and passed to the daughter cells [1]
so every cell in the population carries the human gene and can express it [1]
bacteria have a rapid reproduction rate, so after two days there are enormous numbers of them and the total yield is commercially useful [1]
⚠ If you missed marks here: The second mark is the one that is nearly always missing. Multiplying the cells only helps if the gene multiplies with them, and it does so because the plasmid is copied at every division. Without that sentence the answer never explains why numbers matter.
Question 2 — Enzymes in a Factory
Total: 12 marks
(a) [4]
A drinks company crushes 100 kg of apples and obtains 42 litres of cloudy juice. It then adds pectinase to a second identical batch, holds it at 40°C for 30 minutes, and obtains 71 litres of clear juice. Explain both changes.
Model Answer — 2(a)
pectinase breaks down pectin, which holds the plant cells together in the pulp [1]
so the cells separate and more juice is released from the pulp [1]
the juice is clearer because the pectin that was making it cloudy has been broken down [1]
40°C is at or near the optimum temperature for the enzyme, so it works quickly [1]
⚠ If you missed marks here: Two changes need two explanations. Answers regularly explain the extra volume and then treat the clarity as if it were the same point — it is not, and it is a separate mark. Note that the extra 29 litres is the increase; quoting 71 would be the total.
(b) [4]
The company then tries holding the pulp at 75°C, and finds it obtains only 38 litres, which is less than from untreated pulp. Explain this result, and suggest what the company should do.
Model Answer — 2(b)
at 75°C the pectinase is denatured [1]
the active site changes shape, so pectin no longer fits and the enzyme stops working — permanently [1]
a yield below that of untreated pulp shows the enzyme is contributing nothing, rather than working slowly [1]
the company should run the process at or near 40°C, and could test temperatures between 30°C and 50°C to find the optimum more precisely [1]
⚠ If you missed marks here: The third mark rewards reading your own data properly. A yield below the untreated control is evidence of complete loss of activity, and an answer that says “the enzyme works more slowly at 75°C” has not noticed. Denaturation is permanent — cooling the pulp again would not bring the enzyme back.
(c) [4]
Explain how the enzymes in a biological washing powder remove a grease stain and a blood stain, naming the enzyme responsible in each case.
Model Answer — 2(c)
grease is a fat, broken down by a lipase [1]
into fatty acids and glycerol [1]
blood is largely protein, broken down by a protease [1]
into amino acids and other small molecules; in both cases the products are small and soluble, so they dissolve in the water and are rinsed away [1]
⚠ If you missed marks here: The enzyme names are given away by the substrate — lip-ase acts on lipids, prote-ase on protein — so the marks that are actually at risk are the products. “The enzyme removes the stain” is not an explanation; broken down into small soluble molecules is.
Question 3 — Reading a Fermenter
Total: 12 marks
Fig. 3.1 shows an industrial fermenter with its parts lettered.
Fig. 3.1 — an industrial fermenter, shown in sectionThe parts have been lettered instead of labelled.ABCDEFG
(a) [4]
Fig. 3.1 shows a fermenter. Identify the parts labelled A, B, E and F.
Model Answer — 3(a)
A — the nutrient inlet [1]
B — the pH probe [1]
E — the sterile air inlet (with its filter) [1]
F — the water jacket [1]
⚠ If you missed marks here: Two probes hang from the lid, so look at where each one is drawn and at what enters or leaves at that point. F is the outer layer wrapped around the vessel with water flowing through it — not the vessel wall itself.
(b) [4]
Explain why the stirrer is essential in a large fermenter. Give three separate reasons and explain what would go wrong without it.
Model Answer — 3(b)
it spreads the oxygen bubbles through the liquid instead of letting them rise straight out [1]
it keeps the nutrients evenly distributed, so no region of the vessel runs short [1]
it keeps the temperature even, so there are no hot regions where enzymes could be denatured [1]
without it the organisms would settle at the bottom, where they would have neither oxygen nor nutrients, and growth and production would fall [1]
⚠ If you missed marks here: Three reasons means three, and most answers give one. Notice that the three jobs map onto three of the five controlled conditions — oxygen, nutrient supply and temperature — which is a useful way to remember them.
(c) [4]
Mycoprotein is grown in a fermenter of this kind. State what mycoprotein is and what kind of organism makes it, and explain why waste products must be removed during a long run.
Model Answer — 3(c)
mycoprotein is a protein-rich food, grown as a meat substitute [1]
it is made by a fungus [1]
waste products accumulate and can be toxic to the organism, slowing or stopping growth [1]
they are often acidic, so the pH falls away from the optimum and the enzymes work less well [1]
⚠ If you missed marks here: The prefix “myco-” means fungus, which makes the second mark recoverable even if you have forgotten it. For the last two marks give two distinct effects of waste — toxicity and pH change — rather than one effect written twice.
Question 4 — Four Flasks and a Loaf
Total: 12 marks
(a) [4]
Yeast was added to four flasks of glucose solution. Flask 1 was sealed; flask 2 had air bubbled through it; flask 3 was sealed and boiled for five minutes first; flask 4 was sealed and contained no yeast. After 24 hours, ethanol was found only in flask 1. Explain the result in each flask.
Model Answer — 4(a)
flask 1: no oxygen, so the yeast respires anaerobically and produces ethanol and carbon dioxide [1]
flask 2: oxygen present, so the yeast respires aerobically, giving carbon dioxide and water and no ethanol [1]
flask 3: boiling has killed the yeast and denatured its enzymes, so no respiration takes place [1]
flask 4: no yeast, so no respiration — this flask is the control, showing that the glucose does not turn into ethanol on its own [1]
⚠ If you missed marks here: Flask 4 is the mark most often lost, because it looks like a pointless flask. It is the control, and it is what allows you to say the ethanol came from the yeast rather than from the glucose breaking down by itself. In flask 3, note that both statements are correct and each uses the right word: the yeast is killed, its enzymes are denatured.
(b) [4]
Bread dough was left to rise at 30°C. Explain what makes the dough rise, why the risen loaf has holes in it, and why the baked bread contains no alcohol.
Model Answer — 4(b)
the yeast respires anaerobically, producing carbon dioxide [1]
the gas is trapped as bubbles in the stretchy dough, which makes it rise [1]
the holes in the baked loaf are those trapped bubbles, which expand further in the heat before the loaf sets [1]
the ethanol evaporates in the heat of the oven, so the bread is not alcoholic [1]
⚠ If you missed marks here: The last mark is the one Cambridge likes best, because the natural assumption is that no ethanol was ever made. It was — the reaction is identical to the one used to make biofuel — and it simply evaporates.
(c) [4]
A student writes: “Yeast is a bacterium that ferments sugar into alcohol. In bread the alcohol makes the dough rise.” Identify two errors and correct each one.
Model Answer — 4(c)
error: yeast is not a bacterium [1]
correction: yeast is a fungus [1]
error: it is not the alcohol that makes the dough rise [1]
correction: the carbon dioxide makes the dough rise; the ethanol evaporates in the oven [1]
⚠ If you missed marks here: Each error needs identifying and correcting, so a bare “this is wrong” earns half. Both of these errors are on the standard list of misconceptions for this topic, and both cost marks in real papers every year.
Question 5 — Definitions and Uses
Total: 10 marks
(a) [4]
Define genetic modification, and give two of the four examples of its use named in the syllabus.
Model Answer — 5(a)
changing the genetic material of an organism [1]
by removing, changing or inserting individual genes [1]
example 1: inserting a human gene into bacteria so they produce a human protein [1]
example 2: inserting a gene into a crop plant to give resistance to herbicides, or to insect pests, or to improve nutritional qualities [1]
⚠ If you missed marks here: The definition is worth two marks and is two short phrases — the cheapest marks in the topic if you have learned it word for word. The phrase individual genes is what stops the definition sliding into “moving chromosomes about”.
(b) [3]
Explain why bacteria, rather than a plant or an animal, are used to produce a human protein. Give three reasons.
Model Answer — 5(b)
they have a rapid reproduction rate, so very large numbers are available quickly [1]
they can make complex molecules — their ribosomes assemble amino acids into the human protein [1]
they contain plasmids, which give a ready-made way of getting the gene in; and there are few ethical concerns about growing and manipulating them [1]
⚠ If you missed marks here: Three of the four named reasons, and none of them is “bacteria are simple”. If you are short of ideas, run through the list: fast, complex molecules, ethics, plasmids.
(c) [3]
A student says that inserting a gene into a crop plant is “the same as selective breeding, only faster”. Explain why this is wrong.
Model Answer — 5(c)
selective breeding uses variation that already exists within the species, and combines whole sets of genes by breeding [1]
genetic modification transfers one individual gene, and can take it from a completely different species [1]
so it can produce a feature that no member of the species has ever had, which selective breeding could never do however long it were continued [1]
⚠ If you missed marks here: The third mark is what converts a memorised list of differences into an argument. Speed is a difference, but it is not the important one — the important one is that selective breeding is limited to what already exists in the species.
Question 6 — A Decision About a Crop
Total: 12 marks
(a) [6]
A country is deciding whether to allow farmers to grow genetically modified soya that is resistant to a herbicide. Discuss the advantages and disadvantages, and give a conclusion.
Model Answer — 6(a)
advantage: weeds can be sprayed off while the crop is growing, because the crop survives the herbicide [1]
advantage: the crop is therefore not competing with weeds for light, water and mineral ions, so yield rises [1]
advantage: fewer or simpler chemical applications may be needed, which is cheaper for the farmer and may mean less fuel used and less run-off into rivers [1]
disadvantage: the resistance gene can spread by pollen to wild relatives, producing weeds the herbicide no longer controls [1]
disadvantage: growing one variety widely gives the crop little genetic variation, so a new disease could destroy all of it; and seed usually has to be bought each year [1]
conclusion: a judgement that refers back to the evidence given, for example that the case is strongest where the crop has no close wild relatives growing nearby [1]
⚠ If you missed marks here: Notice that the sixth mark is for the conclusion itself, and that it must follow from what you have written rather than being an opinion announced at the start. Three points on each side, each explained, then a decision — that is the shape of a full-mark discuss answer.
(b) [3]
Ten years after the modified soya is introduced, a wild plant growing at the edges of the fields is found to survive the herbicide. Explain how this could have happened.
Model Answer — 6(b)
pollen from the modified soya was carried to a closely related wild plant [1]
fertilisation occurred, so the offspring inherited the herbicide-resistance gene [1]
when the fields were sprayed, those resistant plants survived and reproduced while others died, so the resistant form spread through the wild population [1]
⚠ If you missed marks here: Two ideas are needed, and answers usually give only the first: the gene has to get across by pollination, and then the spraying itself selects for the plants that carry it. Without the second stage the resistant plants would stay rare.
(c) [3]
Suggest three pieces of information a government would want before deciding whether to allow the crop, and explain why each one matters.
Model Answer — 6(c)
whether there are closely related wild plants growing near the fields, because that determines the risk of the gene spreading by pollen [1]
the effect on other species — the insects, birds and other organisms that depend on the plants in and around the fields, because removing weeds changes the whole food web [1]
the yield and cost compared with unmodified varieties, including the cost of buying seed each year, because that decides whether farmers actually benefit [1]
⚠ If you missed marks here: This is a “suggest” question, so other sensible answers are creditable — long-term health monitoring and consumer labelling would both earn a mark. What is not creditable is a bare list: each piece of information must come with a reason it matters.
Question 7 — Protein, Gene and Patient
Total: 10 marks
Fig. 7.1 shows a bacterial cell with its structures lettered.
Fig. 7.1 — a bacterial cellNot drawn to scale.ABCDEE
(a) [4]
Fig. 7.1 shows a bacterial cell. Name the structures labelled C and D, and explain why a genetic engineer works on E rather than on D.
Model Answer — 7(a)
C — the cytoplasm [1]
D — the main circular DNA [1]
E is a plasmid: small, circular and separate from D, so it can be removed and returned on its own [1]
D carries the genes the bacterium needs in order to live, so cutting into it risks damaging or killing the cell [1]
⚠ If you missed marks here: The last mark is the one that shows real understanding. It is not simply that the plasmid is convenient — it is that the alternative is dangerous, because the main circular DNA is the cell’s working set of instructions.
(b) [3]
A student claims that because a modified bacterium contains a human gene, the protein it makes will not be identical to human protein. Explain whether the student is right.
Model Answer — 7(b)
the student is wrong [1]
the sequence of bases in the gene determines the sequence of amino acids in the protein [1]
since the gene is human, the amino acid sequence is the human one, so the protein is identical to the one a human cell would make [1]
⚠ If you missed marks here: The reasoning matters more than the verdict here: a bare “the student is wrong” scores one mark out of three. The chain from bases to amino acids to protein is recall from 17.1, and this is exactly the kind of link a Paper 4 makes between topics.
(c) [3]
Explain why a person treated with a protein made in this way still has to be given it regularly, rather than being cured once.
Model Answer — 7(c)
the protein is used up or broken down in the body, so its effect does not last [1]
the person’s own cells still cannot make enough of it, because their own genes have not been changed [1]
so the supply has to be replaced from outside as often as the body needs it [1]
⚠ If you missed marks here: The second mark is the important one: modifying a bacterium does nothing to the patient’s own genes. Treating the symptom is not the same as changing the cause, and recognising that distinction is what this question is testing.

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IGCSE Biology 0610 — Paper 4 Theory (Extended) — Topic 21: Biotechnology and Genetic Modification — Challenge Exam 3
For examination preparation only. Not an official Cambridge Assessment paper.