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IGCSE Biology Paper 4 (Theory / Extended)

Topic 21: Biotechnology and Genetic Modification -- Challenge Exam 2
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 — Lactase and Lactose-Free Milk
Total: 12 marks
(a) [3]
Name the enzyme used to make lactose-free milk, name the sugar it acts on, and name the two products it forms.
Model Answer — 1(a)
enzyme: lactase [1]
substrate: lactose [1]
products: glucose and galactose [1]
⚠ If you missed marks here: Watch the spelling: lactose is the sugar and lactase is the enzyme, and the “-ase” ending means enzyme every time. Both products are needed for the third mark; naming only glucose is worth nothing.
(b) [4]
A student writes: “Lactose-free milk has had all of its lactose removed by filtering, which is why it contains less sugar and tastes less sweet.” Identify and correct two errors in this sentence.
Model Answer — 1(b)
error 1: the lactose is not removed [1]
correction: it is broken down by lactase into glucose and galactose, and both of those remain in the milk [1]
error 2: the milk does not taste less sweet [1]
correction: it tastes slightly sweeter, because glucose and galactose taste sweeter than the lactose they came from [1]
⚠ If you missed marks here: Each error needs both an identification and a correction, so a bare “that is wrong” earns half. The taste is not a detail — it is the evidence that the lactose was broken down rather than taken out, since nothing was added and nothing was removed.
(c) [5]
Milk was treated with lactase at six temperatures for the same length of time, and the mass of glucose produced was measured: 10°C, 0.4 g; 20°C, 1.1 g; 30°C, 2.0 g; 40°C, 2.6 g; 50°C, 1.2 g; 60°C, 0.1 g. Describe and explain these results, and suggest one reason why a dairy might still choose to run the process at 10°C.
Model Answer — 1(c)
the mass of glucose rises from 10°C to a maximum at 40°C, then falls sharply [1]
the rise happens because molecules gain kinetic energy, so enzyme and substrate collide more often and more successfully [1]
the fall happens because the lactase is denatured above its optimum [1]
the active site changes shape, so lactose no longer fits and the reaction stops [1]
a dairy might still use 10°C because milk kept cold does not spoil — bacteria in it grow much more slowly — even though the enzyme works slowly too [1]
⚠ If you missed marks here: The last mark is an application mark, and it rewards you for thinking about the product rather than the enzyme. Warm milk is an excellent growth medium for the bacteria that turn it sour, so a dairy will accept a slow reaction in exchange for milk that keeps.
Question 2 — Ethanol from Sugar Cane
Total: 12 marks
(a) [4]
Sugar cane juice is mixed with yeast in a sealed tank to make ethanol for fuel. Explain why the tank is sealed, and state what would happen to the yield of ethanol if air were allowed in.
Model Answer — 2(a)
sealing keeps oxygen out [1]
so the yeast respires anaerobically: glucose → ethanol + carbon dioxide [1]
if air were let in the yeast would respire aerobically instead [1]
giving carbon dioxide and water, so the yield of ethanol would fall, possibly to nothing [1]
⚠ If you missed marks here: The commonest wrong answer is that oxygen would kill the yeast. It would not — yeast grows perfectly well in air, which is precisely the problem. The issue is which kind of respiration it carries out, and that determines whether you get ethanol at all.
(b) [4]
The ethanol produced is burned in vehicle engines, which releases carbon dioxide. Explain why ethanol made in this way is still described as a biofuel and is treated differently from petrol.
Model Answer — 2(b)
the ethanol is made from sugar produced by a crop, sugar cane [1]
the carbon in that sugar was taken from carbon dioxide in the air by the crop during photosynthesis [1]
so the carbon dioxide released on burning was removed from the air recently, within the last growing season [1]
whereas the carbon in petrol comes from a fossil fuel, and was removed from the air millions of years ago, so burning it adds carbon dioxide that had been locked away [1]
⚠ If you missed marks here: This question is really about the carbon cycle from 19.4 dressed up as biotechnology. An answer that says a biofuel “releases no carbon dioxide” is simply wrong — it does. The point is where the carbon came from and how recently.
(c) [4]
Explain, for bread-making and for biofuel production, which product of the reaction is wanted and what happens to the other one.
Model Answer — 2(c)
bread: the carbon dioxide is wanted [1]
it is trapped as bubbles in the dough so the dough rises; the ethanol evaporates in the oven [1]
biofuel: the ethanol is wanted [1]
it is separated and burned; the carbon dioxide is a by-product that escapes [1]
⚠ If you missed marks here: This contrast is the most frequently examined idea in 21.2, so make sure you can write all four halves of it quickly. The half most often left out is what happens to the ethanol in bread — it evaporates in the oven, which is why bread is not alcoholic.
Question 3 — Growing a Fungus for Penicillin
Total: 12 marks
(a) [4]
Penicillin is produced in a fermenter. Name the type of organism that makes it, state what penicillin is used for, and explain why penicillin is of no use against a viral infection.
Model Answer — 3(a)
penicillin is made by a fungus [1]
it is an antibiotic, used to treat infections caused by bacteria [1]
antibiotics work by damaging structures or processes that only bacterial cells have [1]
a virus has none of those — it reproduces inside the body cells of the host — so the antibiotic has nothing to act on [1]
⚠ If you missed marks here: The first mark catches most candidates: it is a fungus, not a bacterium, and the name Penicillium is the clue. The last two marks are recall from 15.1, and this is exactly the kind of link a real Paper 4 makes between topics.
(b) [4]
In one fermenter run the mass of fungus rose steeply between hours 0 and 30 and then stayed almost constant, while the mass of penicillin produced stayed near zero until hour 30 and then rose steeply. Suggest an explanation for each of these two patterns.
Model Answer — 3(b)
the fungus stops growing when something becomes limiting — waste products accumulate and are toxic, or the pH drifts, or oxygen becomes limiting in a dense culture [1]
so the rate of division falls until as many cells die as are produced, and the mass stays constant [1]
penicillin is not needed for growth, so while nutrients are plentiful the fungus uses its energy and materials for reproduction instead [1]
once growth slows, the energy and raw materials go into making penicillin rather than new cells, so the yield rises steeply [1]
⚠ If you missed marks here: The second half of this question is a “suggest”, so several sensible answers are accepted — but they must be reasoned rather than asserted. Notice the practical consequence: the run cannot be stopped when the culture stops growing, because that is when the product finally starts to appear.
(c) [4]
Describe how the temperature and the pH of a fermenter are kept constant, and explain why each of them must be.
Model Answer — 3(c)
temperature: a probe monitors it and a water jacket carries away the heat released by respiration [1]
because the enzymes of the organism have an optimum temperature, and above it they are denatured [1]
pH: a pH probe monitors it and small amounts of acid or alkali are added [1]
because enzymes also have an optimum pH, and the acidic waste released by the culture would otherwise make the pH fall [1]
⚠ If you missed marks here: Each condition needs both halves: the hardware that controls it and the reason it matters. Answers that give four pieces of hardware and no reasons, or four reasons and no hardware, score two out of four.
Question 4 — Why Bacteria
Total: 12 marks
(a) [4]
Give the four reasons the syllabus gives for bacteria being useful in biotechnology and genetic modification.
Model Answer — 4(a)
rapid reproduction rate [1]
ability to make complex molecules [1]
few ethical concerns over their manipulation and growth [1]
the presence of plasmids [1]
⚠ If you missed marks here: The answer almost everybody writes — that bacteria are small, simple or easy to grow — is not one of the four and earns nothing. Learn them as a list of four short phrases: fast, complex molecules, ethics, plasmids.
(b) [4]
A bacterial culture starts with 500 cells and doubles every 15 minutes. Calculate how many cells there are after 2 hours, showing your working. Then explain why a real culture would not reach this number.
Model Answer — 4(b)
2 hours = 120 minutes, so there are 120 ÷ 15 = 8 doublings [1]
number = 500 × 28 = 500 × 256 [1]
= 128 000 cells [1]
in a real culture growth would slow because nutrients run out, waste products accumulate, or oxygen becomes limiting — which is why a fermenter controls all of these [1]
⚠ If you missed marks here: The starting number is not 1 here, so the answer is 500 × 28 and not 28. Read what the culture starts with before you begin. The last mark ties the arithmetic back to the fermenter, which is where the marks in this topic usually connect.
(c) [4]
Explain how the structure of a bacterial cell makes it suitable for making a human protein. Refer to two named structures.
Model Answer — 4(c)
ribosomes assemble amino acids into proteins [1]
so once the bacterium carries the human gene it can build the human protein itself — this is the “ability to make complex molecules” [1]
plasmids are small circles of DNA separate from the main circular DNA [1]
so a plasmid can be given the human gene outside the cell and then taken up, without altering the DNA the bacterium needs to live [1]
⚠ If you missed marks here: Naming a structure is only half of each mark — the other half is what it lets the bacterium do. A list of bacterial cell parts with no functions attached will score one or two out of four.
Question 5 — Reading the Chain
Total: 10 marks
Fig. 5.1 shows the six-step process with three steps left blank.
Fig. 5.1 — producing a human protein in bacteriaThree steps have been left blank.1Cut the gene outa restriction enzyme cutsthe human gene out of human DNA,leaving sticky ends2?you write this step3Join them togetherDNA ligase joins the geneinto the plasmid, forming arecombinant plasmid4?you write this step5Let them multiplythe bacteria containing therecombinant plasmid multiply6?you write this step
(a) [5]
Fig. 5.1 shows the process of genetic modification with three steps left blank. Write out the three missing steps in full, in the correct order, and state which enzyme is involved in each of steps 1 and 3.
Model Answer — 5(a)
step 2: the same restriction enzyme cuts the bacterial plasmid [1]
leaving sticky ends that are complementary to those on the gene [1]
step 4: the recombinant plasmids are inserted into bacteria [1]
step 6: the human gene is expressed, so the bacteria make the human protein [1]
step 1 uses a restriction enzyme (which cuts) and step 3 uses DNA ligase (which joins) [1]
⚠ If you missed marks here: The word complementary in step 2 is a mark on its own, and it is the one most often missing. For the last mark, check the direction before you write: a restriction enzyme cuts and DNA ligase joins. Getting those the wrong way round is the most-reported error in the topic.
(b) [3]
Explain what is meant by a sticky end, and why the sticky ends on the gene and on the plasmid must be complementary.
Model Answer — 5(b)
a sticky end is a short single-stranded overhang left when the restriction enzyme makes a staggered cut [1]
complementary means that what sticks out on one end is exactly the gap on the other [1]
so the gene pairs with and fits into the plasmid; ends of a different shape would not join, which is why the same enzyme is used on both [1]
⚠ If you missed marks here: You are not expected to know any base sequences — they are outside this syllabus — so describe the ends as shapes that fit together. The word staggered is worth having, because it is what explains why an overhang exists at all.
(c) [2]
The syllabus states that the specific details of getting the recombinant plasmid into the bacterium are not required. Explain what this means for how you should answer a question on this process.
Model Answer — 5(c)
the step must still be included, because it is one of the six marking points [1]
but writing “the recombinant plasmids are inserted into bacteria” is enough, and time spent describing a method cannot earn any marks [1]
⚠ If you missed marks here: Both halves matter. Leaving the step out loses a mark, and inventing a method wastes time you will want for the questions where the marks actually are. Knowing what not to write is part of the technique in this topic.
Question 6 — Modified Rice
Total: 12 marks
(a) [6]
A variety of rice has been genetically modified so that it makes a substance that the human body converts into vitamin A. It is intended for regions where rice is the main food and vitamin A deficiency causes blindness. Discuss the advantages and disadvantages of introducing this crop.
Model Answer — 6(a)
advantage: it could reduce vitamin A deficiency and the blindness it causes, in people whose diet is mostly rice [1]
advantage: it needs no change of diet or of farming method — people eat the same food they already eat [1]
advantage: the modification does not reduce yield, and other modifications could be combined with it, such as pest resistance [1]
disadvantage: the modified gene could spread by pollen to other rice varieties or to wild relatives [1]
disadvantage: growing one modified variety widely reduces genetic variation, so a new disease could affect the whole crop [1]
disadvantage: seed may have to be bought each year, which is difficult for poor farmers; and some people object to the technology or want food labelled so they can choose [1]
⚠ If you missed marks here: The command word is discuss, so both sides are needed and a one-sided answer cannot reach full marks. Notice that the advantages here are specific to this crop — an answer that only recites general points about yield and insecticide has not engaged with the situation described.
(b) [3]
A student answers the question above by writing six paragraphs on why genetic modification is dangerous and should be stopped. Explain why this answer would not score highly, and state what it would need in order to do so.
Model Answer — 6(b)
the command word is discuss, which requires both advantages and disadvantages [1]
marks are reserved for the advantages, so a one-sided answer cannot reach more than about half however well it is written [1]
it also needs a conclusion drawn from the evidence given, rather than an opinion stated at the start [1]
⚠ If you missed marks here: Examiners are marking the balance and the reasoning, not the position you take. A calm answer that gives both sides and then decides will always beat a passionate answer that gives one.
(c) [3]
Explain why genetic modification, and not selective breeding, was used to produce this rice.
Model Answer — 6(c)
selective breeding can only use variation that already exists within the species [1]
no rice variety makes the substance, so there is nothing to select for and breed from [1]
genetic modification can transfer an individual gene from a different species, and does it in one generation rather than many [1]
⚠ If you missed marks here: The second mark is the one that turns a memorised comparison into an answer to this question. If the feature does not exist anywhere in the species, selective breeding has nothing to work with, however many generations you allow it.
Question 7 — Testing a Washing Powder
Total: 10 marks
(a) [5]
Design an investigation to find out whether a biological washing powder removes a protein stain better than a non-biological one at 30°C. Describe what you would do and how you would judge the result.
Model Answer — 7(a)
use identical squares of the same cloth, each with the same volume of the same stain, left for the same time before washing [1]
wash each square in the same volume of water at 30°C, with the same mass of powder [1]
wash for the same length of time, with the same amount of stirring [1]
include a control square washed in water alone [1]
judge the result by comparing each square against a printed colour scale, or by using a light meter [1]
⚠ If you missed marks here: The two marks people miss are the control and the method of judging. Without a water-only square you cannot show that the powder did anything, and without a scale or a meter your result is one person’s opinion of which square looks whitest.
(b) [3]
Explain why the same investigation carried out at 70°C would give a different result.
Model Answer — 7(b)
at 70°C the enzymes in the biological powder would be denatured [1]
the active site changes shape, so the stain molecules no longer fit and the enzymes stop working [1]
so the biological powder would lose its advantage, and the non-biological powder might remove more of the stain, because hot water loosens and dissolves stains more effectively [1]
⚠ If you missed marks here: The third mark is where this question is decided. It is not enough to say the biological powder gets worse — the non-biological one gets better, and above about 55°C it wins. That crossover is the whole point of the comparison.
(c) [2]
Suggest two reasons why washing at 30°C rather than 70°C is an advantage for the person doing the washing.
Model Answer — 7(c)
less energy is needed to heat the water, so the wash is cheaper [1]
lower temperatures are gentler on the fabric and on any dyes in it, so clothes last longer and colours fade less [1]
⚠ If you missed marks here: Heating the water is where almost all the energy in a wash goes, so this is a real and substantial saving rather than a marketing claim. Either of these two answers earns its mark; vague statements such as “it is better for the environment” need the reason attached.

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IGCSE Biology 0610 — Paper 4 Theory (Extended) — Topic 21: Biotechnology and Genetic Modification — Challenge Exam 2
For examination preparation only. Not an official Cambridge Assessment paper.