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IGCSE Biology Paper 4 (Theory / Extended)

Topic 21: Biotechnology and Genetic Modification -- Challenge Exam 1
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 — A Bacterial Cell and Why It Is Useful
Total: 12 marks
Fig. 1.1 is not labelled. Work from the position and the size of each structure.
Fig. 1.1 — a bacterial cellNot drawn to scale.ABCDEE
(a) [4]
Fig. 1.1 shows a bacterial cell with its structures lettered A to E. Name the structures A, B, D and E.
Model Answer — 1(a)
A — the cell wall [1]
B — the cell membrane [1]
D — the main circular DNA [1]
E — a plasmid [1]
⚠ If you missed marks here: D and E are where the marks go. Both are DNA, and the difference between them is size and separateness — D is the main circular DNA of the cell, E is a small circle of DNA that is separate from it. Writing “chromosome” for D is risky, because a bacterium does not have chromosomes in the ordinary sense.
(b) [4]
Explain why the presence of structure E makes bacteria particularly useful for genetic modification.
Model Answer — 1(b)
a plasmid is a small circle of DNA [1]
it is separate from the main circular DNA of the cell [1]
so it can be removed and cut open and a gene inserted into it, without disturbing the DNA the cell needs to live [1]
and the bacterium can take a plasmid up again afterwards [1]
⚠ If you missed marks here: The mark most often missed is the second one. “Separate” is not decoration — it is the entire reason a plasmid is usable, because it can be worked on and returned on its own. An answer that calls a plasmid “part of the bacterial DNA” has removed the reason it is useful and usually loses two marks at once.
(c) [4]
A single bacterium is placed in a flask of nutrient solution at 37°C and divides every 20 minutes. Calculate the number of bacteria present after 4 hours, showing your working, and state one assumption you have made.
Model Answer — 1(c)
4 hours = 240 minutes, so the number of doublings is 240 ÷ 20 = 12 [1]
number of bacteria = 212 [1]
= 4096 [1]
assumption: nothing limits growth — for example that nutrients do not run out, or that waste products do not build up, or that no cells die [1]
⚠ If you missed marks here: Two different errors cost marks here. The first is multiplying rather than doubling, which gives an answer in the hundreds instead of the thousands — a doubling population rises as a power of two. The second is leaving out the assumption: in a real culture growth flattens off, which is exactly why a fermenter controls nutrient supply and waste products.
Question 2 — One Reaction, Two Industries
Total: 12 marks
(a) [3]
Write the word equation for anaerobic respiration in yeast, and name the two industries described in this question that use it.
Model Answer — 2(a)
glucose → ethanol + carbon dioxide [1]
bread-making [1]
production of ethanol for biofuel [1]
⚠ If you missed marks here: The equation must have both products. Writing “glucose → alcohol” loses the mark, and writing lactic acid confuses yeast with muscle — muscle produces lactic acid, yeast produces ethanol and carbon dioxide.
(b) [4]
The same reaction is used in both industries, but the useful product is different. Explain, for each industry, which product is useful and what happens to the other one.
Model Answer — 2(b)
bread-making: the useful product is the carbon dioxide [1]
it is trapped as bubbles in the dough, so the dough rises; the ethanol evaporates in the oven [1]
biofuel: the useful product is the ethanol [1]
it is separated from the mixture and burned as a fuel; the carbon dioxide is the by-product and escapes [1]
⚠ If you missed marks here: This contrast is examined more often than anything else in 21.2, and the mark scheme wants all four halves of it. The commonest incomplete answer names both useful products and never says what happens to the other one — that is two marks lost for a sentence you could have written in five seconds.
(c) [5]
A baker measured how far identical samples of dough rose in 40 minutes at five temperatures. The results were: 5°C, 2 mm; 20°C, 14 mm; 35°C, 31 mm; 55°C, 9 mm; 75°C, 0 mm. Explain the shape of these results as fully as you can.
Model Answer — 2(c)
the height risen is a measure of the carbon dioxide produced, and therefore of the rate of anaerobic respiration in the yeast [1]
from 5°C to 35°C the rate increases because molecules have more kinetic energy, so enzyme and substrate collide more often and more successfully [1]
above 35°C the rate falls because the yeast enzymes are denatured [1]
the active site changes shape, so the substrate no longer fits [1]
the optimum is at or close to 35°C — it cannot be stated more precisely because the readings are 15°C or 20°C apart [1]
⚠ If you missed marks here: Two marks are lost here more than any others. The first is writing that the cold has “denatured” the enzymes — low temperature only slows them, and they recover when warmed. The second is claiming the optimum is exactly 35°C; with readings this far apart, all you can honestly say is that it is at or near 35°C, and saying so is itself a mark.
Question 3 — Industrial Enzymes and a Fair Test
Total: 12 marks
Fig. 3.1 shows the results of an investigation into pectinase.
Fig. 3.1 — juice obtained from 100 g of apple pulpEach sample stood for 30 minutes at the temperature shown before the juice was collected.020406080volume of juice/ cm³42A58B71C49D38EA no pectinase · B pectinase 20°C · C pectinase 40°C · D pectinase 60°C · E pectinase 80°C
(a) [4]
Fig. 3.1 shows the volume of juice obtained from five identical 100 g samples of apple pulp. Sample A had no pectinase added; samples B to E had pectinase added and were then held at the temperature shown. Describe and explain the results.
Model Answer — 3(a)
adding pectinase increases the yield — B gives 58 cm³ against 42 cm³ for A [1]
because pectinase breaks down the pectin that holds the plant cells together, so more juice is released [1]
the yield rises to a maximum of 71 cm³ at 40°C because the enzyme works faster as the temperature rises [1]
above 40°C the yield falls because the pectinase is denatured; at 80°C the yield is below that of A, so the enzyme is contributing nothing at all [1]
⚠ If you missed marks here: The fourth mark is the one that separates candidates. Sample E gives less juice than the sample with no enzyme, which is evidence that the enzyme has stopped working completely rather than merely working slowly. An answer that says “the enzyme is slower at 80°C” has misread its own data.
(b) [4]
State the purpose of sample A, and describe two other variables that must have been kept the same for this to be a fair test.
Model Answer — 3(b)
sample A is the control [1]
it shows how much juice is released without the enzyme, so any extra juice can be attributed to the pectinase [1]
any valid controlled variable, e.g. the mass of apple pulp (100 g in each) [1]
a second valid controlled variable, e.g. the time allowed, the volume or concentration of pectinase, the variety and ripeness of the apples, or the method of crushing [1]
⚠ If you missed marks here: Naming the control is worth a mark; saying what it is for is worth a second, and most answers stop after the first. For the controlled variables, be specific — “keep everything else the same” is not an answer, and neither is naming the temperature, which is the variable being changed.
(c) [4]
A biological washing powder removes 86% of a protein stain at 40°C but only 6% at 80°C. A non-biological powder removes 30% at 40°C and 74% at 80°C. Explain these four results.
Model Answer — 3(c)
the biological powder contains enzymes, in this case a protease, which breaks the protein stain down into small soluble molecules that dissolve and are rinsed away [1]
at 40°C the enzymes are near their optimum temperature, so most of the stain is removed [1]
at 80°C the enzymes are denatured, so the 6% removed is what the detergent alone can do [1]
the non-biological powder contains no enzymes, so it is unaffected by denaturation and improves at the higher temperature because hot water dissolves and loosens the stain more effectively [1]
⚠ If you missed marks here: Four results need four explanations, and it is the last one that is usually missing. The non-biological powder is not simply a worse product — it beats the biological one above about 55°C, and explaining why is where the fourth mark lives.
Question 4 — Inside a Fermenter
Total: 12 marks
Fig. 4.1 shows a fermenter used to grow a fungus for mycoprotein.
Fig. 4.1 — an industrial fermenter, shown in sectionThe parts have been lettered instead of labelled.ABCDEFG
(a) [5]
Fig. 4.1 shows an industrial fermenter. State the five conditions that must be controlled inside it, and for each one name the part of the apparatus involved.
Model Answer — 4(a)
temperature — monitored by the temperature probe and controlled by the water jacket [1]
pH — monitored by the pH probe, with acid or alkali added [1]
oxygen — supplied through the sterile air inlet and distributed by the stirrer [1]
nutrient supply — added through the nutrient inlet [1]
waste products — removed through the outlet [1]
⚠ If you missed marks here: Five conditions, five marks — and sterility is not one of them. Filtering the air keeps other microorganisms out, which matters, but it is not on the list of five, so writing it instead of one of the five costs a mark.
(b) [4]
Explain why the temperature inside a large fermenter has to be lowered rather than raised, and explain what would happen to the culture if the cooling failed.
Model Answer — 4(b)
the culture respires aerobically on a very large scale, and respiration releases heat [1]
so heat builds up faster than it escapes, and the water jacket carries it away [1]
if cooling failed the temperature would rise above the optimum and the enzymes of the organism would be denatured [1]
the active site changes shape, reactions stop, and the culture dies, so no more product is made [1]
⚠ If you missed marks here: The first mark is the one people never think of: a fermenter has a heating problem, not a cooling problem, because respiration itself is the heat source. And use the word denatured for the enzymes — the organisms can be killed, but their enzymes are molecules and are denatured.
(c) [3]
In one run the mass of organism rose steeply for 20 hours and then stopped rising, although nutrients continued to be supplied and the temperature stayed constant. Suggest three reasons for this.
Model Answer — 4(c)
waste products have accumulated and are toxic to the organism [1]
the pH has fallen away from the optimum because the waste is acidic, so the enzymes work less well [1]
oxygen has become limiting in a dense culture — it is used faster than it can dissolve, so aerobic respiration slows [1]
⚠ If you missed marks here: The stem has closed two doors on purpose. Nutrients are still being added and the temperature is unchanged, so any answer built on either scores nothing however well it is written. Read what a stem rules out before you decide what to say — that habit is worth marks on every paper.
Question 5 — The Six-Step Process
Total: 10 marks
(a) [6]
Outline the process by which bacteria are genetically modified so that they make a human protein. Write your answer as six numbered steps.
Model Answer — 5(a)
a restriction enzyme cuts the human gene out of human DNA, leaving sticky ends [1]
the same restriction enzyme cuts the bacterial plasmid, leaving complementary sticky ends [1]
DNA ligase joins the human gene into the plasmid [1]
forming a recombinant plasmid [1]
the recombinant plasmids are inserted into bacteria, and the bacteria multiply [1]
the human gene is expressed, so the bacteria make the human protein [1]
⚠ If you missed marks here: Six marks, so six lines — numbering them lets you see at a glance whether all six are there. The two marks lost most often are the word complementary in step 2 and the word recombinant in step 3. And check the enzymes before you move on: a restriction enzyme cuts, DNA ligase joins. Swapping them costs two marks at once.
(b) [2]
Explain why the plasmid must be cut with the same restriction enzyme as the human DNA.
Model Answer — 5(b)
the same enzyme cuts in the same way, so the sticky ends it leaves on the plasmid are complementary to those on the gene [1]
so the two pieces pair together and the gene fits into the plasmid; a different enzyme would leave ends of a different shape and the gene would not fit [1]
⚠ If you missed marks here: This is a two-mark question with two distinct ideas in it: the ends are complementary, and that is what makes them fit. Answers that say only “so they match” give the conclusion without the reason and score one at most.
(c) [2]
A student writes: “DNA ligase cuts the gene out and a restriction enzyme sticks it into the plasmid.” Identify the error and write a corrected version.
Model Answer — 5(c)
the two enzymes have been given each other’s jobs: a restriction enzyme cuts and DNA ligase joins [1]
corrected: a restriction enzyme cuts the gene out of the human DNA, and DNA ligase joins it into the plasmid [1]
⚠ If you missed marks here: Naming an enzyme only earns anything if it is doing its own job. This swap is the single most-reported error in Topic 21, and it is worth checking every answer you write for it before you move on.
Question 6 — Modified Crops, and What Happens Next
Total: 12 marks
(a) [6]
Discuss the advantages and disadvantages of genetically modifying crop plants such as soya, maize and rice.
Model Answer — 6(a)
advantage: higher yield, because less of the crop is eaten by insect pests [1]
advantage: less insecticide needs to be sprayed, which is cheaper and affects fewer other species; or herbicide resistance lets weeds be sprayed off so the crop is not competing for light, water and mineral ions [1]
advantage: improved nutritional quality, for example rice modified to make a substance the body converts into vitamin A, in regions where deficiency causes blindness [1]
disadvantage: the modified gene can spread by pollen to wild relatives, so a weed may become herbicide resistant [1]
disadvantage: insect pests may become resistant over generations by natural selection, or insects that are not pests may be harmed, reducing biodiversity [1]
disadvantage: reduced genetic variation if one variety is grown everywhere, so a new disease could destroy the whole crop; or the cost of buying new seed each year [1]
⚠ If you missed marks here: The command word is discuss, which means both sides. Four advantages and no disadvantages cannot score more than about half, however well argued. Explain each point rather than listing it: “higher yield” alone is weak, “higher yield because less of the crop is eaten by pests” is a mark.
(b) [3]
A farmer says that genetic modification is “just a quicker version of selective breeding”. Give three ways in which the two processes differ.
Model Answer — 6(b)
genetic modification moves an individual gene; selective breeding works with whole organisms and all their genes [1]
genetic modification can transfer a gene between different species; selective breeding can only use variation already present within one species [1]
genetic modification produces a result in one generation; selective breeding takes many generations [1]
⚠ If you missed marks here: The middle difference is the one that matters most, because it is the thing selective breeding can never do — which is why a human gene can end up in a bacterium. If you can give only one difference, give that one.
(c) [3]
Insect pests feeding on a modified maize crop were killed for the first six years. By year nine the crop was being damaged again, and the modified gene was shown to be still present and working. Explain what has happened.
Model Answer — 6(c)
there was variation among the insects, and a few happened to be resistant to the substance the maize makes [1]
the modified crop acted as a selection pressure: the non-resistant insects died and the resistant ones survived [1]
the survivors reproduced and passed the allele on, so over several generations the resistant form became common in the population [1]
⚠ If you missed marks here: This is natural selection, and it needs all three stages: variation already present, selective survival, and the allele becoming more frequent over generations. Note the wording of the stem — the gene is still working, so any answer suggesting the modification has failed is contradicting the information given. And insects do not “learn” or “get used to” anything; individuals do not change, populations do.
Question 7 — From a Gene to a Medicine
Total: 10 marks
(a) [4]
Insulin used to be extracted from the pancreas of cattle and pigs. It is now made by genetically modified bacteria grown in fermenters. Explain two advantages of the modern method.
Model Answer — 7(a)
the protein is made from the human gene, so it is identical to human insulin [1]
so it works properly in the human body, and is less likely to cause a reaction than a protein from another species [1]
bacteria reproduce rapidly, so very large quantities can be produced [1]
production does not depend on the number of animals slaughtered, and no animals need to be used, which raises fewer ethical concerns [1]
⚠ If you missed marks here: Two of the four reasons from 21.1 are hiding in this answer: the ability to make a complex molecule — and specifically the human version of it — and the rapid reproduction rate. Answers that say only “it is cheaper” earn little, because cheapness is a consequence rather than a biological reason.
(b) [3]
The modified bacteria are grown in a fermenter rather than in an open tank. Explain why the air entering the fermenter is passed through a filter, and why the vessel is sterilised before the run but not during it.
Model Answer — 7(b)
the filter removes other microorganisms from the air, which would otherwise compete for the nutrients or contaminate the product [1]
the vessel is sterilised before the run so that no unwanted organisms are present when the culture is added [1]
it is not sterilised during the run because the heat would denature the enzymes of the organism being grown, killing the culture [1]
⚠ If you missed marks here: The third mark is the one this question is really about. Heating the vessel during the run sounds like a sensible way to keep it clean, and it would destroy exactly what you are trying to grow. Sterility during a run comes from the filter, not from heat.
(c) [3]
A student asks why the bacteria are left to multiply for two days before the insulin is collected, rather than the insulin being collected as soon as the plasmid has been inserted. Explain.
Model Answer — 7(c)
each bacterium makes only a very small amount of the protein [1]
the bacteria reproduce rapidly, and each daughter cell receives a copy of the recombinant plasmid, so it also carries the human gene [1]
so after two days there are enormous numbers of bacteria all expressing the gene, and the total yield of protein is commercially useful [1]
⚠ If you missed marks here: The second mark is the one that is usually missing: the plasmid is copied and passed on every time a cell divides, which is why the whole population ends up carrying the gene rather than just the original cell. Without that step the numbers would not help at all.

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IGCSE Biology 0610 — Paper 4 Theory (Extended) — Topic 21: Biotechnology and Genetic Modification — Challenge Exam 1
For examination preparation only. Not an official Cambridge Assessment paper.