This is the shortest topic on the syllabus and one of the easiest to underscore, because there is so little to know that the examiner has to separate candidates on precision of language instead. A restriction enzyme cuts and DNA ligase joins — swap them and you lose two marks at once. Enzymes are denatured, never killed; the yeast can be killed, its enzymes cannot. Lactose-free milk has had its lactose broken down into glucose and galactose, not removed. Bread keeps the carbon dioxide and biofuel keeps the ethanol, from exactly the same reaction. Twelve traps, six data-led walkthroughs, six lookalike pairs, a three-part concept map, six answers that read well and score badly, and ten full practice questions — every one aimed at a place where a sensible-sounding sentence earns nothing at all.
Twelve traps that cost marks on Topic 21 challenge papers, spread across 21.1 to 21.3. Every one is an answer that sounds right and that mark schemes refuse.
Six challenge-level questions with real data, worked through in the order you should actually think about them. Try each part before revealing the next step.
Fig. W1 shows the juice obtained from five identical 100 g samples of apple pulp. Sample A had no pectinase; B to E had pectinase and were held at the temperature shown. (a) Calculate the increase in yield produced by pectinase at its best temperature. (b) Explain the shape of the results. (c) Sample E gave less juice than sample A. What does that tell you? [7]
Sample A has no enzyme. That is the number everything else is measured against, so the increase produced by the enzyme is always this bar subtracted from that one, never the height of the bar on its own.
The best enzyme result is 71 cm³, at 40°C. So the increase is 71 − 42 = 29 cm³. Quoting 71 gives the total, not the increase, and that single slip is the commonest arithmetic error on this kind of question.
Warming the mixture gives the molecules more kinetic energy. Enzyme and substrate collide more often, and more of those collisions are successful, so the reaction runs faster and more pectin is broken down in the 30 minutes allowed.
Notice what is not happening: there is no more enzyme at 40°C than at 20°C. Answers that say “the heat makes more enzyme” lose the mark.
Past the optimum, the shape of the active site changes. Pectin no longer fits, so the enzyme stops catalysing the reaction — and cooling the pulp back down would not bring the activity back, because denaturation is permanent.
Use the word denatured. Not destroyed, not killed, not broken.
If the enzyme at 80°C were merely working slowly, E would still be somewhere above A — slow is better than nothing. E is below A, which means the enzyme is contributing nothing at all: it has been completely denatured.
The small extra drop below A is just the hot pulp behaving slightly differently, and you would not be expected to explain it. What you are expected to notice is the comparison with the control.
Fig. W2 shows how two washing powders removed a protein stain at different temperatures. (a) State which powder contains enzymes and justify your choice. (b) Explain the shape of each curve. (c) Give the temperature at which the two powders perform equally, and explain the practical advice you would give a manufacturer. [7]
Powder P rises to a peak near 40°C and then falls away almost to nothing. Powder Q rises steadily throughout. Only one of those is the shape of an enzyme-catalysed process, because only an enzyme can be denatured.
So P is the biological powder. Justify it with the shape, not with the fact that P is better at 40°C — the curves cross, so “P is better” is not true everywhere and is not a justification.
Up to about 40°C, more kinetic energy means more successful collisions between the protease and the protein of the stain, so more of the stain is broken into small soluble molecules that dissolve and are rinsed away.
Above about 50°C the enzymes are denatured: the active site changes shape and the stain molecules no longer fit. What is still removed at 80°C is what the detergent alone can do.
Q contains no enzymes, so nothing in it is destroyed by heat. It improves steadily because hot water dissolves grease and loosens dirt better than cold water does, and the detergent works more effectively at higher temperatures.
This is the half of the answer that is usually missing. Q is not simply a worse product — above the crossing point it is the better one.
The curves cross at roughly 55°C. Below that, the biological powder wins; above it, the non-biological one does.
The advice follows: sell P for washes at 40°C or below, and label it clearly so that people do not use it hot. That is not a drawback but the selling point — a 40°C wash uses far less energy than a 70°C wash and is gentler on fabric and colours.
A fermenter growing a fungus for mycoprotein was monitored for 60 hours. The mass of fungus rose steeply for 20 hours and then stayed constant, although nutrients continued to be supplied and the temperature was held at 30°C throughout. (a) Suggest three reasons why growth stopped. (b) Explain why the temperature had to be held down rather than up. (c) A technician suggests heating the vessel to 100°C each night to keep it sterile. Evaluate this. [8]
Nutrients are still being supplied, and the temperature was constant. So “the food ran out” and “it got too hot” are both dead, however well written.
This is deliberate. The examiner is checking whether you read the stem before choosing your answer, and it is the single cheapest habit to acquire.
A growing culture releases waste products, which accumulate. They can be toxic to the organism, and they are often acidic, so the pH drifts away from the optimum and the enzymes work less well.
That is two of the three reasons, and they are two of the five controlled conditions — which is a good sign you are on the right track.
The nutrients are being replaced. The oxygen is not being replaced any faster than before, and a much denser culture uses it much faster. Once oxygen is limiting, aerobic respiration slows, less energy is available, and growth slows with it.
A fourth acceptable answer is simply space: the vessel has a fixed volume and the organisms are competing for room.
Respiration in a very large culture releases heat, so the vessel warms itself. The water jacket carries that heat away; without it the temperature would rise past the optimum and the enzymes of the organism would be denatured.
Which is exactly why the technician’s suggestion fails. Heating to 100°C would sterilise the vessel and destroy the culture at the same time. Sterilisation happens before the run; during it, sterility comes from filtering the incoming air.
Yeast was added to flasks of glucose solution. Flask 1 was sealed. Flask 2 had air bubbled through it. Flask 3 was boiled for five minutes and then sealed. Flask 4 was sealed and contained glucose solution only, with no yeast. After 24 hours ethanol was detected in flask 1 only. Explain the result in each flask, and state the purpose of flask 4. [6]
Sealed, so no oxygen. The yeast therefore respires anaerobically: glucose → ethanol + carbon dioxide. This is the only flask that had both a living organism and an absence of oxygen, and it is the only one that produced ethanol.
With air bubbling through, the yeast respires aerobically, giving carbon dioxide and water. There is no ethanol, and the yeast is perfectly healthy — better fed than in flask 1, in fact.
The trap here is writing that the oxygen killed the yeast. It did not. Yeast grows very well in air, which is exactly why a biofuel vessel has to be sealed.
Boiling killed the yeast and denatured its enzymes. Both statements are correct, and each uses the right word for the right kind of thing — the yeast is an organism, the enzymes are molecules.
This flask also shows that the glucose does not turn into ethanol simply by being warm, which is a second useful comparison.
No yeast, so nothing to respire, so no ethanol. That is the point: flask 4 shows that the ethanol in flask 1 came from the yeast rather than from the glucose breaking down on its own.
Whenever an experiment has a flask that seems to be missing the important ingredient, that is the control, and there is almost always a mark for saying what it rules out.
A company wishes to produce human insulin using bacteria. (a) Outline the six steps of the process. (b) Explain why the same restriction enzyme must be used at steps 1 and 2. (c) Explain why the insulin produced is identical to human insulin. [10]
Number them 1 to 6 down the page before you write anything else. It lets you see at a glance whether all six are there, and it lets the marker find each one without hunting through prose.
Six words to hang them on: cut, cut, join, insert, multiply, express.
1. A restriction enzyme cuts the human insulin gene out of human DNA, leaving sticky ends.
2. The same restriction enzyme cuts the bacterial plasmid, leaving complementary sticky ends. That word — complementary — is the one candidates leave out, and it is a mark.
3. DNA ligase joins the human gene into the plasmid, forming a recombinant plasmid.
4. The recombinant plasmids are inserted into bacteria. That sentence is the whole of step 4 — the syllabus says the specific details are not required, so do not invent a method.
5. The bacteria containing the recombinant plasmid multiply. 6. The human gene is expressed, so the bacteria make the human protein.
For (b): the same enzyme cuts in the same way, so the two sets of sticky ends are complementary — what sticks out on one is exactly the gap on the other, and the gene fits. A different enzyme would leave ends of the wrong shape.
For (c): the sequence of bases in the gene determines the sequence of amino acids in the protein. The gene is human, so the amino acid order is the human one, so the protein is the human protein.
A farmer grew an insect-resistant maize variety for eight years. For the first six, almost no crop was lost to the pest. In year seven damage reappeared, and by year nine it was as bad as before the modified variety was introduced. Tests showed the inserted gene was still present and still being expressed. (a) Explain what has happened. (b) Explain why planting a small area of unmodified maize alongside the crop would have slowed it. [7]
The gene is present and expressed, so the maize is still making the substance. Any answer that says the modification stopped working, or that the crop bred with an unmodified variety, is contradicting information you were given.
So the change is in the insects, not in the plants.
Within any insect population there is variation. A small number happened to carry an allele that made them resistant to the substance the maize produces. They were not made resistant by the crop — they were already there, and rare.
This is the step answers skip, and without it the rest is not natural selection but something closer to magic.
The modified maize is a selection pressure. Non-resistant insects die; resistant ones survive, feed and reproduce, passing the allele to their offspring.
Over generations the proportion of resistant insects rises, which is why the damage reappears gradually rather than suddenly. Individuals do not change; the population does.
A patch of unmodified maize lets a population of non-resistant insects survive alongside. They breed with the resistant survivors, so the resistant allele stays diluted rather than sweeping through the population.
It is a strategy for slowing selection, not a control experiment — and it is a good example of a Topic 21 question that can only be answered with Topic 18 biology.
Six pairs that look almost identical and have different answers. In this topic the distinction is nearly always a single word, and that word is nearly always the mark.
Click each node. The whole topic is three frameworks: why bacteria and how a gene gets into one, one enzyme idea doing four industrial jobs, and one reaction plus one vessel plus one six-step chain.
Six answers of the kind that read fluently and score badly. Find the fault before you reveal it.
Ten Cambridge-style challenge questions, each drawing on more than one sub-topic. Write your answer first, then reveal the model answer and the examiner notes.