Biotechnology and genetic modification

• Core, in two sentences: yeast turns glucose into ethanol and carbon dioxide, giving biofuel and risen bread; genetic modification changes DNA by removing, changing or inserting individual genes. Extended adds one more: a fermenter controls temperature, pH, oxygen, nutrient supply and waste products. Everything else in this chapter hangs off those three.
• Thirteen statements: 7 Core and 6 Supplement. The split is uneven by subtopic and worth knowing before you plan your revision — 21.2 is mostly Core at the front and entirely Supplement at the back, and the whole of the fermenter account sits on the Supplement side.
• If a Topic 21 question is worth more than three marks, it is almost certainly asking for a sequence or a balance. Decide which before you write a word.
• If you are a Core candidate and fix only two things before the exam, fix these: the gas that raises bread is carbon dioxide, and a herbicide-resistant crop survives the spray rather than killing weeds. If you are an Extended candidate, add a third: a running fermenter is cooled, not heated. Those three errors between them account for more lost Topic 21 marks than everything else on this page.
• If you can reproduce tables 4 and 5 from memory, you have most of the marks in Topic 21 already. Table 4 is the recall half of the topic; table 5 is the distinction the evaluation questions rest on.
• If drill 1 is the one you keep getting wrong, the fix is a single question, asked every time: has a gene been moved?
• If your mind goes blank on a fermenter question, ask: what problem does this part solve? Overheating, pH drift, no oxygen, uneven mixing, or invasion. Those five problems generate every feature on the diagram.
• Three visits, spaced a day, a week and a month apart, beat six visits in one evening. The forgetting between visits is what makes them work.

• State the two reasons bacteria are useful in biotechnology and genetic modification — a rapid reproduction rate and the ability to make complex molecules — and connect each to a production consequence rather than stating it alone.
• Write the word equation for anaerobic respiration in yeast: glucose → ethanol + carbon dioxide.
• Describe the role of anaerobic respiration in yeast in producing ethanol for biofuels, including why the vessel is closed, and say what makes a fuel produced this way renewable.
• Describe the role of anaerobic respiration in yeast in bread-making, from mixing with a sugar source to the effect of baking, naming carbon dioxide as the gas that raises the dough.
• Describe how pectinase increases the yield and the clarity of fruit juice, naming pectin and where it is found.
• Describe how proteases and lipases in biological washing powders remove protein and fat stains, and explain both the energy benefit and the temperature limitation.
• Plan an investigation into a biological washing powder, with its variables, controls, repeats, results table, graph and matched improvements — because 21.2.4's verb is investigate.
• State the definition of genetic modification in the syllabus wording: changing the genetic material of an organism by removing, changing or inserting individual genes.
• Outline the four named examples: human genes into bacteria to produce human proteins; genes into crop plants to confer resistance to herbicides; to confer resistance to insect pests; and to improve nutritional qualities.
• Explain why a herbicide-resistant crop does not itself kill weeds, and why “resistant” always means resistant to something particular.
• Calculate a rate of production as quantity divided by time, with correct units.
• Calculate a percentage increase or decrease from supplied values and quote it to a sensible number of significant figures.
• Identify an anomalous result and say what should be done about it.
• Discuss the two further reasons bacteria are useful — the few ethical concerns over their manipulation and growth, and the presence of plasmids — and describe a plasmid as a small circular DNA molecule that can receive a selected gene.
• Explain how lactase is used to make lactose-free milk, naming both products of the reaction.
• Distinguish lactose intolerance from milk-protein allergy, and explain why lactose-free milk helps only the first.
• Describe how a fermenter is used for the large-scale production of useful products by bacteria and fungi, and give the function of each feature that serves a controlled condition.
• Name the three required products — insulin, penicillin and mycoprotein — say which kind of organism makes each, and explain why two are separated from the culture while the third is the culture.
• Describe and explain why temperature, pH, oxygen, nutrient supply and waste products are each controlled, and what happens to yield if control is lost.
• Explain why cooling — not heating — is what a large running fermenter needs, and why the oxygen answer depends on the organism and the product.
• Explain what a restriction enzyme does, what a sticky end is, and why using the same enzyme on both molecules is what makes the gene fit the gap.
• Outline the process in the syllabus's six lettered parts, naming restriction enzymes, sticky ends, complementary sticky ends, DNA ligase, the recombinant plasmid, insertion into bacteria, their multiplication and expression of the gene — in order.
• Use the word vector correctly in its genetic sense, without confusing it with a disease vector.
• Trace the gene-flow sequence from GM pollen to a compatible wild relative, and say why it is possible rather than automatic.
• Explain the appearance of resistant pest populations as natural selection, without claiming the crop causes the mutation.
• Discuss balanced advantages and disadvantages of genetically modifying crops, using soya, maize and rice, each point linked to its mechanism.
• Write a conditional conclusion — a judgement that states what it depends on — instead of an absolute one, and distinguish correlation from causation when reading supplied data on yields or pest frequency.

Where you meet this outside biology. Cloudy apple juice and clear apple juice usually start as the same pressed liquid; the clear version has been treated with pectinase. The same chemistry works the other way round in a kitchen: pectin is what makes jam set, which is why fruits naturally rich in pectin set easily and others need pectin added. Same molecule, opposite intention.

• “Biotechnology means genetic modification.” Why wrong It collapses a whole field into one of its techniques, and makes it impossible to answer a question about bread or washing powder.
• “All industrial fermentation is anaerobic.” Why wrong Many fermenters have a sterile air inlet precisely because the organisms inside them respire aerobically. If oxygen were never wanted, the inlet would not exist.
• “Selective breeding inserts genes.” Why wrong Nothing is inserted. Parents are chosen, they reproduce, and their alleles recombine as they would in any cross. The change is in which combinations survive, over generations.
• “Enzymes are microorganisms.” Why wrong An enzyme is a protein molecule — thousands of times smaller than a bacterium, not alive, and incapable of reproducing. Microorganisms may be used to make the enzyme; the enzyme itself is the isolated product.
• “Microorganisms make these products for us.” Why wrong Yeast is not baking bread. It is respiring, because respiring releases the energy it needs to live, and the carbon dioxide is a waste product it has no use for. Human beings arrange conditions so that a by-product of the organism's own metabolism becomes useful. Writing as though the organism intends the outcome is treated as a real error, not a stylistic one.
• “The bacteria learn to make insulin.” Why wrong Bacteria do not learn anything. Their DNA has been changed: a gene they did not have has been inserted, and they read it as they read any other gene.
• “The modified bacteria are injected into the patient.” Why wrong The purified protein is what is used. The bacteria stay in the fermenter, and separating the insulin from them is precisely why a purification stage exists.
• “The whole human genome is put into the bacterium.” Why wrong One gene is transferred. The definition in 21.3.1 says individual genes, and this is the clearest illustration of why that phrase is in it.
• “The insulin comes out of the fermenter ready to use.” Why wrong It is mixed with bacteria, culture medium and everything else in the vessel. It must be separated and purified, and its quality checked, before it is prepared for medical use.
• “All biotechnology is genetic modification.” Why wrong It collapses a whole field into one technique. Bread, brewing and every enzyme application in this chapter involve no gene transfer at all. Say instead Biotechnology is the use of living organisms, cells or enzymes to make useful products. Genetic modification is one technique used within it.
• “All fermentation is anaerobic.” Why wrong True of yeast fermentation, but an industrial fermenter is a vessel, not a process, and many processes run in one are aerobic — which is why the sterile air inlet exists. Say instead In yeast, fermentation is anaerobic respiration. In a fermenter, whether oxygen is supplied depends on the organism and the product.
• “Yeast produces oxygen, which makes bread rise.” Why wrong Oxygen is not a product of any kind of respiration. It is a reactant in aerobic respiration and absent from anaerobic respiration altogether. Say instead Yeast respires anaerobically, producing carbon dioxide and ethanol. The carbon dioxide is trapped as bubbles and makes the dough rise.
• “The ethanol makes the bread rise.” Why wrong Ethanol is a liquid. It cannot form the bubbles that expand in the dough. Say instead Carbon dioxide, a gas, raises the dough. Most of the ethanol evaporates during baking.
• “Baking kills the yeast enzymes.” Why wrong Enzymes are protein molecules and are not alive, so they cannot be killed. Say instead Baking kills the yeast and denatures its enzymes.
• “Fermenters should be kept as hot as possible.” Why wrong Above the optimum, enzymes denature and cells die. And a large fermenter is already overheating from the heat its own microorganisms release. Say instead The culture is held near a suitable temperature, and cold water in the cooling jacket removes the excess heat produced by respiration.
• “The fermenter is heated to make the reactions go faster.” Why wrong The control problem in a running fermenter is removing heat, not adding it. Say instead Respiration releases thermal energy, so the culture is cooled to keep it near the optimum for its enzymes.
• “Stirring gives the bacteria more oxygen.” Why wrong Stirring cannot create oxygen. The oxygen came from the sparger; the impeller only moves it about. Say instead Stirring distributes the oxygen, nutrients, microorganisms and heat evenly through the culture.
• “Sterilisation means adding antibiotics to the culture.” Why wrong Sterilisation happens before the culture starts and removes all microorganisms, including the one you are about to add. Say instead The vessel and medium are sterilised and the incoming air is filtered, and then the chosen microorganism is inoculated into the sterile vessel.
• “Enzymes are tiny microorganisms.” Why wrong An enzyme is a protein molecule, thousands of times smaller than a bacterium. It does not respire, grow or reproduce. Say instead An enzyme is a protein that catalyses a reaction. Microorganisms may be used to produce it, but the enzyme itself is an isolated molecule.
• “The enzymes in washing powder eat the stains.” Why wrong Eating is something an organism does. An enzyme catalyses a reaction on a substrate. Say instead Protease hydrolyses protein in the stain into amino acids, and lipase hydrolyses fats into fatty acids and glycerol. The small soluble products dissolve and wash away.
• “Pectinase digests the whole plant cell wall.” Why wrong It acts on pectin. The cellulose of the cell wall is a different substance and needs a different enzyme. Say instead Pectinase breaks down the pectin in the cell walls and between the cells, so the cells separate and their contents are released.
• “Lactose-free milk is suitable for anyone who reacts badly to milk.” Why wrong It addresses lactose only. A milk-protein allergy is a response to the protein, which is entirely unchanged by lactase. Say instead Lactase-treated milk is suitable for many people with lactose intolerance. It does not help with milk-protein allergy, because the protein is still present.
• “Lactase is the sugar in milk.” Why wrong One letter, two different kinds of molecule. The sentence has an enzyme being digested by a sugar. Say instead Lactose is the sugar; lactase is the enzyme that hydrolyses it into glucose and galactose.
• “The bacteria learn to make insulin.” Why wrong Bacteria do not learn. Their DNA has been changed. Say instead The human insulin gene has been inserted into the bacterial DNA, and the bacteria express it as they would any other gene.
• “The modified bacteria are injected into patients.” Why wrong The product is a purified protein. Separating the insulin from the bacteria is an explicit step in the process. Say instead The insulin is separated from the culture and purified before being prepared for medical use.
• “The whole human genome is inserted into the bacterium.” Why wrong The definition of genetic modification specifies individual genes, and a plasmid could not carry a genome anyway. Say instead One selected gene — the gene that controls insulin production — is inserted.
• “Selective breeding is a kind of genetic modification.” Why wrong No gene is moved in selective breeding. The population changes because of which organisms reproduced, over many generations. Say instead Selective breeding chooses parents with desired features and breeds them; genetic modification directly inserts, removes or changes a gene.
• “Herbicide-resistant crops kill the weeds around them.” Why wrong The crop does nothing to the weeds. The herbicide kills them; the modification only lets the crop survive the spray. Say instead The crop survives being sprayed with a particular herbicide, so that herbicide can be applied to the whole field to control the weeds.
• “Insect-resistant crops mean pests can never be a problem again.” Why wrong The pest population is still present and now under selection. Resistant individuals survive and reproduce, and resistance spreads. Say instead Crop losses fall, but the crop acts as a selection pressure, so resistant pests may become more common over generations.
• “The GM crop causes the pests to mutate.” Why wrong Variation existed before the crop was planted. Selection changes how common an allele is; it does not create it. Say instead A few pests already carried a resistance allele. The crop acted as a selection pressure, so those individuals survived and reproduced and the allele became more common.
• “GM genes spread to every plant and animal nearby.” Why wrong An allele moves the way any allele moves — by successful reproduction between compatible plants. Transfer between distant species is exactly what requires a laboratory. Say instead Pollen may reach a compatible wild relative; if cross-pollination and fertilisation occur, the offspring may carry the allele and the feature may spread in that population.
• “All GM is unsafe.” — and — “GM has no risks.” Why wrong Both are absolute claims about a whole technology on the evidence of one application, and each ignores everything on the other side of the balance sheet. Say instead Name the application, give the mechanism, give a benefit and a risk that follow from it, and conclude with what the judgement depends on.
• Yeast produces oxygen.
• Fermenters are heated as hot as possible.
• Enzymes are killed / enzymes eat stains.
• Herbicide-resistant crops kill weeds.
• All GM is unsafe / GM has no risks.

• Read the command words down the map. Topic 21 uses six of them: state once, describe five times, explain once, outline twice, discuss twice and investigate once. The two discuss statements are both Supplement, and both want the same thing — both sides, each with a mechanism, ending in a judgement that names what it depends on. A list answers only part of discuss, however accurate the list is.
• A timing note for Paper 4. A discuss question tempts you to write a page. It does not need one. Whatever it is worth, read the mark allocation and answer to it: three clearly separated advantages with mechanisms, three disadvantages with mechanisms, and two sentences of conditional conclusion is a complete answer to a six-mark discuss, and takes about eight minutes. The extra page costs time you need elsewhere in the paper.
• Two words worth spelling carefully. Write fermenter, not fermentor, and write plasmid, not plastid — a plastid is a plant organelle and belongs in a different topic entirely.
• Use the precise term, not a paraphrase. On the cold side write kinetic energy. On the hot side write denatured and active site. Each of those names the cause exactly; “the enzymes slow down” and “the enzymes stop working” describe the same two halves without distinguishing them, which is the whole point of the question.
• Give the products by name. “Lactase breaks down lactose” is one mark at best. “Lactase hydrolyses lactose into glucose and galactose” is a complete answer, and naming both products is the detail that separates it from a vague one.
• Mechanism first, benefit second. If you find yourself writing two benefits and no mechanism, go back and add what the enzyme does to the molecule — that clause is what the benefit rests on, and it is the half most often left out of these answers.
• Practise it as a pairing, not as a label. A fermenter diagram is naturally asked about by pointing at a part and saying “name it and state its function”, and the function is the half that carries the reasoning. Practise saying the function out loud while pointing at the diagram until the pairing is automatic — and finish each one with the condition it serves, because that is the sentence 21.2.7 is actually asking for.
• If a question does raise it. Nothing stops an unfamiliar-context question mentioning a contaminated fermenter and asking you to suggest why the yield fell — that is data interpretation, which is assessed. The word worth having ready is competition. What is not assessed is reciting sterilisation procedure, and you should not spend revision time on it.
• Notice the shape of the list. One example is a microorganism making a medicine; three are crops in a field. If a question gives you a four-mark “outline examples of genetic modification”, that division is the plan for your answer: one sentence on bacteria and a human protein, then one each on herbicide resistance, insect-pest resistance and nutritional quality. Four examples, four marks, and nothing about plasmids or enzymes needed anywhere in it.
• Mechanism before benefit, every time. All three modifications follow the same two-sentence shape: a gene is inserted that does X; because of X, the benefit is Y. Writing only Y states the outcome without the biology that produced it, which is the half of the answer this topic is actually testing.
• Four words, and the word that connects them. The four names are restriction enzyme, sticky ends, DNA ligase and recombinant plasmid, and an answer that uses all four in the right places has the statement. The connecting word is same. Write “the same restriction enzyme is used to cut the plasmid” and you have explained why the ends are complementary without needing another sentence.
• Six parts, so write six separate statements. Running three parts into one long sentence buries two of them, so number them on your page if it helps. The two most easily dropped are (e) and (f) — it is tempting to describe getting the plasmid into the bacterium and then jump straight to “insulin is collected”, leaving out both the multiplication and the expression, which are the two parts that actually explain where the protein came from.
• A sentence worth having ready in an evaluation. “This risk is not unique to genetic modification: repeated use of a single insecticide selects for resistance in the same way.” It shows you are comparing the technology with the realistic alternative rather than with a perfect one, which is what evaluate asks for.
• Naming a crop costs four words. “A herbicide-resistant crop” is fine; “a herbicide-resistant soya crop” is better, because soya is one of the three crops 21.3.4 names and it anchors the answer to a real case. Do not, however, invent detail about a named variety you have not been given — naming the crop is not the same as claiming to know its yield figures.
• The three parts most easily left out. The control — without a no-powder run you cannot separate the enzymes from the hot water, so the whole comparison is weaker. The end-point definition — saying how you decided the stain had gone. And matched improvements: an improvement is only an improvement if it fixes a limitation you actually named, so write them as a pair.
• Significant figures. Give your answer to about the same number of significant figures as the data you were given. If a table shows 46 cm³ and 10 minutes, answer 4.6 cm³/min, not 4.600000 cm³/min. Writing more figures than the data support is treated as a misunderstanding of the measurement, not as extra care.
• One habit that lifts every answer in this topic. Before writing, look at the mark allocation and decide how many separate statements you owe. Then write that many, each as its own clause. It suits this topic particularly well, because almost every question in it is a sequence or a balance.
• Marking yourself honestly. Award a mark only where the marking point actually appears in your own words. The easiest way to over-mark yourself in this topic is to credit an answer that gives the benefit but never gave the mechanism — look for the because in your own sentence before you tick.

• Topic 21 behaves differently from the physiology topics around it. There is very little to draw from memory, almost no calculation unique to the topic, and a great deal of writing that has to be structured rather than merely correct. Knowing the shape of the questions is worth as much here as knowing the content.
• Three habits do most of the work in this topic, and they are all about structure rather than recall.
• Name the mechanism before the benefit. “Pectinase increases juice yield” is one statement. “Pectinase breaks down the pectin in the plant cell walls, so more juice is released” is two, and the second one earns the first.
• Say which condition, and why that one. Fermenter questions are marked on the pairing. A control named without its reason is half an answer, and a reason given for the wrong control is none.
• Keep possible and certain apart. In a discuss question, write may, could and if. A risk stated as a certainty is treated as an error, not as a strong opinion.