Hi Tara. This is the last one. Every other topic on 0610 is now behind you, which means this guide can lean on any of them without apology — and it will, because Topic 21 is almost entirely made of ideas you already own, put to industrial use.
Here is the shape. 21.1 asks why bacteria, of all things, and the answer is four specific reasons, none of which is “they are simple”. 21.2, 21.3 and 21.4 are all one syllabus sub-topic, split three ways here because Cambridge has bundled three unrelated processes together: yeast and its one reaction; three industrial enzymes; and the fermenter that grows the organisms. 21.5 is genetic modification — the definition, the four named uses, and the six-step chain that is the highest-value thing in the topic. 21.6 is the checklist for the night before.
Three warnings that will each save you marks. First: a restriction enzyme cuts, and DNA ligase joins. Swapping them is the error examiners report most often here, and it costs two marks at once. Second: enzymes are denatured, never killed — the yeast can be killed, its enzymes cannot. Third: when a question says discuss, it wants both sides and a judgement, and the most confident one-sided answer in the room cannot score more than half.
Four Reasons, and None of Them Is “Because They Are Simple”
Almost everything in this topic is done by bacteria or to bacteria, so Cambridge starts by asking why they are the organism of choice. There are four reasons on your syllabus, two Core and two Supplement, and each of them is a mark. What is not a mark is the answer everybody writes first — that bacteria are useful because they are small, or simple, or easy. Say what they actually do.
| The reason | What it means | Why it matters industrially |
|---|---|---|
| Rapid reproduction rate Core | A bacterium divides to give two bacteria, and in good conditions it can do that roughly every 20 minutes. | A few cells become billions overnight. You do not have to wait a generation for your product, and you never run out of the organism. |
| Ability to make complex molecules Core | Bacteria have ribosomes and the rest of the machinery for assembling amino acids into proteins. | Given the right gene they will build a large protein such as human insulin — something no simple industrial process can do. |
| Few ethical concerns Supplement | People raise far fewer objections to growing and manipulating bacteria than to the same work on animals. | Work can go ahead, and be scaled up, with fewer restrictions than research using animals attracts. |
| Presence of plasmids Supplement | A plasmid is a small circle of DNA, separate from the main circular DNA of the cell. | It is a ready-made way in. A gene can be put into a plasmid and the plasmid can be taken up by the bacterium. |
You met the last of those in 2.1, in the list of bacterial cell parts: cell wall, cell membrane, cytoplasm, ribosomes, circular DNA and plasmids. At the time the plasmid was just another word to learn. This is the topic where it turns out to matter.
A plasmid is a small circle of DNA that is separate from the main circular DNA. An answer that says a plasmid is “part of the bacterial DNA” or “a piece of the chromosome” loses the mark, because being separate is the whole point — it can be taken out, worked on and put back without touching the rest of the cell.
And notice the shape: circular, both of them. Bacteria have no nucleus and no linear chromosomes, which is why the word chromosome is best avoided here altogether.
This is the kind of arithmetic Cambridge uses to make the phrase “rapid reproduction rate” mean something. Do it in doublings, never in multiplications of 20.
In a real fermenter the population would not keep doubling. Nutrients run out, waste products build up and the pH drifts, which is exactly why a fermenter controls those things. You will meet all of that in 21.4 — and a question that asks why the real curve flattens off is asking you to name those limits.
Fast · Complex · Ethical · Plasmids. Fast reproduction, complex molecules, few ethical concerns, and plasmids to carry a gene in. Four reasons, four marks, and the last two are the Supplement ones you will be the only person in the room to remember.
Cambridge puts yeast, the industrial enzymes and fermenters together in one sub-topic, 21.2 Biotechnology, but they are three unrelated processes with nothing in common except that a living thing is doing the work. They are split across three sections here so that each one gets taught properly — yeast now, industrial enzymes in 21.3, fermenters in 21.4.
One Reaction You Already Know
Yeast is a fungus. Not a bacterium — that is worth fixing now, because a question that says “name the microorganism” will not accept bacterium, and the fermenter objective in 21.4 deliberately says bacteria and fungi.
When yeast respires anaerobically — without oxygen — it does this, and you met it in 12.3:
That is the whole of the science in this section. Two industries use it, they are examined constantly, and the examiners are testing one thing: which of the two products you actually want.
Ethanol for Biofuels
Sugar from a crop — sugar cane, maize, sugar beet — is dissolved in water and yeast is added. Air is kept out, so the yeast respires anaerobically and converts the sugar to ethanol. The ethanol is then separated from the mixture and burned as a fuel, on its own or mixed with petrol.
Here the ethanol is the useful product. The carbon dioxide is simply the by-product, and it bubbles off.
Carbon Dioxide for Bread
Yeast is mixed with flour, sugar and warm water and the dough is kneaded and left in a warm place. The yeast respires the sugar anaerobically. The carbon dioxide is trapped as bubbles in the stretchy dough, so the dough rises. When the loaf goes into the oven, the bubbles expand further and the heat kills the yeast, so the rising stops — and the ethanol evaporates, which is why bread is not alcoholic. The holes you can see in a slice of bread are those trapped bubbles.
Here the carbon dioxide is the useful product. The ethanol is lost.
Same organism, same reaction, same two products — opposite one kept. In bread you want the carbon dioxide and lose the ethanol; in biofuel you want the ethanol and lose the carbon dioxide. If a question in this sub-topic looks hard, check first whether it is really just asking you which product is wanted.
Fermentation is a fine word, and on its own it is a weak answer. The process is anaerobic respiration, and the mark is usually for the word anaerobic — it is what makes the yeast produce ethanol instead of the carbon dioxide and water it would produce with oxygen.
Similarly, do not write that the yeast “makes alcohol” without saying why oxygen is excluded. If oxygen were present, the yeast would respire aerobically and there would be no ethanol at all.
Two different explanations are needed — one for the rise and one for the fall — and a single sentence about “the best temperature” will not get either mark.
Both sentences are correct and they are not interchangeable. The yeast is a living organism, so it can be killed. Its enzymes are protein molecules, so they are denatured. Use the right word for the right thing and you will never lose the mark that the word “killed” costs most candidates.
Three Enzymes Doing Three Industrial Jobs
This is the part of 21.2 where everything you learned about enzymes in 5.1 and 5.2 comes back and earns money. Three uses are named on your syllabus — pectinase in fruit juice, enzymes in biological washing powders, and lactase for lactose-free milk — and every one of them is examined through the same two ideas: an enzyme breaks a large molecule into smaller ones, and it has an optimum temperature above which it is denatured.
Pectinase in Fruit Juice Production
Plant cell walls and the material between plant cells contain pectin, which holds the cells together. Crush an apple without doing anything else and you get a thick, cloudy pulp that only releases some of its juice.
Pectinase breaks down the pectin. The cells come apart, the pulp collapses, and the juice runs out freely. Two things improve as a result: the volume of juice obtained from the same mass of fruit goes up, and the juice is clearer, because the fragments that made it cloudy have been broken down.
The bar for “no pectinase” is the control. Without it you could not say that the extra juice came from the enzyme rather than from standing for 30 minutes. If a question asks why that sample was included, that is the answer — and if a question asks you to design the experiment, that is the bar you must remember to include.
Notice the last bar as well. At 80°C the yield is lower than with no enzyme at all. The enzyme is denatured, so it does nothing; the small extra drop is just the pulp behaving slightly differently when hot. An answer that says “the enzyme works more slowly at 80°C” misses the point — it has stopped working, permanently.
Biological Washing Powders
A washing powder described as biological contains enzymes. The stains that are hardest to shift are large insoluble molecules stuck to the fibres:
- Proteases break down protein stains — blood, egg, grass, sweat — into amino acids and other small soluble molecules.
- Lipases break down fat stains — grease, butter, oil — into fatty acids and glycerol.
The products are small and soluble, so they dissolve in the water and are rinsed away. That is the whole mechanism, and the mark is for the words broken down into small soluble molecules, not for “the enzymes remove the stain”.
Two answers are needed, not one, and the second one is where the marks are.
“Investigate the use of biological washing powders” is in the syllabus, so it can be set as an experiment to plan. A safe design: identical squares of the same cloth, each with the same volume of the same stain, left for the same time before washing. Wash each square in the same volume of water containing the same mass of powder, for the same length of time, with the same amount of stirring.
Change one thing only — the temperature, or the type of powder. Include a square washed in water alone as a control. Judge the result by comparing each square against a printed colour scale, and get a second person to judge them as well, because deciding how clean something looks is subjective. That word is worth a mark on its own when you are asked to evaluate the method.
Lactase and Lactose-Free Milk
Milk contains the sugar lactose. Digesting it needs the enzyme lactase, and a large proportion of the world’s adults do not produce enough of it. Undigested lactose passes into the large intestine, where bacteria act on it, causing pain, wind and diarrhoea.
The solution is to do the digestion before the milk is drunk. Lactase is added to the milk, and it breaks the lactose down into glucose and galactose, which are small, soluble and absorbed without any lactase of your own. The result is sold as lactose-free milk.
Two consequences follow, and both are examined:
- The milk tastes slightly sweeter, because glucose and galactose taste sweeter than the lactose they came from. No sugar has been added — one sugar has been broken into two.
- People who cannot digest lactose can drink it without the symptoms, so they keep milk in their diet.
“Lactose-free milk has had the lactose removed.” It has not. The lactose is broken down into glucose and galactose by lactase, and both of those products stay in the milk. Nothing is filtered out and nothing is thrown away.
If you can write “lactase breaks lactose down into glucose and galactose” without hesitating, you have this objective. Note the spelling too: lactose is the sugar, lactase is the enzyme, and the “-ase” ending means enzyme every time.
Pectin→ase breaks down pectin. Lact→ase breaks down lactose. Prote→ase breaks down protein. Lip→ase breaks down lipids, which are fats. Nothing in this section requires you to memorise which enzyme does what — the name is the answer, as long as you write the products as well as the substrate.
Everything in this section is Supplement content, which for you means it is all examinable and most of the class will skip it.
A Very Large Tank With Very Tight Controls
A fermenter is a vessel used for the large-scale production of useful products by bacteria and fungi. Three products are named on your syllabus, and it is worth knowing which organism makes each:
| Product | Made by | What it is for |
|---|---|---|
| Insulin | Genetically modified bacteria | The hormone that lowers blood glucose. Used to treat people whose own supply is inadequate. |
| Penicillin | A fungus (Penicillium) | An antibiotic — it kills bacteria, which is why it does nothing against a virus. You met that in 15.1. |
| Mycoprotein | A fungus | A protein-rich food, grown as a meat substitute. |
Notice that only the first of those involves genetic modification. Penicillin and mycoprotein come from fungi doing what they already do naturally, on an industrial scale. A question that assumes every fermenter contains modified bacteria has misread the syllabus.
The Five Conditions, and the Hardware That Controls Each One
This is the objective that carries the marks: describe and explain the conditions that need to be controlled in a fermenter. There are five, and a good answer says for each one why it matters and how it is controlled.
| Condition | Why it must be controlled | How it is controlled |
|---|---|---|
| Temperature | The enzymes of the organism have an optimum temperature. Too cold and growth is slow; too hot and the enzymes are denatured and the culture dies. Respiration in a huge culture releases a great deal of heat, so the tank warms itself. | A temperature probe monitors it and a water jacket carries the excess heat away. Cold water flows in and warm water flows out. |
| pH | Enzymes also have an optimum pH. Waste products released by the culture are often acidic, so the pH drifts downwards during the run and the enzymes work less well. | A pH probe monitors it continuously and small amounts of acid or alkali are added to hold it steady. |
| Oxygen | Most of these cultures respire aerobically, and aerobic respiration releases far more energy than anaerobic. That energy is needed both for growth and for making the product. | Sterile air is bubbled in at the bottom through a filter, and the paddle stirrer spreads the bubbles through the whole tank. |
| Nutrient supply | The organism needs a carbon source, usually a sugar, plus other nutrients. If they run out, growth stops and so does production. | Nutrients are added through the nutrient inlet, and the stirrer makes sure they reach every part of the tank rather than settling. |
| Waste products | Waste builds up as the culture grows. It can be toxic to the organism and it changes the pH, so growth slows and eventually stops. | Waste is drawn off through the outlet, and in a continuous process fresh medium replaces it. |
T · pH · O · N · W — Temperature, pH, Oxygen, Nutrients, Waste. Five fingers, five conditions. Then for each one ask the two questions the mark scheme asks: why does it matter? and what piece of hardware deals with it? A six-mark question on a fermenter is almost always three conditions × two marks.
How a fermenter is used, start to finish
- Sterilise. The empty fermenter is cleaned and sterilised with steam. The nutrient medium (water, a sugar such as glucose, and a source of nitrogen) is sterilised too. This kills any microorganisms already present, so none can compete for the nutrients or contaminate the product.
- Inoculate. A small, pure culture of the chosen organism is added: genetically modified bacteria for insulin, the fungus Penicillium for penicillin, and a fungus for mycoprotein.
- Grow under control. The organism grows and reproduces. The probes, the water jacket, the sterile air supply, the pH control and the stirrer hold the five conditions at their best values, so growth and production are as fast as possible.
- Harvest. After a set time the contents are run off (a batch process). Alternatively, some liquid is drawn off continuously and replaced with fresh medium (a continuous process).
- Separate and purify the product. Penicillin is released by the fungus into the liquid: the fungus is filtered off, and the penicillin is extracted from the liquid and purified. Insulin is made by the bacteria: it is separated from the bacteria and the liquid, and purified so that it is safe to inject. Mycoprotein is the fungus itself: the fungal mass is filtered out of the liquid and then processed into food.
Why use a fermenter at all? It makes very large amounts. The product is the same every time, because the conditions are the same every time. And it works all year round, whatever the weather.
If you are asked why the fermenter has a paddle stirrer, there are three creditable answers and you should give more than one: it keeps the oxygen bubbles spread through the liquid rather than letting them rise straight out; it keeps the nutrients evenly distributed so no region runs short; and it keeps the temperature even, so there are no hot spots near the middle where heat builds up.
It also stops the microorganisms settling at the bottom, where they would have neither oxygen nor nutrients.
Before the run, the vessel is sterilised and the air entering it is filtered, so no other microorganisms get in to compete for nutrients or contaminate the product. This is why the diagram says sterile air inlet.
What does not happen is heating the tank during the run to keep it sterile. That would denature the enzymes of the very organism you are growing. It is a favourite distractor because it sounds sensible, and it kills the culture.
Note also that sterility is not one of the five named conditions. If the question says “state the five conditions that are controlled”, sterility is not one of your five.
This is an AO2 question: nothing here is a fact you can recall, so work from what limits a growing population.
The Definition, Word for Word
by removing, changing or inserting individual genes.
Learn that sentence. It is a mark on its own and it is also a defence against the commonest muddle in this sub-topic — the idea that genetic modification means moving a whole chromosome, or breeding two organisms together. Neither. One gene at a time.
The Four Examples the Syllabus Names
| Gene inserted into… | What it gives the organism | Why anyone wants it |
|---|---|---|
| Bacteria — a human gene | The bacteria make a human protein. | Human insulin can be produced in enormous quantity in a fermenter. It is identical to the human protein, so it works properly in a human body. |
| Crop plants — herbicide resistance | The crop survives a herbicide that kills the weeds around it. | Weeds can be sprayed off without harming the crop, so the crop is not competing for light, water and mineral ions. |
| Crop plants — insect pest resistance | The crop resists insect pests. | Less of the crop is eaten, so yield rises, and less insecticide has to be sprayed. |
| Crop plants — improved nutritional quality | The crop makes a substance it did not make before. | Rice has been modified to make a substance the body converts into vitamin A, in regions where deficiency causes blindness. |
The Six-Step Process
This is the part of the topic that most rewards being organised. Cambridge sets it as an ordered chain, marks it one step at a time, and the example is always the same: making a human protein in bacteria. Six steps, six marking points. Anything you can say about the process that is not one of these six steps is worth nothing, so do not spend time on it.
The whole chain, with every step in place.
Watch what a complete answer looks like before you try one. Read it left to right along the top row, then left to right along the bottom row.
1 — a restriction enzyme cuts the human gene out of human DNA. The cut is staggered, not straight across, so each end of the cut is left with a short single-stranded overhang. Those overhangs are called sticky ends.
2 — the same restriction enzyme cuts the bacterial plasmid open. This is the step candidates skip, and it is where the cleverness of the method lives. Because it is the same enzyme, it cuts in the same way, so the plasmid is left with sticky ends that are complementary to the ones on the gene. What sticks out on one is exactly the gap on the other.
3 — DNA ligase joins the human gene into the plasmid. The sticky ends pair up because they are complementary, and ligase seals the joins. The plasmid now contains DNA from two sources, so it is called a recombinant plasmid.
4 — the recombinant plasmids are inserted into bacteria. Your syllabus says in as many words that the specific details are not required, so do not learn a method and do not invent one. “Inserted into bacteria” is the whole marking point.
5 — the bacteria containing the recombinant plasmid multiply. This is where the rapid reproduction rate from 21.1 pays off: one modified cell becomes billions, and every one of them carries the human gene.
6 — the human gene is expressed, so the bacteria make the human protein. The word expressed is the mark. It means the gene is actually being used to make its protein rather than just sitting there.
A restriction enzyme cuts. DNA ligase joins. Swapping those two is the error examiners report most often in this topic, and it wrecks two marks at once because the steps then come out in the wrong order.
A hook that works: ligase is a ligature — a stitch, something that ties two things together. And a restriction enzyme restricts the DNA by chopping it into pieces.
Step 3 has been removed. Write it out before you open the answer.
You need the enzyme, what it does, and the name of what you end up with. Three things, and the mark scheme wants all three.
DNA ligase joins the human gene into the plasmid, forming a recombinant plasmid.
Check your version against three separate things. Did you name DNA ligase, not just “an enzyme”? Did you say it joins rather than sticks or attaches? And did you use the word recombinant, which is the name of the product and a mark in its own right?
One more check that catches people out: the gene goes into the plasmid, not into the bacterium. That happens at step 4, and running the two together loses one of them.
Steps 1 and 2 have been removed — the two cutting steps.
Careful here. These are two separate marking points even though the same enzyme does both, and the second one has an extra word in it that the first does not.
1 — a restriction enzyme cuts the human gene out of the human DNA, leaving sticky ends.
2 — the same restriction enzyme cuts the bacterial plasmid, leaving complementary sticky ends.
The extra word in step 2 is complementary, and it is the reason the whole method works. If you cut the plasmid with a different restriction enzyme, its sticky ends would be a different shape and the gene would not fit. That is why an examiner will accept “the same enzyme” as a marking point on its own.
A second check: it is sticky ends, plural, and they are single-stranded overhangs left by a staggered cut. If your answer said the enzyme cuts the DNA straight across, the rest of the process cannot happen.
All six steps have gone. Write the whole chain out on paper first — six numbered lines, in order — and then fill in the five key terms below.
- A restriction enzyme cuts the human gene out of human DNA, leaving sticky ends.
- The same restriction enzyme cuts the bacterial plasmid, leaving complementary sticky ends.
- DNA ligase joins the human gene into the plasmid, forming a recombinant plasmid.
- The recombinant plasmids are inserted into bacteria.
- The bacteria containing the recombinant plasmid multiply.
- The human gene is expressed, so the bacteria make the human protein.
Spelling is not marked strictly, but these five terms are, so it is worth getting them exactly right: restriction enzyme, sticky ends, DNA ligase, recombinant plasmid, expressed.
Genetically Modifying Crops — Both Sides
Your syllabus says discuss the advantages and disadvantages of genetically modifying crops, including soya, maize and rice. “Discuss” is a command word with a precise meaning: give both sides, supported, and then say what you conclude from the evidence you have given. Cambridge is marking the balance and the reasoning, not your opinion, and a passionate answer for or against that gives only one side cannot score more than half.
| Advantages | Disadvantages |
|---|---|
| Higher yield. Insect-resistant maize loses less of the crop to pests, so more food comes off the same area of land. | Genes can spread. Pollen from a herbicide-resistant crop can reach a wild relative, so a weed inherits the resistance and the herbicide no longer controls it. |
| Less insecticide. A crop that resists its pests may need little or no spraying, which is cheaper for the farmer and affects fewer other species. | Effects on other insects. A crop that makes an insecticidal substance may also harm insects that are not pests, including pollinators, reducing biodiversity. |
| Weeds controlled without harming the crop. Herbicide-resistant soya can be sprayed while it is growing, so the crop is not competing with weeds for light, water and mineral ions. | Pests can become resistant. Insects that survive the modified crop reproduce, so over years the pest population becomes resistant and the advantage is lost — the same natural selection you met in 18.3. |
| Improved nutritional quality. Rice modified to make a substance the body converts into vitamin A could reduce deficiency in regions where rice is the main food. | Cost and dependence. Modified seed is usually bought each year from the company that produced it, which can be difficult for farmers with little money. |
| Crops can be grown in more places, and a larger harvest from the same land can mean less new land is cleared. | Reduced genetic variation. If everyone grows one modified variety, the crop is a monoculture with little variation, so a new disease could destroy all of it — the risk you met in 18.1. |
| Fewer applications of chemicals can mean less run-off into rivers and less fuel used driving machinery over the fields. | Uncertainty and choice. Long-term effects on health and on ecosystems are not fully known, and some people object to the technology in principle or want the food labelled so they can choose. |
Pick two advantages and two disadvantages and give each one a reason, not just a label. “Higher yield” on its own is weak; “higher yield, because less of the crop is eaten by insect pests” is a mark.
Then write a conclusion that refers back to what you said. Something like: “The nutritional and yield advantages are large where food is short, but the risk of resistance spreading to wild plants is real, so the case is strongest for crops grown where there are no close wild relatives.” That is a judgement drawn from evidence, which is what the command word asks for.
What loses marks: listing four points on one side and none on the other; writing that GM is dangerous or that GM is safe without support; and confusing genetic modification with selective breeding.
Selective breeding (18.4) uses variation that already exists within a species. You choose the individuals with the feature you want, breed them, and repeat over many generations. Nothing new is introduced.
Genetic modification transfers an individual gene, can move it between different species, and does it in one generation. That is why a human gene can end up in a bacterium, which no amount of selective breeding could ever achieve.
A question that asks you to compare them is asking for those three differences: what is moved, whether it crosses species, and how long it takes.
Cut · Cut · Join · Insert · Multiply · Express. Six words, six marks. Then hang the detail on each: cut what, with what, leaving what; join with what, forming what; insert into what; multiply why; express to make what.
The Night Before
Topic 21 is short. There is less to know here than in almost any other topic on the syllabus, which means the marks are decided almost entirely by whether you use the exact word. This section is the list of words, plus the shape of the answers they belong in.
The Vocabulary That Scores
| Say this | Not this | Why it matters |
|---|---|---|
| a restriction enzyme cuts | ligase cuts | The swap is the most-reported error in this topic and costs two marks, because the steps then come out in the wrong order. |
| DNA ligase joins | a restriction enzyme joins / ligase sticks | “Joins” is the marking word. “Sticks” and “attaches” are usually let through, but there is no reason to gamble. |
| complementary sticky ends | matching / identical sticky ends | Complementary is the printed word, and it is the reason the same enzyme must be used on both. |
| a recombinant plasmid | a modified plasmid / a new plasmid | A mark on its own. It names a plasmid holding DNA from two sources. |
| the gene is expressed | the gene works / the gene switches on | Expressed means the gene is being used to make its protein. It is the final marking point of the six. |
| the enzyme is denatured | the enzyme is killed / dies / breaks | Enzymes are molecules. Molecules are denatured. Only organisms can be killed — the yeast can be killed, its enzymes cannot. |
| lactose is broken down into glucose and galactose | the lactose is removed | Nothing is taken out of lactose-free milk. One sugar is split into two, and both stay in the milk. |
| a plasmid is separate from the main circular DNA | a plasmid is part of the bacterial DNA | Being separate is what makes it usable. An answer that makes it part of the main DNA has removed the reason it is useful. |
| anaerobic respiration in yeast | fermentation | Fermentation is a loose word. The mark is usually for anaerobic, because that is what makes ethanol appear instead of water. |
Bread and biofuel use the same reaction and keep opposite products. In bread the useful product is the carbon dioxide and the ethanol evaporates in the oven. In biofuel the useful product is the ethanol and the carbon dioxide is the by-product.
If a question about yeast looks harder than it should, check whether this is all it is asking. It very often is.
Shaping the Answer to the Command Word
| Command word | What the marker wants | A Topic 21 example |
|---|---|---|
| State | One short fact. No explanation, no time spent. | “State the enzyme used to join the gene into the plasmid.” → DNA ligase. |
| Describe | What happens, in order, without needing reasons. | “Describe how bread dough rises.” → yeast respires anaerobically, produces carbon dioxide, the gas is trapped in the dough, the dough rises. |
| Explain | Why it happens. Every sentence needs a because. | “Explain why a biological washing powder is used at 40°C.” → because above about 50°C the enzymes are denatured, the active site changes shape and the stain no longer fits. |
| Outline | The steps, briefly, in the right order. This is the six-step question. | “Outline the process of genetic modification.” → six numbered lines and nothing else. |
| Discuss | Both sides, each supported, then a judgement drawn from them. | “Discuss the advantages and disadvantages of GM crops.” → two of each, explained, then a conclusion. |
| Suggest | Apply what you know to something unfamiliar. There is often more than one creditable answer. | “Suggest why growth stopped although nutrients were still added.” → waste build-up, pH change, oxygen becoming limiting. |
Number them. One step per line. That way the marker can find each of the six marking points without hunting, and you can see at a glance whether you have written six or only four.
It also protects you. If you are unsure about one step, the other five are still clearly there and can still be credited — whereas a paragraph that runs the steps together can lose several marks to a single muddle in the middle.
The name of a particular restriction enzyme. Not on your syllabus, and naming one wrongly looks worse than not naming one at all.
The base sequence of a sticky end. Explicitly excluded. Describe the shape instead.
How the plasmid gets into the bacterium. The syllabus says the specific details are not required. Write “inserted into bacteria” and move on.
Anything about modifying animals or people. The syllabus limits genetic modification to bacteria producing human proteins and to crop plants.
The time you save by not writing these is time you can spend on the “discuss” question, where most of the marks in this topic are actually lost.
21.1 Bacteria: fast, complex molecules, few ethical concerns, plasmids.
21.2 yeast Anaerobic respiration: glucose → ethanol + carbon dioxide. Bread keeps the CO₂, biofuel keeps the ethanol.
21.2 enzymes Pectinase → more and clearer juice. Proteases and lipases in washing powder → small soluble molecules, low temperature. Lactase → lactose broken into glucose and galactose.
21.2 fermenters Insulin, penicillin, mycoprotein. Temperature, pH, oxygen, nutrients, waste.
21.3 GM Cut · Cut · Join · Insert · Multiply · Express. Restriction enzyme cuts, ligase joins. Advantages and disadvantages, both sides.