Contents: 8 sections
Cambridge IGCSE Co-ordinated Sciences 0654 and Combined Science 0653 · Core and Extended
Syllabus points
- Explain why bacteria are useful in biotechnology and genetic engineering.
- Describe the role of anaerobic respiration in yeast in bread making and brewing.
- Describe the use of enzymes in biological washing powders and in food production.
- Describe the use of a fermenter to grow a product such as penicillin, and the conditions that must be controlled.
- Outline genetic modification, using the production of human insulin by bacteria as the example.
- Discuss the advantages and disadvantages of genetically modifying crop plants.
Why bacteria
Bacteria are the workhorses of biotechnology for reasons worth being able to list, because "why bacteria" is a standard question:
- They reproduce very rapidly, so a useful strain can be scaled up in hours.
- They have few ethical objections attached to their use, compared with animals.
- They have simple biochemical pathways that can be altered.
- Their genetic code is the same as in every other organism, so a human gene put into a bacterium is read correctly and produces the human protein.
- They contain plasmids, small circles of DNA that are easy to cut open, add a gene to, and put back.
That fourth point is the one that makes genetic modification possible at all. The code is universal, so a bacterium given the human insulin gene makes real human insulin, not a bacterial approximation of it.
Yeast
Yeast is a single-celled fungus that respires anaerobically when oxygen runs short:
glucose → alcohol (ethanol) + carbon dioxide, releasing energy.
The same reaction is used for two different products.
Bread making wants the carbon dioxide. Yeast is mixed into dough with sugar, and the bubbles of gas it produces make the dough rise. The alcohol evaporates during baking, and the heat kills the yeast.
Brewing wants the alcohol. Yeast is added to a sugary liquid and kept in anaerobic conditions so that fermentation continues. In beer making the sugar comes from germinating barley, in which the seed's own amylase converts stored starch to maltose.
Being able to say which product is wanted in which process is the whole of some questions.
Enzymes outside the body
Biological washing powders contain protease and lipase. Protease digests protein stains such as blood, egg and grass, and lipase digests fatty and oily stains. Both break large insoluble molecules into small soluble ones that wash away.
The advantage is that clothes can be washed at a lower temperature, which saves energy and protects delicate fabrics. The limitation follows from what enzymes are: a very hot wash denatures them, so a biological powder used at high temperature works no better than a plain one.
In food production, pectinase is used to increase the yield and clarity of fruit juice, because it breaks down the pectin in plant cell walls and releases the juice held inside.
Fermenters
A fermenter is a large sterile vessel in which microorganisms are grown on an industrial scale, for instance to produce the antibiotic penicillin from the fungus Penicillium.
The conditions inside are controlled, and each control has a reason:
| Controlled | Why |
|---|---|
| Temperature, by a water jacket | Keeps the organism's enzymes near their optimum. Respiration releases heat, so the jacket usually cools rather than warms |
| pH, monitored and adjusted | Keeps the enzymes near their optimum pH |
| Oxygen, bubbled in as sterile air | Needed for aerobic respiration and growth |
| Nutrients, added as needed | Supply the raw materials for growth and for the product |
| Stirring, by paddles | Keeps the organisms, nutrients, oxygen and heat evenly distributed |
| Sterility, everything sterilised first | Stops unwanted microorganisms competing for the nutrients or contaminating the product |
The temperature detail catches people out. It is natural to assume the jacket heats the vessel, but a dense culture of respiring organisms generates a great deal of heat, and without cooling the temperature would rise past the optimum and denature the enzymes.
Genetic modification
Genetic modification is changing an organism's genetic material by removing, changing or inserting individual genes.
The standard example is human insulin made by bacteria, and it is best learnt as a sequence:
- The human insulin gene is cut out of human DNA using restriction enzymes, which cut DNA at specific sequences and leave short unpaired "sticky ends".
- A bacterial plasmid is cut open with the same restriction enzyme, so its sticky ends match.
- The gene is joined into the plasmid using ligase, giving a plasmid carrying a human gene.
- The plasmid is put back into a bacterium.
- The bacterium is grown in a fermenter, and as it multiplies every daughter cell carries the gene, so all of them make human insulin, which is then extracted and purified.
Using the same restriction enzyme in steps 1 and 2 is the detail that makes it work, and the one most often left out. The sticky ends only pair up if the cuts match.
Insulin made this way is genuinely human insulin, so it works better and provokes fewer allergic reactions than the animal insulin extracted from pigs and cattle that was used before. It can also be made in unlimited quantity, and it raises none of the religious or ethical objections that animal-derived insulin does for some patients.
Genetically modified crops
Crops are modified to give herbicide resistance, so weeds can be killed without harming the crop; insect resistance, so less pesticide is needed; improved nutritional value, such as rice modified to make vitamin A; and better resistance to drought or disease.
| Advantages | Disadvantages |
|---|---|
| Higher yields, so more food from the same land | Long-term effects on health are not yet fully known |
| Less pesticide sprayed on the land | Modified genes could spread to wild plants, for instance passing herbicide resistance to weeds |
| Crops can grow in poorer conditions | Reduced biodiversity if one modified variety replaces many local ones |
| Improved nutritional content can address deficiency diseases | Farmers may become dependent on companies that own the seed |
This is an evaluation question rather than a recall question, so a good answer gives points on both sides and then says which is the stronger, rather than listing everything and stopping.
Common mistakes
- Saying yeast respires aerobically in brewing. Fermentation is anaerobic.
- Saying bread rises because of the alcohol, or that beer is made from the carbon dioxide.
- Saying biological washing powders work best in a very hot wash, when heat denatures the enzymes.
- Saying a fermenter is heated, when it usually has to be cooled.
- Forgetting that everything in a fermenter must be sterilised first.
- Saying restriction enzymes join DNA. They cut it; ligase joins it.
- Using different restriction enzymes on the human DNA and the plasmid, so that the sticky ends would not match.
- Saying bacteria make an insulin-like substance. The genetic code is universal, so they make human insulin.
- Saying genetic modification and selective breeding are the same thing. Selective breeding works through whole organisms over generations; genetic modification moves an individual gene directly.
- Answering an evaluation question with advantages only.