Anaerobic respiration in yeast — one equation, two uses
Anaerobic respiration in yeast — a single-celled fungus — converts glucose into ethanol and carbon dioxide, and both Core uses come from that one equation. Biofuel: yeast ferments sugar solution from a crop such as sugar cane; the ethanol is distilled off and burned. Bread-making: the same yeast in warm dough releases carbon dioxide, trapped as bubbles, which makes the loaf rise; the ethanol evaporates during baking. The pathway only runs without oxygen.
Industrial enzymes — pectinase, washing powders, lactase
Pectinase breaks down pectin in plant cell walls, so crushed fruit gives a higher juice yield and clearer juice. Biological washing powders carry protease, lipase and amylase, digesting protein, fat and starch stains into soluble products that rinse away; enzymes work at cool wash temperatures, saving energy, but water hot enough to denature them stops the powder working. (Extended) Lactase hydrolyses lactose into glucose and galactose for lactose-free milk.
Fermenters (Extended) — three products, five conditions
A fermenter is a large sterile vessel for growing microorganisms at industrial scale so a useful product can be harvested. Named products: insulin from genetically modified bacteria carrying a recombinant plasmid, penicillin from Penicillium, mycoprotein from a fungus such as Fusarium. Five conditions are controlled — temperature (a cooling jacket, the culture heats itself by respiring), pH, oxygen, nutrient supply, and removal of waste carbon dioxide.
Drawn from real examiner reports.
Penicillium, not just "a fungus"
Asked to name the organism used to produce penicillin, the answer is the genus Penicillium. Writing "a fungus" names a whole kingdom rather than an organism, and writing "penicillin" names the product, not the thing that makes it. Neither scores. Examiners flag both substitutions specifically, so learn the genus name.
Flagged s23 P23 Q40; s22 P43 Q1b ii
The insulin gene goes into a plasmid
The human insulin gene is inserted into a bacterial plasmid — a small circular loop of DNA separate from the main chromosome — forming a recombinant plasmid that the bacterium then takes up. Answers that put the gene straight into the bacterial chromosome, or that describe the whole transfer without ever using the word plasmid, lose the mark.
Flagged s23 P23 Q39
Fermenter nutrients are not gases
(Extended) Nutrients means a carbon/energy source such as sugar plus a nitrogen source, together with mineral ions. Oxygen and carbon dioxide are gases; oxygen is a separate controlled condition and neither counts as a nutrient here. If you name a mineral, add the ions qualifier: "magnesium ions", never bare "magnesium".
Flagged s22 P43 Q1b iii
Sterilising is not "keeping it clean"
(Extended) Sterilising the fermenter and its nutrient medium kills contaminating microorganisms that would otherwise compete with the culture for nutrients, or make unwanted and harmful substances in the product. Vague answers about cleanliness or "stopping germs" state the aim without the reason, and examiners record them as uncredited.
Flagged s22 P43 Q1b iv
Name the waste gas: carbon dioxide
(Extended) The gas released through the outflow valve is carbon dioxide, produced by the culture respiring. Naming it carries the answer, because both reasons depend on it: carbon dioxide is toxic to the culture once it accumulates, and the trapped gas raises the pressure inside a sealed vessel. "Gas" alone was not credited.
Flagged s22 P43 Q1b v
Biofuels do release carbon dioxide
Burning ethanol releases carbon dioxide exactly as burning any fuel does. The carbon-neutral argument is that this release is roughly balanced by the carbon dioxide the crop absorbed by photosynthesis while it grew, so the net addition is small — not zero. Writing "biofuels do not emit carbon dioxide" contradicts the chemistry and scores nothing.
Flagged w22 P42 Q4a ii
"Fermentation" alone is not enough
Asked how biofuel is produced from biomass, candidates write "fermentation" and stop. Name the process in full: anaerobic respiration in yeast, converting sugar from the crop into ethanol and carbon dioxide. Aerobic respiration is the wrong answer — with oxygen present the yeast makes carbon dioxide and water, and no ethanol at all.
Flagged s23 P33 Q8d i
Name the organism, not the kingdom
Every naming mark in 21.2 wants the specific term: yeast, pectinase, protease, lipase, amylase, lactase, Penicillium. "A fungus", "bacteria", "an enzyme" and "gas" are categories rather than names, and the digest records each of them being written in place of the answer.
One enzyme, one substrate
Each named enzyme has exactly one substrate and one use: pectinase/pectin/juice, lactase/lactose/milk, protease/protein, lipase/fat, amylase/starch. Read the stain or substrate in the question first, then choose the enzyme — naming the wrong one loses the mark.
Describe asks how, explain asks why
"Describe the use of pectinase" wants the sequence — enzyme, substrate, product, effect on yield. "Explain the use of lactase" wants the reason underneath: lactase hydrolyses lactose because its active site is complementary in shape to that substrate.
Fermenter answers end at yield
(Extended) For any "why must this be controlled" question, run the chain to the end: condition, effect on enzymes or respiration, effect on growth rate, effect on product yield. Stopping at "the enzymes denature" leaves the last mark on the table.
Biotechnology is the use of living organisms (or their products, such as enzymes) to make useful products for people.
Yeast and anaerobic respiration: yeast is a single-celled fungus. Under anaerobic (oxygen-free) conditions, it respires anaerobically:
Pectinase in fruit juice production: pectinase breaks down pectin in plant cell walls, releasing more juice (increased yield) and reducing cloudiness (clearer juice, easier to filter).
Biological washing powders contain enzymes active at lower wash temperatures than non-biological powders need:
| Enzyme | Stain digested | Product |
|---|---|---|
| Protease | Protein (blood, grass, egg) | Soluble amino acids |
| Lipase | Fat/grease | Soluble fatty acids + glycerol |
| Amylase | Starch (food, gravy) | Soluble sugars |
(Extended) Lactase and lactose-free milk: the enzyme lactase is added to milk, hydrolysing the milk sugar lactose into glucose and galactose. People who are lactose intolerant lack sufficient lactase in their own gut, so pre-treating milk with lactase lets them digest it without the sugar reaching the gut undigested (which would otherwise cause discomfort).
Describe the use of pectinase in fruit juice production.
(a) State the word equation for anaerobic respiration in yeast. [2]
(b) Describe the use of pectinase in fruit juice production. [2]
