Fermentation: yeast vs Lactobacillus
Three microorganisms, three fermentation products. Yeast respires anaerobically, turning glucose into ethanol and carbon dioxide: the carbon dioxide makes bread dough rise (the ethanol bakes off) and is the alcohol in beer and wine. Lactobacillus turns lactose (the milk sugar) into lactic acid only — no carbon dioxide — and the acid lowers milk pH, coagulating the proteins into yoghurt. Key contrast: yeast gives two products; Lactobacillus gives one.
Fermenters control five conditions
An industrial fermenter is a sterile, sealed vessel growing microbes for useful products (e.g. mycoprotein from Fusarium, penicillin). Five conditions are held at optimum: aseptic technique (no competitors), nutrients (carbon and nitrogen source), temperature (a water jacket removes respiration heat), pH, and oxygen/aeration for aerobic respiration. The stirrer spreads nutrients, heat and air evenly; wrong conditions denature enzymes and cut yield.
Fish farms, glasshouses and pest control
Fish farming raises protein by controlling abiotic factors (temperature, oxygen, pH). Glasshouses trap warmth, raise carbon dioxide and allow year-round growth. Fertilisers add nitrates for amino acids and proteins, but run-off causes eutrophication. Pest control: chemical pesticides act fast but bioaccumulate and breed resistance; biological control is specific and pollution-free but slower.
Drawn from real examiner reports.
Lactose is the sugar, not lactase
In yoghurt, Lactobacillus ferments lactose — the milk sugar and the substrate — into lactic acid. Writing "lactase" describes an enzyme, not a sugar; the one-letter change reverses the meaning and loses the mark. You must also name both lactose and lactic acid, and describe the bacteria's role — not the commercial process of making yoghurt.
Jun 2023 P1BR Q1(b)
Lactic acid lowers pH, never raises it
Lactic acid is an acid, so it lowers the milk pH (makes it more acidic). Milk proteins coagulate as the pH falls, not as it rises. Writing that the pH rises, or that the milk turns alkaline, is a reversal error that scores zero. Memorise the chain: Lactobacillus + lactose → lactic acid → lower pH → proteins coagulate.
Jun 2023 P1BR Q1(b)
Enzymes denature; organisms die
Use each term for the right thing. Enzymes are proteins, so they denature — the active site changes shape. Whole microorganisms die only if the temperature becomes too extreme. Never write "enzymes die" or "bacteria denature". The water jacket removes the heat of respiration to hold the optimum temperature so enzymes do not denature; it does not "kill bacteria".
Jun 2024 P1BR Q8(b)
The stirrer spreads heat and air too
A common partial answer credits the stirrer only for mixing nutrients. It has two further roles: distributing temperature evenly through the broth, and breaking up air bubbles so the microbes get maximum oxygen contact for aerobic respiration. Name all three — nutrients, even temperature, oxygen — to gain full marks on a fermenter question.
Jun 2024 P1BR Q8(b)(i)-(ii)
Abiotic control is not predator number
Abiotic means a non-living physical or chemical factor. The abiotic conditions controlled in fish farming are water temperature, dissolved oxygen (aeration) and pH, and sometimes light or salinity. A biotic answer — predator number, fish density, disease — scores zero when the question asks for an abiotic factor, and vague "water quality" earns nothing either.
Jun 2024 P2B Q6(b)(iv)
Pesticide kills pests; herbicide kills weeds
Keep these apart. A pesticide kills animal pests such as insects; a herbicide kills weeds (unwanted plants). A fertiliser is different again — it feeds the crop with mineral ions, it kills nothing. Mixing them up, such as "the pesticide kills the weeds" or "fertiliser kills pests", loses marks on pest-control and crop-yield questions.
Evaluate: one advantage, one disadvantage
On "evaluate" or "advantages and disadvantages" questions comparing chemical pesticides with biological control, give one clear advantage AND one linked disadvantage for each method, every point carrying a biological reason — not just "bad for the environment".
Feed conversion: suggest a reason
A low feed-conversion index means faster growth per unit of food. When asked why, SUGGEST a biological reason — more digestive enzymes, or less energy lost to movement, respiration or faeces — rather than just quoting the numbers back at the examiner.
A rate always needs its unit
In a fermentation or growth-rate calculation, rate = change ÷ time — and always state the unit (for example cm³/hour or g/day). A correct number with no unit loses a mark; show your working so partial credit survives a wrong final value.
Accuracy vs reliability in practicals
Keep the practical terms apart. Measuring gas volume in a syringe improves accuracy, because bubble sizes vary; repeating the run and taking a mean improves reliability. Do not offer repeats when the question asks about accuracy, or vice versa.
All three processes below involve anaerobic respiration (fermentation) by microorganisms. Learn which organism produces which product.
| Microorganism | Product made from | Products of fermentation | Process name |
|---|---|---|---|
| Yeast (Saccharomyces cerevisiae) | Sugars in dough (bread) | (carbon dioxide) + ethanol | Anaerobic respiration |
| Yeast | Glucose from malted barley or fruit juice (beer / wine) | Ethanol + | Anaerobic respiration |
| Lactobacillus | Lactose (milk sugar) | Lactic acid only | Lactic acid fermentation |
Word equation for yeast fermentation:
glucose → ethanol + carbon dioxide
Word equation for Lactobacillus fermentation:
lactose → lactic acid
How yoghurt forms: Lactobacillus converts lactose into lactic acid, which lowers the pH of the milk. The lower pH causes milk proteins to coagulate (thicken/curdle), producing the gel-like texture of yoghurt. The starter culture is added to warm, pasteurised milk; pasteurisation first kills competing bacteria.
Bread rising: produced by yeast fermentation becomes trapped in the dough, forming bubbles that cause it to rise. Ethanol evaporates during baking. Sufficient warmth is provided so the yeast enzymes work efficiently (not so hot that they denature).
An industrial fermenter is a large sealed vessel used to grow microorganisms (e.g. Fusarium for mycoprotein) under controlled conditions to maximise product yield.
Five conditions controlled and why each matters:
Dual role of the stirrer: (1) distributes nutrients and temperature evenly throughout the broth; (2) breaks up air bubbles to increase oxygen availability.
Role of the water jacket: removes the heat of respiration (fermentation is exothermic) to prevent overheating and enzyme denaturation.
Fish (e.g. salmon, tilapia) are kept in enclosed water (cages or tanks) to provide a controlled source of protein. Abiotic conditions that are controlled:
Other management practices: feed provided in measured amounts; predators excluded; disease treated with antibiotics or vaccines; selective breeding of fast-growing strains.
Glasshouses and polythene tunnels:
Fertilisers: supply mineral ions (especially nitrates) that the soil lacks. Nitrates provide the nitrogen needed by plants to synthesise amino acids and proteins, which are essential for growth. Without adequate nitrates, plants show stunted growth and yellowing.
Eutrophication (the key risk of fertilisers): excess fertiliser washes into waterways → algal bloom → algae block light → underwater plants die → decomposers (bacteria) break down dead material → decomposers respire aerobically → dissolved depleted → aquatic animals die.
Pest control — chemical pesticides vs biological control:
| Feature | Chemical pesticides | Biological control |
|---|---|---|
| Speed | Fast-acting — kills pests quickly | Slower — population of control agent must build up |
| Specificity | Often non-specific — can kill non-target organisms (bees, natural predators) | Specific — predator/parasite targets only the pest species |
| Persistence | Can persist in environment; bioaccumulate through food chains | Biodegradable; no chemical accumulation |
| Resistance | Pest populations can evolve resistance over time | No chemical resistance develops |
| Introduced-species risk | None | Introduced control agent may become a pest itself if it has no natural enemies |
| Effectiveness | Reliable short-term control | Reduces but does not eliminate pests |
Bioaccumulation (biological magnification): a persistent pesticide is absorbed by producers, passed to primary consumers, then secondary/tertiary consumers — each step concentrating the chemical, so top predators accumulate the highest (lethal) doses.
Explain the role of yeast in bread-making and why the dough rises.
Yeast is used in the production of beer. State the substrate used by yeast and name the TWO products formed during this process.
