People have used yeast for thousands of years to make bread, and now use it to make fuel. Enzymes extracted from microorganisms get more juice out of fruit, remove stains at low temperatures and (Extended) make milk that lactose-intolerant people can drink.
Yeast respires anaerobically: glucose → ethanol + carbon dioxide. In biofuel production the ethanol is the useful product; in bread-making the carbon dioxide makes the dough rise. Pectinase breaks down pectin in fruit cell walls, so more juice is released and it is clearer. Biological washing powders contain enzymes that digest stains into small soluble molecules at low temperatures. Extended: lactase breaks lactose into glucose and galactose to make lactose-free milk.
Fig. 1: the same anaerobic respiration in yeast is used two ways: bakers keep the carbon dioxide, biofuel producers keep the ethanol
Section 01
Yeast: bread and biofuel.
Choose bread-making or biofuel (ethanol), set the temperature slider (5 to 60 °C) and press Start. Both processes rely on anaerobic respiration in yeast. Try 35 °C, then 5 °C and 60 °C.
Bread-making
Biofuel (ethanol)
Sugar source
sugar in the dough, and sugars from flour starch
sugars from crops such as sugar cane or maize
Useful product
carbon dioxide
ethanol
What happens to it
bubbles trapped in the dough make it rise
separated from the mixture and used as a fuel
The other product
ethanol evaporates during baking
carbon dioxide escapes
Temperature: the dough is left somewhere warm (about 35 °C in the sim) so the yeast enzymes work quickly. Cold slows them; too hot denatures them.
Baking: the high oven temperature kills the yeast and evaporates the ethanol.
Examiner trap
In bread-making the dough rises because of carbon dioxide, not ethanol and not oxygen. For biofuel, the useful product is ethanol.
Section 02
Pectinase: more, clearer juice.
Two funnels hold apple pulp. One has pectinase added, the other water as a control. Press Filter for 20 minutes and compare the volume of juice collected on the graph. Then try a high temperature.
Pectin is a substance in plant cell walls that helps hold the cells together.
Pectinase is an enzyme that breaks down pectin, so the cell walls break down more easily and the cells release more juice.
Clearer juice: pectin makes juice cloudy, so breaking it down also clarifies the juice.
Result in the sim: at about 45 °C the pectinase pulp gives far more juice (about 40 cm3) than the control (about 15 cm3). At 70 °C the enzyme is denatured and gives little more than the control.
Fair test: same mass of pulp, same volume of enzyme or water, same temperature and same time.
Section 03
Biological washing powders.
The cloth carries three stains: egg (protein), butter (fat) and gravy (starch). Choose biological or non-biological powder, set the temperature and press Wash for 30 minutes. The bar chart shows the percentage of each stain left.
Stain
Made of
Enzyme in the powder
Products
egg, blood, grass
protein
protease
amino acids
butter, grease
fat
lipase
fatty acids and glycerol
gravy, sauces
starch
amylase
simple reducing sugars
How they work: the enzymes digest large, insoluble stain molecules into small, soluble molecules that dissolve and wash out of the cloth.
Low temperature: they work well at about 40 °C, which saves energy and money and is gentler on delicate fabrics. Above about 60 °C the enzymes are denatured.
Investigating: change one variable (powder type or temperature), and keep the stain, cloth, volume of water, mass of powder and washing time the same. Measure how much stain is left.
Examiner trap
Very hot water does not help a biological powder: above about 60 °C its enzymes are denatured, so it cleans no better than a non-biological powder.
Section 04
Lactase and lactose-free milk.
Extended. Press Pour milk through. Milk runs down a column packed with beads that hold the enzyme lactase. Each lactose molecule is drawn as a blue and a pink part joined together. Compare the LACTOSE IN and LACTOSE OUT counters.
Lactose is the sugar in milk. Lactase breaks it down: lactose → glucose + galactose.
Lactose intolerance: some people do not make enough lactase, so lactose passes undigested to the large intestine, where bacteria ferment it and cause pain, wind and diarrhoea. Lactose-free milk lets them drink milk without these symptoms.
Immobilised enzyme: the lactase is held in beads in the column, so it stays behind and can be used again, and the milk is not contaminated with enzyme.
In the sim: LACTOSE IN keeps rising, but LACTOSE OUT stays at or near zero: almost every lactose molecule is split as it passes the beads.
Examiner trap
Lactase does not remove the sugar from milk. It breaks lactose into two smaller sugars, glucose and galactose, which the body can absorb.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the bread-making sim, what makes the dough rise?
Quick check+10 XP
In the pectinase sim, why does the pulp with pectinase give more juice than the control?
Quick check+10 XP
In the washing powder sim at 40 °C, which enzyme removes the egg stain?
Quick check+10 XP
Extended: in the lactase column, what happens to the lactose molecules as they pass the beads?
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which product of anaerobic respiration in yeast is used as a biofuel?
Exam style+20 XP
A biological washing powder removes stains well at 40 °C but poorly at 80 °C. The best explanation is that at 80 °C:
Exam style+20 XP
Extended: why do people who are lactose intolerant benefit from lactose-free milk?
Exam style+20 XP
Which variable must be kept constant when comparing juice yield from apple pulp with and without pectinase?
Skill unlocked
Yeast and enzymes in industry, mastered.
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