IGCSE · Practice questions · Yeast and enzymes in industry

Yeast and enzymes at work.

Six original Cambridge-style questions on anaerobic respiration in yeast for bread and biofuel, pectinase in fruit juice production, washing powder investigations and lactose-free milk.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Which product, which enzyme.

Yeast: glucose → ethanol + carbon dioxide; bread uses the CO₂, biofuel uses the ethanol.
Pectinase: breaks down pectin in cell walls: more juice, clearer juice.
Washing powders: protease, lipase, amylase digest stains to small soluble molecules; work at low temperatures.
Lactase (Extended): lactose → glucose + galactose; lactose-free milk for lactose-intolerant people.

Explain

[4 marks]

Describe the role of anaerobic respiration in yeast during bread-making.

Yeast is mixed with flour, water and sugar to make dough, which is left in a warm place. ✓
The yeast respires anaerobically, breaking down sugar into ethanol and carbon dioxide. ✓
Bubbles of carbon dioxide are trapped in the dough, making it rise. ✓
During baking the ethanol evaporates and the heat kills the yeast. ✓

Recall

[3 marks]

Describe how yeast is used to produce ethanol for biofuel.

Sugars are obtained from crops such as sugar cane or maize. ✓
Yeast respires the sugar anaerobically, producing ethanol and carbon dioxide. ✓
The ethanol is separated from the mixture (by distillation) and used as a fuel, for example mixed with petrol. ✓

Data

[4 marks]

Apple pulp was treated with pectinase or with water and filtered for 20 minutes.

Treatment	Volume of juice / cm³	Appearance
pectinase at 45 °C	38	clear
water at 45 °C	15	cloudy
boiled pectinase at 45 °C	16	cloudy

(a) Calculate the percentage increase in juice volume caused by pectinase compared with water. (b) Explain the results.

(a) (38 − 15) ÷ 15 × 100 = 153% (accept 150%). ✓
(b) Pectinase breaks down pectin in the fruit cell walls, so more juice is released. ✓
Breaking down pectin also makes the juice clearer. ✓
Boiling denatures the pectinase, so the boiled enzyme gives about the same result as water (it acts as a control). ✓

Analysis

[4 marks]

Plan an investigation to compare how well a biological and a non-biological washing powder remove egg stains at 40 °C.

Stain identical squares of the same cloth with the same volume of egg and let them dry. ✓
Wash one square in each powder, using the same mass of powder in the same volume of water at 40 °C (water bath), for the same time, stirred in the same way. ✓
Measure the stain left (for example compare against a colour chart, or time how long it takes for the stain to disappear). ✓
Repeat several times for each powder and calculate a mean. ✓

Explain

[3 marks]

Explain why biological washing powders can be used at lower temperatures than non-biological powders and why this is an advantage.

The enzymes (protease, lipase, amylase) digest stains into small soluble molecules that wash away, even at about 30 to 40 °C. ✓
Above about 60 °C the enzymes would be denatured, so they are designed for low temperatures. ✓
Washing at lower temperatures saves energy (and money) and does less damage to delicate fabrics. ✓

Explain

[4 marks]

Extended. Explain how lactase is used to produce lactose-free milk and why this milk is useful.

Milk is passed over the enzyme lactase (often immobilised in beads in a column). ✓
Lactase breaks down lactose into glucose and galactose. ✓
Some people are lactose intolerant: they do not produce enough lactase, so lactose in milk causes pain, wind and diarrhoea. ✓
Lactose-free milk lets them drink milk and gain its nutrients without these symptoms. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.