OxfordAQA International GCSE Biology: Digestion and enzymes (9201)

This guide covers section 3.2.4 (Digestion, parts a to c) of the OxfordAQA International GCSE Biology (9201) specification, Version 5.2, for exams May/June 2018 onwards. It includes the digestive system, enzymes, the three digestive enzymes, acid and bile, the temperature and pH required practical, and microbial enzymes. Use it with the revision notes and the practice questions. Related pages: the Bioenergetics study guide, the Bioenergetics revision notes and the Bioenergetics practice questions. The course hub is OxfordAQA IGCSE Biology and the printable list is the checklist. To find weak spots first, try the free diagnostics.

What you need to be able to do

Section	You must be able to
3.2.4 a	Explain why starch, proteins and fats must become soluble; say where absorption and water uptake happen; say what faeces are; label the digestive system
3.2.4 b	Describe enzymes as biological catalysts that are large proteins; explain denaturation and the effect of pH; explain why digestive enzymes work outside cells; describe the required practical
3.2.4 c	Give where amylase, protease and lipase are made and act; explain hydrochloric acid and bile
3.2.4 c (microbes)	Give examples of enzymes from microorganisms used at home and in industry; use data about enzyme properties

3.2.4 a: Why food must be digested

Starch (a carbohydrate), proteins and fats are insoluble. They are too big to pass through the wall of the gut into the blood. Digestion converts them into soluble substances that can be absorbed into the bloodstream in the wall of the small intestine.

Food molecule	Broken down into
Starch	Sugars
Proteins	Amino acids
Fats (lipids)	Fatty acids and glycerol

In the large intestine much of the water mixed with the food is absorbed into the blood. What remains is indigestible food, which makes up the bulk of the faeces. Faeces leave the body through the anus.

The digestive system

You must recognise these parts on a diagram. Food passes along a continuous tube, with glands alongside.

Part	Role in this unit
Salivary glands	Make saliva containing amylase
Oesophagus	Carries food to the stomach
Stomach	Makes protease and hydrochloric acid
Liver	Makes bile
Gall bladder	Stores bile
Pancreas	Makes amylase, protease and lipase
Duodenum	First part of the small intestine; receives bile and pancreatic juice
Small intestine	Makes enzymes; digestion finishes and soluble products are absorbed
Large intestine	Absorbs water; forms faeces
Anus	Faeces leave the body

Liver, gall bladder and pancreas are not on the food path. They send secretions into the duodenum.

Worked example 1: why the large intestine matters

A person has 9.0 litres of fluid enter the gut in a day (food, drink and secretions). The faeces contain 0.2 litres of water.

Water absorbed = 9.0 − 0.2 = 8.8 litres. Percentage absorbed = 8.8 ÷ 9.0 × 100 = 98% (2 significant figures).

This shows why damage to the large intestine causes watery faeces: the water is not reabsorbed.

3.2.4 b: Enzymes

Enzymes are large proteins that act as biological catalysts. A catalyst increases the rate of a chemical reaction. In the gut, enzymes speed up the breakdown of large molecules into small molecules so they can be absorbed.

A catalyst is not used up, so a small amount of enzyme can break down a lot of food.

Shape and the active site

Each enzyme has an active site. The substrate fits into it, the reaction happens and the products leave. The shape of an enzyme is vital for its function. If the active site is altered, the substrate does not fit.

High temperatures denature the enzyme. The bonds that hold the protein in shape break, so the shape of the active site changes. A denatured enzyme cannot catalyse the reaction, and cooling it again does not restore it.

Temperature

• At low temperature, particles move slowly, so enzyme and substrate meet and react less often. The rate is low.
• As temperature rises, the rate rises.
• Above the best temperature, denaturation begins and the rate drops quickly towards zero.

A cold enzyme is only slow. A too-hot enzyme is permanently damaged.

pH

Different enzymes work best at different pH values. Each has a pH at which its rate is greatest. Moving away from that pH reduces the rate, and extreme values can denature the enzyme. This is why stomach enzymes and small intestine enzymes need different conditions (see 3.2.4 c).

Enzymes that work outside cells

Digestive enzymes are made by specialised cells in glands and in the lining of the gut. They pass out of the cells into the gut, where they meet the food and catalyse the breakdown of large molecules into smaller ones. Most enzymes work inside cells, but digestive enzymes act outside the cells that make them.

Worked example 2: reading a rate from time

An amylase solution is mixed with starch at four temperatures. The time for the starch to disappear (iodine no longer turns blue-black) is recorded.

Temperature (°C)	Time (s)	Rate = 1000 ÷ time
20	280	3.57
30	160	6.25
40	80	12.5
50	125	8.00

A shorter time means a faster reaction, so rate is proportional to 1 ÷ time. The rate at 40 °C is 12.5, the highest of the four.

Percentage change in rate from 30 °C to 40 °C = (12.5 − 6.25) ÷ 6.25 × 100 = +100%, so the rate doubled. From 40 °C to 50 °C = (8.00 − 12.5) ÷ 12.5 × 100 = −36%.

Explanation: from 20 to 40 °C, more collisions per second give a faster rate. At 50 °C some enzyme molecules are denatured, so the rate falls. The best temperature lies near 40 °C, but the data cannot say exactly where, because no temperature between 40 and 50 °C was tested.

Worked example 3: rate from an amount

A starch solution contains 1.20 g of starch. After 5 minutes with amylase, 0.45 g remains.

Starch broken down = 1.20 − 0.45 = 0.75 g. Mean rate = 0.75 ÷ 5 = 0.15 g per minute. Percentage digested = 0.75 ÷ 1.20 × 100 = 62.5%.

Required practical: temperature, pH and the rate of digestion

You investigate how different temperatures and different pH values affect the rate of digestion.

A typical method for temperature

1. Put equal volumes of amylase solution and starch solution in separate test tubes.
2. Stand the tubes in a water bath at the chosen temperature for several minutes, so both reach it.
3. Mix them and start the stopwatch.
4. Every 30 seconds, remove a drop and add it to iodine solution on a spotting tile.
5. Record the time when the iodine stops turning blue-black.
6. Repeat at other temperatures. Repeat each one at least three times and calculate a mean.

For pH, keep the temperature fixed and add buffer solutions of different pH values to each tube before mixing.

Variables

Type	What
Independent	Temperature, or pH
Dependent	Time taken for the starch to disappear (convert to rate)
Control	Volume and concentration of enzyme and starch, the buffer or temperature not being tested, drop size, stirring

Worked example 4: evaluating the method

A student tests a protease on cloudy milk-protein solution. The time for the cloudiness to clear is recorded at five pH values.

pH	1	2	3	5	7
Time (s)	150	45	110	400	no change in 600
Rate (1000 ÷ time)	6.67	22.2	9.09	2.50	0

The rate is highest at pH 2, so the enzyme works best in acid conditions, like a stomach protease. At pH 7 there is no reaction, because the enzyme is not active (or is denatured) at that pH.

Ways to improve: test more pH values between 1 and 3 to find the best value more precisely; repeat and take means; use a water bath to keep the temperature constant; wait for the tubes to reach temperature before mixing. A drop of iodine every 30 seconds limits the timing to 30 seconds, so a clearer end point or shorter intervals would help.

Safety: wear eye protection and take care with hot water, acid and iodine.

3.2.4 c: The digestive enzymes

Enzyme	Made in	Acts in	Reaction
Amylase	Salivary glands, pancreas, small intestine	Mouth and small intestine	Starch into sugars
Protease	Stomach, pancreas, small intestine	Stomach and small intestine	Proteins into amino acids
Lipase	Pancreas, small intestine	Small intestine	Lipids (fats) into fatty acids and glycerol

Notice that the place where an enzyme is made is not always where it acts. Pancreatic enzymes travel to the duodenum.

Hydrochloric acid

The stomach also produces hydrochloric acid. Enzymes in the stomach work most effectively in acid conditions. A stomach protease would work poorly at the alkaline conditions of the small intestine, and the reverse is also true.

Bile

The liver produces bile, which is stored in the gall bladder before being released into the small intestine. Bile has two jobs.

1. It neutralises the acid added to food in the stomach. This gives alkaline conditions in which the small intestine enzymes work most effectively.
2. It emulsifies fats, breaking large drops of fat into smaller droplets. This increases the surface area of the fat for lipase to act upon.

Bile is not an enzyme and does not chemically digest fat. It only breaks up the drops.

Worked example 5: emulsification and surface area

Treat a fat drop as a cube of side 2 cm. Bile breaks it into eight cubes of side 1 cm.

	Surface area	Volume	Surface area to volume
One cube, side 2 cm	6 × 2 × 2 = 24 cm²	2³ = 8 cm³	24 ÷ 8 = 3
Eight cubes, side 1 cm	8 × 6 × 1 × 1 = 48 cm²	8 × 1³ = 8 cm³	48 ÷ 8 = 6

The volume of fat is unchanged. The total surface area doubles (48 ÷ 24 = 2). Lipase acts at the surface of fat, so digestion is faster.

Microbial enzymes at home and in industry

Some microorganisms produce enzymes that pass out of the cells. These enzymes have many uses at home and in industry. You should be able to give examples and relate data to enzyme properties.

Enzyme	Use
Protease and lipase	Biological washing powders: remove protein and fat stains
Lactase	Makes lactose-free milk by breaking down lactose
Pectinase	Helps extract and clear fruit juice
Amylase	Breaks starch down into sugars in food manufacture

Because the microorganisms release the enzyme into their surroundings, it can be collected from the liquid they grow in.

Worked example 6: relating data to properties

A protease from a bacterium is tested for use in a washing powder. Relative activity (100 is the highest) is measured at four temperatures.

Temperature (°C)	20	40	60	80
Relative activity	15	60	100	35

• The enzyme works best at 60 °C.
• From 20 to 40 °C the activity rises fourfold (60 ÷ 15 = 4).
• From 60 to 80 °C the activity falls by 65% ((35 − 100) ÷ 100 × 100), because denaturation begins.
• A wash at 40 °C would use the enzyme at 60% of its best activity. A hotter wash would suit it better, but not above 60 °C.

Common errors

• Writing that high temperature “kills” an enzyme. Enzymes are not alive. Say denatures.
• Saying the substrate shape changes. It is the shape of the active site that changes.
• Saying bile contains enzymes, or that bile digests fat. Bile emulsifies fat and neutralises acid.
• Writing that cold temperatures denature enzymes. They only slow the reaction.
• Forgetting that protease is made in three places, and that amylase is also made in the pancreas and small intestine.
• Treating a longer time as a faster rate. Rate is proportional to 1 ÷ time.
• Writing “the enzyme is used up”. Enzymes are catalysts and are not used up.

Official syllabus

OxfordAQA International GCSE Biology (9201), Version 5.2, for exams May/June 2018 onwards, published by OxfordAQA. This guide covers section 3.2.4 Digestion, parts a to c.