OxfordAQA International GCSE Biology: Digestion and enzymes (9201) – Revision Notes

These notes condense 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. For full explanations and worked examples, use the study guide. Then test yourself with the practice questions. The wider unit is in the Bioenergetics revision notes. Course pages: the OxfordAQA IGCSE Biology hub and the printable checklist. Not sure what to revise first? Take a free diagnostic.

3.2.4 a: The big picture

• Starch, proteins and fats are insoluble. They are broken into soluble substances so they can be absorbed into the bloodstream in the wall of the small intestine.
• Large intestine: much of the water mixed with the food is absorbed into the blood.
• What is left is indigestible food, which makes up the bulk of the faeces. Faeces leave through the anus.

Food	Products
Starch	Sugars
Protein	Amino acids
Fat (lipid)	Fatty acids and glycerol

Diagram labels to recognise

Salivary glands, oesophagus, stomach, liver, gall bladder, pancreas, duodenum, small intestine, large intestine, anus.

Label	One fact
Salivary glands	Make amylase
Stomach	Protease and hydrochloric acid
Liver	Makes bile
Gall bladder	Stores bile
Pancreas	Amylase, protease, lipase
Duodenum	Receives bile and pancreatic juice
Small intestine	Enzymes; absorption
Large intestine	Water absorbed

Food only passes through the tube: mouth, oesophagus, stomach, duodenum, small intestine, large intestine, anus.

3.2.4 b: Enzymes

• Enzymes are large proteins that act as biological catalysts.
• A catalyst increases the rate of a reaction. It is not used up.
• In digestion, enzymes speed up the breakdown of large molecules into small ones for absorption.
• Shape is vital. High temperature denatures the enzyme and changes the shape of the active site. The substrate cannot fit. Denaturation is not reversed by cooling.
• Different enzymes work best at different pH values.
• Digestive enzymes are made in specialised cells in glands and the gut lining, then pass out of the cells into the gut to meet the food.

Temperature and pH in steps

1. Low temperature: slow, because there are fewer successful collisions.
2. Warmer: faster.
3. Above the best temperature: the enzyme denatures and the rate falls to zero.
4. Away from the best pH: the rate falls. Extreme pH can denature.

3.2.4 c: Which enzyme, where

Enzyme	Made in	Acts in	Reaction
Amylase	Salivary glands, pancreas, small intestine	Mouth, small intestine	Starch into sugars
Protease	Stomach, pancreas, small intestine	Stomach, small intestine	Protein into amino acids
Lipase	Pancreas, small intestine	Small intestine	Lipid into fatty acids and glycerol

Acid and bile

	Hydrochloric acid	Bile
Made in	Stomach	Liver
Stored in	Not stored	Gall bladder
Job	Gives acid conditions where stomach enzymes work best	Neutralises the acid, giving alkaline conditions for small intestine enzymes
Extra	Not an enzyme	Not an enzyme; also emulsifies fat

Emulsification: bile breaks large fat drops into small droplets. The volume is unchanged, but the surface area increases, so lipase has more surface to act on.

Surface area reminder

A cube of side 3 cm has surface area 6 × 3 × 3 = 54 cm². Split into 27 cubes of side 1 cm, the total surface area is 27 × 6 = 162 cm². That is three times greater, with the same volume.

Worked reminder: water absorption (3.2.4 a)

Fluid entering the large intestine is 7.5 litres in a day and 0.15 litres leave in the faeces. Water absorbed = 7.5 − 0.15 = 7.35 litres. As a percentage: 7.35 ÷ 7.5 × 100 = 98%.

Reading enzyme data

Use this order for any table or graph about an enzyme.

1. Find the highest rate (or shortest time). Quote the temperature or pH and the value.
2. Describe the trend before the peak: the rate increases.
3. Describe the trend after the peak: the rate decreases.
4. Explain the rise using more collisions (temperature) or conditions nearer the best pH.
5. Explain the fall using denaturation of the active site.
6. Use a figure from the data, with units, and a calculation if asked.

Worked reminder: a microbial enzyme

A lipase from a fungus gives these relative activities: 30 °C is 40, 45 °C is 90, 60 °C is 25.

• Best temperature of those tested: 45 °C.
• Rise from 30 to 45 °C: 90 ÷ 40 = 2.25 times.
• Fall from 45 to 60 °C: (25 − 90) ÷ 90 × 100 = −72%.
• Suitable for a cool wash. It would lose most of its activity at 60 °C.

Command words for this unit

Word	What to write
State	One fact, no reason
Describe	What happens, in order, using data if given
Explain	A fact plus the biology behind it, for example “denatured so the active site changes shape”
Calculate	Show the working and give units
Suggest	Apply what you know to an unfamiliar case
Evaluate	Give points for and against, then a judgement

Short model answers

• Why does bile help digestion of fat? It emulsifies fat into small droplets. This increases the surface area for lipase, so the fat is broken down faster.
• Why is the pancreas important? It makes amylase, protease and lipase, which pass into the small intestine.
• Why do the enzymes in the stomach work poorly in the small intestine? The conditions are no longer acidic. Bile has neutralised the acid, and the enzymes work best at a different pH.

Required practical: temperature and pH

Aim: investigate how temperature and pH affect the rate of digestion.

Item	Detail
Independent variable	Temperature or pH
Dependent variable	Time for the substance to be digested
Control variables	Volumes, concentrations, the other factor (pH or temperature), drop size
Measure	Rate = 1000 ÷ time (units are arbitrary, so state that you have used this)
Reliability	Repeats and a mean; use a water bath; use buffer solutions for pH

Method in steps

1. Bring enzyme and substrate separately to the chosen temperature (or add buffer for pH).
2. Mix and start the timer.
3. Test a drop on iodine at regular intervals (starch and amylase).
4. Record the time at which the end point is reached.
5. Repeat, calculate the mean time, then convert to rate.

Worked reminder: times of 62 s, 58 s and 60 s have a mean of (62 + 58 + 60) ÷ 3 = 60 s. The rate is 1000 ÷ 60 = 16.7. Compare this with a second tube at 30 s: 1000 ÷ 30 = 33.3, which is twice as fast.

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

Microbial enzymes

• Some microorganisms produce enzymes that pass out of their cells.
• These enzymes are used at home and in industry.

Enzyme	Example use
Protease, lipase	Biological washing powder (protein and fat stains)
Lactase	Lactose-free milk
Pectinase	Fruit juice extraction and clearing
Amylase	Breaking starch into sugars in food manufacture

For data questions: find the highest value to give the best temperature or pH, describe the trend before and after it, and link a fall to denaturation.

Must-know distinctions

• Denatured (shape lost, permanent) and slowed (cold, reversible).
• Made in and acts in. Pancreatic enzymes act in the small intestine.
• Neutralise (bile, acid) and emulsify (bile, fat).
• Digestion (breaking down) and absorption (moving into the blood).
• Absorbed water (large intestine, into the blood) and faeces (the indigestible food that remains).

Quick self-test

1. Name the three food groups that are insoluble and must be digested.
2. State the products of lipase action.
3. Where is bile made, and where is it stored?
4. Name three places that make protease.
5. Explain why the stomach and the small intestine need different conditions.
6. A reaction takes 50 s. Calculate the rate as 1000 ÷ time.
7. A rate rises from 8 to 12 units. Calculate the percentage increase.
8. Explain why bile makes lipase work faster.
9. Give two uses of microbial enzymes.
10. A student reads off the time as 90 s at one pH and 30 s at another. How many times faster is the second?

Answers

1. Starch, proteins and fats.
2. Fatty acids and glycerol.
3. The liver makes it and the gall bladder holds it.
4. Stomach, pancreas and small intestine.
5. Stomach enzymes work best in acid (hydrochloric acid). Bile neutralises the acid, giving alkaline conditions where the small intestine enzymes work best.
6. 1000 ÷ 50 = 20.
7. (12 − 8) ÷ 8 × 100 = 50%.
8. Bile emulsifies fat into smaller droplets, which increases the surface area for lipase.
9. For example, protease and lipase in biological washing powders; lactase for lactose-free milk.
10. 90 ÷ 30 = 3 times faster.

Where marks are usually lost

• Writing “killed” instead of denatured, or saying the enzyme “dies”.
• Saying the enzyme changes shape when the active site is meant.
• Saying bile is an enzyme, or that it digests fat.
• Mixing up where an enzyme is made and where it acts.
• Calling a longer time a faster rate.
• Describing a graph without the highest point and a figure from the data.
• Calling the small intestine the place where water is mainly absorbed. That is the large intestine.
• Forgetting control variables such as volume, concentration and the factor that is not being tested.
• Saying that low temperature denatures enzymes.

Official syllabus

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