AQA GCSE Combined Science Foundation (8464), Foundation tier · Biology › Organisation › Animal tissues, organs and organ systems
Practise The human digestive system. 13 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How the organs of the digestive system work together to digest and absorb food, how enzymes work, and what bile does. Expect recall of enzymes and their products, 'explain' questions on the lock and key model and denaturing, and questions on two required practicals: food tests and the effect of pH on amylase.
Key facts
Amylase: starch → sugars (salivary glands, pancreas, small intestine)
Protease: proteins → amino acids (stomach, pancreas, small intestine)
Lipase: lipids → fatty acids + glycerol (pancreas, small intestine)
Bile: made in liver, stored in gall bladder; alkaline and emulsifies fat
Benedict's (heated): blue → brick red = sugar
Iodine: orange-brown → blue-black = starch
Biuret: blue → purple = protein
Sudan III: red-stained layer on top = lipid
Denatured = active site has changed shape, so the substrate no longer fits
Amylase practical: rate = 1000 ÷ time (shorter time = faster rate)
Notes
Digestion and digestive enzymes
diagram
The digestive system is an organ system: several organs work together to digest food (break large, insoluble molecules into small, soluble ones) and absorb the products into the bloodstream.
The digestive system (a simplified front view)
Carbohydrases break carbohydrates down into simple sugars. Amylase is a carbohydrase: starch → sugars. It is made in the salivary glands, pancreas and small intestine.
Proteases: proteins → amino acids. Made in the stomach, pancreas and small intestine.
Lipases: lipids (fats) → fatty acids + glycerol. Made in the pancreas and small intestine.
The stomach also makes hydrochloric acid, giving the acidic pH at which the stomach protease works best.
The products of digestion are used to build new carbohydrates, lipids and proteins. Some glucose is used in respiration.
How enzymes work
diagram
Enzymes are biological catalysts: large protein molecules that speed up reactions without being used up.
Each enzyme has an active site with a specific shape. Only a substrate with the matching (complementary) shape fits, so each enzyme catalyses one specific reaction. This is the lock and key model.
Lock and key: only a substrate with the complementary shape fits the active site.
As temperature rises, the rate increases up to the optimum (about 37 °C for most human enzymes), because the molecules move faster and collide more often.
Enzyme activity and temperature. Above the optimum the enzyme is denatured.
Each enzyme has its own optimum pH: the stomach protease works best in acid; amylase works best at around neutral pH.
Bile
Bile is made in the liver, stored in the gall bladder and released into the small intestine. It is not an enzyme.
It is alkaline, so it neutralises the hydrochloric acid from the stomach.
It emulsifies fat: it breaks large drops into small droplets, which have a much larger surface area.
The alkaline conditions and the larger surface area both increase the rate at which lipase breaks down fat.
Required practicals
Sugars: add Benedict's reagent and heat in a hot water bath. Blue → green, yellow or brick red if sugar is present.
Starch: add iodine solution. Orange-brown → blue-black. Protein: add Biuret reagent. Blue → purple. Lipids: add Sudan III stain; a red-stained layer floats on top.
pH and amylase: mix starch solution, amylase and a pH buffer in a water bath at a constant temperature. Every 30 seconds, put a drop of the mixture into iodine on a spotting tile. Record the time when the iodine stays orange-brown (no starch left). Repeat at other pH values.
The shorter the time, the faster the rate. The pH with the shortest time is closest to the optimum.
How to answer each type of question
Recall enzymes, where they are made and their products
1 to 4 marksGrade 4
Learn each enzyme with its substrate, its products and where it is made.
Give the specific product: 'amino acids', not 'smaller molecules'.
For lipase, give both products: fatty acids and glycerol.
Example. (a) Name the type of food molecule that proteases break down. (b) Name the two products when lipase breaks down lipids. (c) Name one organ that makes amylase.
Give any extra step, e.g. heat in a water bath for Benedict's.
Give the positive result as a colour change 'from ... to ...'.
Example. Describe how a student could test a sample of food for sugar. Give the result if sugar is present.
Show the model answerHide the model answer
Add Benedict's reagent (1) and heat in a hot water bath (1). It changes from blue to brick red (or green / yellow) if sugar is present (1).
Don’t lose marks
Saying bile is an enzyme or that it breaks down fat. It emulsifies fat; lipase breaks it down.
Saying bile is made in the gall bladder. It is made in the liver and stored in the gall bladder.
Saying enzymes are 'killed' at high temperature. Enzymes are not alive: they are denatured.
Saying enzymes are denatured at low temperature. They are just slower, because the molecules move more slowly.
Writing 'the active site changes' but not saying that the substrate no longer fits.
Forgetting to heat the Benedict's test, or giving a colour change the wrong way round.
More tips
Memory tricks
The enzyme name gives away its substrate: lipase → lipids, protease → proteins, carbohydrase → carbohydrates.
Lipids → 'FAG': Fatty Acids + Glycerol.
Food test colours: Benedict's → Brick red (and it needs heat); Biuret → purple; Iodine → Inky blue-black.
Bile does two jobs, 'neutralise and emulsify'. Neither job breaks molecules down, so bile is not an enzyme.
Sense check for rates: a shorter time must give a bigger rate. If it does not, you divided the wrong way round.
Exam technique
In enzyme explanations use the key words: active site, substrate, specific (complementary) shape, denatured.
For the pH practical, examiners often ask how temperature is controlled (water bath) and how pH is controlled (buffer solutions).
Give colour changes as 'from ... to ...' to get the mark.
When a graph of rate against temperature or pH is given, read the optimum from the graph and give its unit.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Name the organs of the digestive systemMouth and salivary glands, oesophagus, stomach, liver, gall bladder, pancreas, small intestine and large intestine.
Grade 4
State what each digestive enzyme producesAmylase turns starch into sugars, proteases turn proteins into amino acids, and lipases turn lipids into fatty acids and glycerol.
Grade 4
Describe the food tests and positive resultsBenedict's turns from blue to brick red when heated with sugar, iodine from orange-brown to blue-black with starch, and Biuret from blue to purple with protein.
Grade 5
Recall where each digestive enzyme is madeAmylase in the salivary glands, pancreas and small intestine; proteases in the stomach, pancreas and small intestine; lipases in the pancreas and small intestine.
Required practicals:Food tests · Enzymes (method, variables and exam tips)
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Name the type of enzyme that breaks down proteins into amino acids.
protease
A student tested some foods for the presence of different nutrients. Name the reagent used to test for starch.
iodine solution
The protease enzyme in the stomach works best at pH 2. Name the substance produced by the stomach that provides this pH.
hydrochloric acid
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Digestive enzymes break down large food molecules into small soluble molecules.
(a) Complete the sentence. Amylase breaks down starch into ____________ .[1]
(b) Name the type of enzyme that breaks down proteins into amino acids.[1]
(c) Lipase breaks down lipids. Name the two products.[2]
(d) Where is bile made? Tick (✓) one box.[1]
Gall bladder
Liver
Pancreas
Stomach
Show the answer and mark scheme
(a)
sugars / maltose
(b)Answer: protease
protease(s)
(c)
fatty acids
glycerol
(d)Answer: Liver
Question 2Medium6 marks
Enzymes are biological catalysts.
(a) Use the lock and key theory to explain why an enzyme can only catalyse one specific reaction.[2]
(b) Explain why the rate of an enzyme-controlled reaction decreases when the temperature rises above the optimum.[3]
(c) The protease enzyme in the stomach works best at pH 2. Name the substance produced by the stomach that provides this pH.[1]
Show the answer and mark scheme
(a)
the active site has a specific shape
only one substrate has a shape that fits / is complementary to the active site