AQA GCSE Combined Science (8464), Higher tier · Biology › Organisation › Animal tissues, organs and organ systems
Practise The human digestive system. 24 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.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Name the organs of the digestive systemMouth and salivary glands, oesophagus, stomach, liver, gall bladder, pancreas, small intestine and large intestine.
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.
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.
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.
6
Explain enzyme action using lock and keyThe substrate fits the specifically shaped active site, so each enzyme catalyses only one reaction.
6
Explain how bile speeds up fat digestionBile is alkaline, so it neutralises stomach acid, and it emulsifies fat into small droplets with a larger surface area for lipase.
7
Explain denaturing by high temperature or pHThe active site changes shape, so the substrate no longer fits and the rate of reaction falls.
7
Calculate rates from the amylase practicalUse the equation given, such as rate = 1000 ÷ time taken for the starch to be digested, and compare rates at each pH.
Notes
Digestion and digestive enzymes
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.
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
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.
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.
Above the optimum temperature, or at a pH far from the optimum pH, the shape of the active site changes. The substrate no longer fits, so the reaction slows or stops. The enzyme is denatured. grade 7+
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.
Cheatsheet
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)
How to answer each type of question
Recall enzymes, where they are made and their products
1 to 4 marks4
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 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).
Explain the effect of temperature or pH on an enzyme
2 to 4 marks7
If a graph is given, describe the trend first and quote the optimum.
Below the optimum temperature: molecules move more slowly, so there are fewer collisions between enzyme and substrate.
Above the optimum, or at the wrong pH: the active site changes shape.
Finish the chain: the substrate no longer fits the active site, so the enzyme is denatured.
Example. A student measured how quickly a protease digested protein at different temperatures. The rate was highest at 40 °C and was zero at 65 °C. Explain why the rate was zero at 65 °C.
Show the model answer
The enzyme was denatured (1). The shape of the active site changed (1), so the protein (substrate) could no longer fit into the active site (1).
Explain how bile helps the digestion of fat
3 to 4 marks6
Emulsifies: large fat drops → small droplets with a larger surface area for lipase.
Alkaline: neutralises hydrochloric acid from the stomach, giving a pH that suits lipase.
End with the effect: fat is digested faster.
Example. Explain how bile increases the rate of fat digestion in the small intestine.
Show the model answer
Bile emulsifies fat into small droplets (1), which gives a larger surface area for lipase to act on (1). Bile is alkaline, so it neutralises the hydrochloric acid from the stomach (1). This gives a pH closer to the optimum for lipase, so fat is broken down faster (1).
Required practical: the effect of pH on amylase
2 to 6 marks6
Independent variable: pH (set with buffer solutions). Dependent variable: time for the starch to be digested.
Control variables: temperature (water bath), volume and concentration of starch and of amylase.
Method: sample every 30 seconds into iodine on a spotting tile until the iodine stays orange-brown.
Use the given equation to turn times into rates, then compare them.
Example. At pH 7 the iodine stopped turning blue-black after 150 seconds. (a) Calculate the rate of reaction at pH 7. Use the equation: rate = 1000 ÷ time. Give your answer to 2 significant figures. (b) Give two variables the student should keep the same.
Show the model answer
(a) 1000 ÷ 150 (1) = 6.7 (1) (b) Any two from: temperature / volume of amylase / concentration of amylase / volume or concentration of starch solution (1) (1)
Shortcuts and 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.
Where marks are lost
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.
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.
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
(b)
the enzyme is denatured
the shape of the active site changes
so the substrate no longer fits (the active site)
(c)Answer: hydrochloric acid
hydrochloric acid
Question 3Hard7 marks
Amylase breaks down starch. A student wanted to investigate how pH affects the time taken for amylase to break down starch. The student was given starch solution, amylase solution, buffer solutions of different pH, iodine solution, a spotting tile and a water bath.
(a) Describe a method the student could use. Your method should produce valid results.[6]
(b) Give the reason why the solutions should be left in the water bath for a few minutes before they are mixed.[1]
Show the answer and mark scheme
(a)
put a drop of iodine solution into each well of a spotting tile
use a measuring cylinder / syringe to put the same volume of starch solution and buffer solution into a test tube
keep the temperature constant by placing the tubes of starch and amylase in a water bath, e.g. at 35 °C, and letting them reach that temperature
add a set volume of amylase to the starch and buffer, mix and start the stopwatch
every 30 s remove a drop of the mixture with a pipette and add it to a fresh drop of iodine
record the time when the iodine stays orange-brown (starch has all been digested)
repeat with buffers of different pH, e.g. pH 5, 6, 7, 8 and 9
repeat each pH and calculate a mean time; rate = 1 ÷ time
Marked with levels of response: the full level descriptors are in the app.
(b)
so the solutions reach the temperature of the water bath before the reaction starts