Edexcel IGCSE Biology · Spec 2.29-2.31
Digestive Enzymes and Bile
How amylase and maltase convert starch into glucose, what protease and lipase produce, where the enzymes are made, and the two roles of bile.
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Digestive Enzymes and Bile
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Explained
The digestive enzymes, and the one substance that is not an enzyme
Digestion breaks large insoluble food molecules into small soluble ones that can be absorbed through the wall of the small intestine. Each type of food molecule has its own enzyme, and each enzyme has its own place in the gut where it works.
Three enzyme families cover the whole topic, and one extra substance, bile, does something related but different.
Carbohydrases
Amylase breaks starch down into maltose. It is made in the salivary glands and in the pancreas, so it acts first in the mouth and again in the small intestine.
Maltase then breaks maltose down into glucose. It is made in the small intestine.
Two enzymes, two steps, and the point of the pair is that starch cannot become glucose in one move. Maltose is still too large to be absorbed, so the second enzyme is not a detail; it is what finishes the job.
Proteases
Protease breaks protein down into amino acids. It is made in the stomach, where it is called pepsin, and also in the pancreas and the small intestine.
Pepsin works at about pH 2, which is why the stomach produces hydrochloric acid. That acid also kills most of the bacteria in food, so it has two jobs.
Lipases
Lipase breaks lipids, meaning fats and oils, down into fatty acids and glycerol. It is made in the pancreas and acts in the small intestine.
Three fatty acids and one glycerol come from each lipid molecule, matching the way the lipid was built.
Bile, which is not an enzyme
Bile is made in the liver, stored in the gall bladder and released into the small intestine. It contains no enzymes and digests nothing itself, and saying that it digests fat is the standard error here.
It does two things, both of which help enzymes work.
It is alkaline, so it neutralises the acid arriving from the stomach and raises the pH of the small intestine to about 8. That is the optimum for the pancreatic enzymes, which would be denatured by stomach acid.
And it emulsifies fats, breaking large fat droplets into many small ones. The total amount of fat is unchanged; what changes is the surface area, which is now far greater, so lipase has more fat exposed to work on and digests it faster.
Emulsification is a physical change and digestion is a chemical one, which is the distinction a question about bile is usually probing. Bile makes the fat easier to attack. Lipase does the attacking.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme awards three marks for explaining how starch becomes glucose, from four points: amylase, that it comes from the salivary glands or the pancreas, that the starch is digested into maltose, and that maltase digests the maltose into glucose.
Its accept list is worth noting for what it treats as equivalent. Released into or digested in the mouth is accepted for the source, and so is the small intestine, duodenum or ileum. Broken down, hydrolysed and turned into are all accepted in place of digested.
So the vocabulary is flexible and the structure is not. Four separate claims are on offer and three are needed, which means an answer naming only amylase and glucose has given the beginning and the end and skipped both middle steps.
Notice that where the enzyme is made is worth a mark of its own, alongside what it does. That is true across this topic, and it is the part most often left out. Learn each enzyme as three facts rather than two: what it breaks down, what it produces, and where it is made.
The word hydrolysed appearing in the accept list is a reminder that these reactions add water. It is not required, but it is precise, and precision is rewarded elsewhere in the same mark scheme.
Putting it together along the gut
In the mouth, amylase begins on starch at about pH 7.
In the stomach, pepsin works on protein at about pH 2, and amylase from the mouth is denatured by the acid.
In the small intestine, bile neutralises the acid and emulsifies the fat, and pancreatic amylase, protease and lipase all work at about pH 8, with maltase finishing the carbohydrate.
Reading the gut in order like this explains why bile has to arrive before the pancreatic enzymes can do anything. Without it the contents are still acidic, and the enzymes that follow would be denatured on arrival.
Spec 2.29-2.31
What you need to know
- Explain how amylase and maltase convert starch into glucose
- Recall the substrates and products of protease and lipase
- Identify where the digestive enzymes are made
Active recall
Quick check
Answer each question before opening the answer.
Which two enzyme steps convert starch into glucose?
Amylase converts starch into maltose; maltase then converts maltose into glucose.
State two functions of bile.
It neutralises stomach acid (it is alkaline) and it emulsifies fats to increase their surface area for lipase.
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