Cells and organisation · GCSE Biology
Digestive enzymes
Locate amylase, protease and lipase in the digestive system for GCSE Biology: where each is made, its substrate, products and optimum conditions.
Amylase: starch → sugars (mouth + pancreas). Protease: protein → amino acids (stomach pepsin + pancreas). Lipase: lipid → fatty acids + glycerol (pancreas). Bile helps lipase, not a enzyme.
The important bits
What you need to know
- 1
Digestive enzymes are biological catalysts that break large insoluble molecules into small soluble products for absorption.
- 2
Amylase breaks starch into maltose and other sugars. Salivary amylase is released in the mouth; pancreatic amylase continues digestion in the duodenum.
- 3
Protease breaks proteins into amino acids. Pepsin is produced in the stomach (works best in acid, pH ~2); trypsin and other proteases from the pancreas work in the slightly alkaline small intestine.
- 4
Lipase breaks lipids (fats and oils) into fatty acids and glycerol. Lipase is made in the pancreas and acts in the duodenum.
- 5
The pancreas produces pancreatic amylase, protease and lipase, released into the duodenum through the pancreatic duct.
- 6
Bile from the liver emulsifies fats into small droplets, increasing surface area for lipase; it also neutralises stomach acid so intestinal enzymes have suitable pH.
- 7
Each enzyme has an optimum pH: pepsin in acid stomach; intestinal enzymes after bile neutralisation. Extreme pH denatures enzymes.
- 8
Enzyme names often end in -ase; the substrate is implied — amylase for starch (amylum), protease for protein, lipase for lipid.
Quotations worth analysing
Short evidence. Real method.
“Amylase breaks starch into sugars; protease breaks proteins into amino acids; lipase breaks lipids into fatty acids and glycerol.”
Triple: enzyme, substrate, product, location. Missing any column loses table marks.
“Pepsin is a protease that works in the acid conditions of the stomach.”
Name acid as the condition. Intestinal proteases need neutral/alkaline pH after bile.
“Bile emulsifies fats; it is not an enzyme.”
Students lose marks crediting bile with digesting fat. Lipase digests; bile emulsifies.
Go deeper
Build the master table
Columns: enzyme, source, where it works, substrate, products, notes. Amylase | salivary glands, pancreas | mouth, duodenum | starch | sugars | acid in stomach stops salivary amylase. Protease | stomach (pepsin), pancreas | stomach, duodenum | protein | amino acids | HCl in stomach. Lipase | pancreas | duodenum | lipid | fatty acids + glycerol | needs emulsification by bile. Drill until automatic. Exam data may add temperature — enzymes are proteins; denature above optimum.
Go deeper
Why location and pH matter together
Pepsin is activated in stomach acid — perfect for protein in chyme. If you dump chyme into duodenum without bile, acid would denature intestinal enzymes. Bile’s alkalinity neutralises acid. Lipase at the lipid-water interface needs small droplets from emulsification — surface area argument again. Lock-and-key: each enzyme’s active site fits one substrate type. “Enzymes digest food” without names scores zero on six-mark digestion questions.
Go deeper
Links to required practical and food tests
Amylase practical: starch + iodine until orange-brown — starch gone. Food tests: iodine for starch, Benedict’s for sugars after digestion. You can argue digestion is turning Benedict’s-positive results from starch breakdown. Evaluation: control pH and temperature; same enzyme volume; end-point time = 1/rate. Connect back to organisation: glands (pancreas, salivary) secrete into the lumen of the digestive system organ.
See the idea in action
Describe the action of lipase and the role of bile (4 marks). Lipase, from the pancreas, breaks lipids in the duodenum into fatty acids and glycerol, which are soluble enough to absorb. Bile from the liver emulsifies fats into small droplets, increasing surface area for lipase action. Bile also neutralises stomach acid, giving a suitable pH for intestinal enzymes. Lipase is an enzyme; bile is not.
Exam technique
Turn knowledge into marks
Every enzyme answer: name, location, substrate, products. Mention pH for pepsin vs intestinal enzymes. Never call bile an enzyme.
Common mistakes
Do not give these marks away
- 01
Locating lipase in the stomach, or amylase only in the pancreas with no mention of saliva.
- 02
Saying bile digests fats, or protease breaks starch.
- 03
Omitting fatty acids and glycerol as lipase products, or amino acids as protease products.
Where is lipase produced and what does it break down?
AStomach — breaks starch into sugars
BPancreas — breaks lipids into fatty acids and glycerol
CSalivary glands — breaks protein into amino acids
DLiver — breaks starch into glucose
Show the answer
Pancreas — breaks lipids into fatty acids and glycerol. Lipase is made in the pancreas and works in the duodenum. Amylase handles starch; protease handles protein; the liver makes bile, not lipase.
Quick questions
If this is the bit you searched
What does amylase digest?
Starch, breaking it into sugars such as maltose. It is made in salivary glands and the pancreas.
Where does pepsin work?
In the stomach, breaking proteins into amino acids in acid conditions (around pH 2).
What are the products of lipase action?
Fatty acids and glycerol, from the breakdown of lipids in the small intestine.
Why is bile needed for fat digestion?
It emulsifies fats into smaller droplets for lipase and neutralises stomach acid so intestinal enzymes work properly.