Lesson focus: Digestive enzymeA protein that acts as a biological catalyst, speeding up reactions without being used up; its active site fits one substrate. catalyse specific hydrolysisBreaking a large molecule into smaller pieces using water. reactions, while gut microorganisms can transform unabsorbed material and interact with health.
Use enzyme specificity to explain digestion and evaluate cautious claims about gut bacteria.
Build on earlier learning
The digestive system: absorptionAbsorption requires small molecules. This lesson explains the enzyme-catalysed reactions that produce many of them and microbial activity on material that remains.
You will be successful when you can…
- Define enzyme and catalyst accurately.
- Match major digestive enzymes to substrates and products.
- Explain specificity with active-site shape.
- Describe gut-microbiome roles without claiming all bacteria are beneficial.
Words that carry the science
- enzyme
- A biological catalyst, usually a protein, that speeds a reaction without being used up.
- substrate
- A reactant molecule that binds to an enzyme’s active site.
- active site
- The region of an enzyme where specific substrates bind and react.
- hydrolysis
- Breaking a chemical bond using water.
- microbiome
- The community of microorganisms and their genes in a particular environment.
- protease
- An enzyme that digests proteins into smaller peptides and amino acids.
Separate enzyme catalysis from microbial contribution
01Enzymes lower the barrier to reaction
Digestive enzymeA protein that acts as a biological catalyst, speeding up reactions without being used up; its active site fits one substrate. catalyse hydrolysisBreaking a large molecule into smaller pieces using water., allowing large food molecules to become smaller products at body temperature. They are not consumed by the reaction, so one enzyme molecule can catalyse repeatedly.
A substrateThe molecule an enzyme acts on, such as starch for the enzyme amylase. binds to an active siteThe part of an enzyme where the substrate fits because their shapes match (are complementary), like a key in a lock. with a compatible shape and chemical environment. Different enzyme groups therefore catalyse different reactions.
02Substrates determine products
Amylase and other carbohydrases digest starch and related carbohydrates toward sugars. proteaseAn enzyme that breaks down proteins into amino acids. digest proteins toward amino acids. Lipase digests lipids into fatty acids and glycerol.
The named equations are simplified summaries because digestion often occurs through several intermediate molecules and enzymes.
03Gut microorganisms form an ecosystem
Microorganisms in the large intestine ferment some fibre and other material that human enzymes cannot digest. They produce compounds that host cells or other microbes can use, and they interact with immune and metabolic systems.
Species and effects vary. “Bacteria are healthy” is too broad: some are harmless or beneficial in context, some can cause disease, and community effects depend on diet, environment and the individual.
Enzyme activity and pH
Each enzyme works fastest at its own optimum pH. Too acidic or too alkaline and the active site changes shape.
Enzyme Substrate Product
Controls
Amylase works in the mouth and small intestine. Optimum about pH 7. Temperature held at 37 °C.
Trace protein to absorption
Explain how dietary protein becomes available for building a muscle-cell protein.
- Protease active sites bind protein or peptide substrates.
- hydrolysisBreaking a large molecule into smaller pieces using water. breaks peptide bonds and produces smaller peptides and amino acids.
- Amino acids are absorbed across the small-intestine epithelium into blood.
- Cells use amino acids to synthesise their own proteins in a genetically controlled sequence.
How to make the reasoning visible: Dietary protein is digested and rebuilt; whole food proteins are not inserted directly into muscles.
Complete the catalyst map
Treat the products as KS3 summaries rather than every intermediate step.
| Enzyme group | Main substrate | Simplified products | Major site or source |
|---|---|---|---|
| Amylase/carbohydrase | Starch/carbohydrate | Sugars | Saliva, pancreas and small intestine |
| Protease | Protein | Peptides and amino acids | Stomach, pancreas and small intestine |
| Lipase | Lipid | Fatty acids and glycerol | Pancreas and small intestine |
| Microbial enzymes | Some fibre and other substrates | Various fermentation products | Large intestine microbiome |
Why does lipase not digest starch?
Explain one way gut bacteria can affect material humans cannot digest directly.
Check your understanding
Answer from memory, then check against the suggested answer.
What is a biological catalyst?
What does protease digest?
What does lipase produce?
A claim says a probiotic “improves everyone’s digestion”. Explain why stronger evidence is needed.
Explain why enzyme specificity involves more than two rigid shapes fitting together.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Enzymes and gut bacteria both help to digest food.
Explain how digestive enzymes work.
(3)Compare the role of gut bacteria with enzymes.
(2)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., contrast the chemical reactions catalysed by named human enzymes with the broader activity of a microbial community.