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An enzyme is a biological catalyst that speeds up specific chemical reactions in living organisms by lowering the activation energy required. Free notes.
The digestive system and enzymes is part of Animal tissues, organs and organ systems in GCSE Edexcel Higher Biology.
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GCSE Edexcel Higher
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An enzyme is a biological catalyst that speeds up specific chemical reactions in living organisms by lowering the activation energy required. Free notes.
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Use this lesson to stay inside Animal tissues, organs and organ systems while connecting the detail back to the wider Topic 1: Key concepts in biology topic.
Read the core explanations from The digestive system and enzymes before testing yourself from memory.
Your body processes around 9 litres of fluid every day through digestion - that's like emptying and refilling a large bucket. But here's the remarkable part: this isn't the work of just one organ. The digestive system is a perfect example of how multiple organs collaborate as an organ system.
Think of it like a factory production line. Each organ has a specific job, but they all work together towards the same goal: breaking down complex food molecules into simple ones your body can absorb. The mouth starts mechanical breakdown, the stomach adds chemical breakdown with acid, and the small intestine completes absorption.
Trap: Many students think digestion only happens in the stomach. Actually, digestion begins in the mouth with mechanical breakdown by teeth and chemical breakdown by enzymes in saliva. The stomach continues the process, but most nutrient absorption happens in the small intestine.
Imagine trying to cut paper with a blunt knife versus sharp scissors. The scissors work because they're designed for that specific job. Enzymes are like molecular scissors - each one perfectly shaped to cut specific chemical bonds.
Here's what makes enzymes special: they're incredibly specific. Amylase only breaks down starch, pepsin only breaks down proteins, and lipase only breaks down fats. This specificity comes from their unique three-dimensional shape, particularly at the active site.
This specificity is crucial for your digestive system. Without it, enzymes might break down the wrong molecules or even damage your own cells. The lock and key model helps us understand this precision.
Picture a padlock and its key. The key only fits because its shape exactly matches the lock's internal structure. If you try the wrong key, nothing happens. The lock and key model explains enzyme action in exactly the same way.
In this model, the enzyme is the lock and the substrate (the molecule being broken down) is the key. The substrate must fit perfectly into the enzyme's active site. Once bound, the enzyme changes the substrate into products, then releases them. The enzyme remains unchanged and ready for the next substrate molecule.
This explains why amylase can't break down proteins - protein molecules don't fit into amylase's active site. It's like trying to open a padlock with a car key - wrong shape, no reaction.
Trap: The lock and key model is simplified. In reality, enzymes are more flexible and the active site can change shape slightly when the substrate binds (called induced fit). But for GCSE, the lock and key model is sufficient to explain enzyme specificity.
Enzymes are sensitive molecules. Change their environment and their activity changes dramatically. Two key factors control how well enzymes work: temperature and pH. Understanding these helps explain why your body maintains such precise internal conditions.
Temperature effects: As temperature increases, molecules move faster and collide more frequently. This increases the rate of enzyme-substrate collisions, speeding up reactions. But there's a catch - too much heat destroys the enzyme's shape.
pH effects: Each enzyme has an optimal pH range. Pepsin works best in the acidic stomach (pH 2), while trypsin works best in the alkaline small intestine (pH 8). Wrong pH changes the enzyme's shape, reducing activity.
When enzymes lose their shape due to high temperature or wrong pH, we say they are denatured. This is permanent damage - the enzyme can't work anymore.
In experiments and exams, you'll need to calculate how fast enzyme reactions occur. The rate tells us how much product forms (or substrate disappears) per unit time. This is crucial for understanding enzyme efficiency.
Worked example: Calculating Rate of Reaction
In an experiment, 20 cm³ of oxygen gas is produced in 5 minutes during the breakdown of hydrogen peroxide by catalase enzyme. Calculate the rate of reaction.
Answer: 4 cm³/minute
Common mistake: Students often forget units in rate calculations. The units tell you what the rate means - cm³/minute means 'cubic centimetres per minute'. Without units, you lose marks even if the number is correct.
Enzyme questions appear frequently in GCSE exams, often worth 4-6 marks. Knowing the science is half the battle - knowing how to write exam answers is the other half. Here's how to maximise your marks.
Command word focus: 'Explain' questions want you to give reasons using scientific principles. Don't just describe what happens - explain WHY it happens. For example: 'Rate increases because higher temperature gives molecules more kinetic energy, so enzyme and substrate collide more frequently.'
6-mark extended response structure: For longer enzyme questions, use this framework: 1) State what enzymes are and their function, 2) Explain the lock and key model, 3) Describe the factor affecting the enzyme (temperature/pH), 4) Explain the effect on reaction rate, 5) Mention what happens at extremes (denaturation), 6) Give a specific example from digestion.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-23.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
An enzyme is a biological catalyst that speeds up specific chemical reactions in living organisms by lowering the activation energy required. Enzymes are not used up in the reaction and can be used repeatedly, and each is specific because of the shape of its active site.
The lock and key model is a simplified model where the enzyme (lock) has a specific active site shape that only fits certain substrate molecules (key). When the substrate fits into the active site, the enzyme changes it into products and releases them, remaining unchanged and ready for the next substrate.
An active site is the specific region of an enzyme molecule where substrate molecules bind. The shape of the active site determines which substrates can bind and react, which is why amylase can break down starch but cannot break down proteins.
An enzyme is denatured when it loses its shape due to high temperature or the wrong pH. This is permanent damage, so the substrate no longer fits the active site and the enzyme cannot work anymore.
Pepsin works best at pH 2 in the acidic stomach. Trypsin, however, works best at pH 8 in the alkaline small intestine, and the wrong pH changes an enzyme's shape, reducing its activity.
Enzymes are specific because of their unique three-dimensional shape, particularly at the active site. Only substrate molecules with the right shape can bind to the active site and react, which stops enzymes breaking down the wrong molecules.
Induced fit is the idea that an enzyme's active site can change shape slightly when the substrate binds, because enzymes are more flexible than the simple lock and key model shows. The lock and key model is still used to explain enzyme specificity.
The rate of an enzyme reaction = change in amount ÷ time taken. For example, if catalase produces 20 cm³ of oxygen gas in 5 minutes, the rate = 20 ÷ 5 = 4 cm³/minute. Always include units in the final answer.
Digestion does not only happen in the stomach: it begins in the mouth with mechanical breakdown by teeth and chemical breakdown by enzymes in saliva. The stomach continues the process, but most nutrient absorption happens in the small intestine.
Most nutrients are absorbed in the small intestine. The mouth starts mechanical breakdown, the stomach adds chemical breakdown with acid, and the small intestine completes absorption.
Amylase breaks down starch, pepsin breaks down proteins and lipase breaks down fats. Each enzyme only breaks down its own substrate because of the specific shape of its active site.
Amylase cannot break down proteins because protein molecules do not fit into amylase's active site. In the lock and key model this is like trying to open a padlock with a car key: wrong shape, no reaction.
A higher temperature increases enzyme activity because molecules move faster and collide more frequently, increasing the rate of enzyme-substrate collisions. Too much heat, however, destroys the enzyme's shape and the enzyme becomes denatured.
Enzymes are not used up in the reactions they catalyse and can be used repeatedly. After the substrate binds and is changed into products, the enzyme releases them and remains unchanged, ready for the next substrate molecule.