Digestive enzymes and biological molecules Revision Notes | GCSE Edexcel Higher Biology

1. The Digestive System - A Team Effort

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.

2. Enzymes - The Chemical Scissors

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.

3. The Lock and Key Model - Perfect Fit

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.

4. Factors Affecting Enzyme Activity

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.

5. Calculating Enzyme Reaction Rates

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.

1. Rate = $\frac{\text{Change in amount}}{\text{Time taken}}$ — This is the standard rate formula for any reaction
2. Change in amount = 20 cm³ Time taken = 5 minutes — Always include units in your working
3. Rate = $\frac{20 \text{ cm}^3}{5 \text{ minutes}}$ — Keep the units - they're part of the answer
4. Rate = 4 cm³/minute — Always include units in your final answer for full marks

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.

6. Exam Technique - Getting Full Marks

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.