EnzymesEdexcel GCSE Biology: Revision notes
Section 1
How Do Enzymes Work?
Enzymes are biological catalysts made of protein. They speed up the rate of specific reactions without being used up themselves, by lowering the activation energy needed.
Each enzyme has a unique 3D shape with an active site — a region whose shape is complementary to a specific substrate molecule (lock and key model): the substrate (key) fits precisely into the active site (lock), forming an enzyme-substrate complex.
Because the active site shape is specific to one substrate shape, enzymes show specificity — a given enzyme will only catalyse a reaction involving its complementary substrate.
Lock and key: only the correctly-shaped key (substrate) fits the lock (active site) — a wrong-shaped key won't work.
Do not say enzymes are 'used up' in a reaction — they are unchanged and can be reused repeatedly.
Section 2
Why Do Enzymes Stop Working? (Denaturation)
Enzymes are proteins, so their function depends entirely on their shape.
- At high temperatures, increased vibration breaks the bonds holding the enzyme's tertiary structure together, changing the active site shape so the substrate no longer fits — the enzyme is denatured and stops working permanently.
- Extreme pH disrupts the bonds maintaining the enzyme's shape in the same way, also denaturing it.
Once denatured, an enzyme cannot return to its original shape or function.
Exam answers must state that the active site changes shape — not just 'the enzyme is destroyed'.
Section 3
Factors Affecting Enzyme Activity
| Factor | Effect on rate |
|---|---|
| Temperature | Rate increases with temperature up to the optimum temperature. Above the optimum, rate falls sharply as the enzyme denatures. |
| Substrate concentration | Rate increases as substrate concentration increases, until all active sites are occupied — then rate plateaus. |
| pH | Rate is highest at the enzyme's optimum pH. Above or below this, rate falls as the enzyme denatures. |
Do not say enzymes 'die' — they are not alive; say they are 'denatured'.
Section 4
Core Practical: Effect of pH on Amylase Activity
Aim: investigate how pH affects the rate of breakdown of starch by amylase.
Method:
- Mix amylase, buffer of set pH and starch solution, starting a timer.
- At regular time intervals, sample onto a spotting tile and add iodine.
- Iodine turns blue-black if starch is present; record the time for iodine to stay browny-orange (all starch gone).
- Repeat at different pH values, keeping temperature, volume and concentration constant.
Analysis: rate = 1 ÷ time taken.
Always state the control variables to show a fair test.
Section 5
Rate Calculations for Enzyme Activity
Rate of reaction can be calculated using:
rate = amount of product formed (or substrate used) ÷ time taken
Where time to completion is measured: rate = 1 ÷ time taken
Units depend on what is measured (cm³/s, mg/min, or s⁻¹).
If starch fully disappears after 40 seconds, rate = 1 ÷ 40 = 0.025 s⁻¹.
Section 6
Enzymes in Synthesis and Breakdown of Food Molecules
Enzymes catalyse both synthesis and breakdown of large food molecules:
- Carbohydrates: synthesised from/broken down into simple sugars
- Proteins: synthesised from/broken down into amino acids
- Lipids: synthesised from/broken down into fatty acids and glycerol
Food Tests:
| Test | Reagent | Positive result |
|---|---|---|
| Starch | Iodine solution | Browny-orange → blue-black |
| Reducing sugars | Benedict's solution, heated | Blue → green/yellow/brick-red precipitate |
| Protein | Biuret test (NaOH + copper sulfate) | Blue → purple/lilac |
| Lipids | Sudan III / ethanol emulsion | Red stain / milky white emulsion |
Section 7
Measuring Energy in Food (Calorimetry)
Energy content of food is measured using calorimetry: burn a food sample under a known mass of water, measure temperature rise, and calculate:
energy (J) = mass of water (g) × temperature change (°C) × 4.2
Divide by food mass for energy per gram. Heat loss to surroundings causes underestimation of true energy content.
If asked to evaluate the method, mention heat loss to the surroundings as a source of error.
Must Know
- Enzymes are protein catalysts with a specific active site (lock and key model), giving specificity.
- High temperature or extreme pH denatures enzymes by changing the active site shape — permanent.
- Rate increases with temperature, substrate concentration and towards optimum pH, then falls/plateaus.
- Rate = product/substrate ÷ time, or 1/time.
- Enzymes synthesise and break down carbohydrates, proteins and lipids.
- Food tests: iodine, Benedict's, Biuret, Sudan III/ethanol.
- Calorimetry: energy = mass of water × temperature change × 4.2.
That's the notes covered.
Carry on to the next subtopic.