Enzymes (AQA GCSE Biology): Revision Notes
Enzymes
What are enzymes?
Enzymes are special protein molecules that speed up chemical reactions in living things. They have complex shapes that are very important for how they work.
Each enzyme has an active site. This is the part where the reaction happens. Think of it like a special shaped hole.
The three-dimensional shape of an enzyme is crucial for its function. Even tiny changes to this shape can completely stop the enzyme from working properly.
Lock and key theory
The lock and key theory helps us understand how enzymes work. It's like a simple puzzle:
- The enzyme is like a lock
- The substrate (the substance that reacts) is like a key
- The substrate fits perfectly into the enzyme's active site
- Once they fit together, the reaction happens
- The products are released and the enzyme can be used again
Only the right substrate will fit into each enzyme's active site. This is called specificity. Other substances won't fit, so they can't react. This is why each enzyme can only catalyse one specific type of reaction.
What happens when enzymes stop working?
If the shape of the active site changes, the enzyme becomes denatured. This means:
- The substrate no longer fits properly
- The enzyme stops working
- It cannot be fixed
Common Mistake to Avoid: Remember that denaturation is usually permanent. Unlike temporary changes, once an enzyme is denatured by extreme conditions, it cannot return to its original shape and function.
Effect of temperature on enzymes
Temperature affects how fast enzymes work by changing the movement and collision rate of molecules.
- Low temperatures: Enzymes work slowly because molecules move slowly
- Higher temperatures: Enzymes work faster because molecules move faster and collide more often
- Optimum temperature: This is the best temperature where the enzyme works fastest
- Too high temperatures: The enzyme gets denatured and stops working completely
The rate of reaction increases with temperature until it reaches the optimum point, then it drops quickly. This creates a characteristic bell-shaped curve when you plot enzyme activity against temperature.
Effect of pH on enzymes
pH measures how acidic or alkaline something is. It affects enzyme shape by altering the chemical bonds that maintain the protein structure.
- Each enzyme has an optimum pH where it works best
- At the optimum pH, the enzyme has its perfect shape
- Wrong pH levels: The active site changes shape and the enzyme becomes denatured
- Different enzymes have different optimum pH levels
The rate of reaction is highest at the optimum pH and drops on both sides. For example, pepsin (stomach enzyme) works best at pH 1.5-2.0, while trypsin (intestinal enzyme) works best at pH 8.0-8.5.
Real example - bile and fat digestion
Worked Example: How Bile Helps Fat Digestion
Step 1: Bile production and storage Bile is made in the liver and stored in the gall bladder.
Step 2: Neutralisation Bile is alkaline, so it neutralises stomach acid when food enters the small intestine.
Step 3: Emulsification Bile breaks fat into tiny droplets, giving fat a much larger surface area.
Step 4: Enzyme action Lipase enzymes can then break down the fat more easily because there's more surface area to work on.
Step 5: Optimal conditions Lipase works best at pH 7.5-8.0 (alkaline conditions), which the bile provides.
Key concepts summary
Key Points to Remember:
- Lock and key theory: Enzymes and substrates fit together perfectly like a lock and key
- Active site: The special shaped part of an enzyme where reactions happen
- Denatured: When an enzyme's shape changes and it stops working permanently
- Optimum conditions: Each enzyme works best at a specific temperature and pH
- Extreme conditions: Very high temperatures or wrong pH levels will denature enzymes permanently
- Specificity: Each enzyme only works with one specific substrate