Enzymes (AQA GCSE Biology Combined Science): Revision Notes
Enzymes
What are enzymes?
Enzymes are special protein molecules that help speed up chemical reactions in living things. They have complex shapes that are very important for how they work.
The most important part of an enzyme is called the active site. This is where the magic happens!
The three-dimensional shape of an enzyme is crucial to its function. Even small changes to this shape can completely stop the enzyme from working properly.
Lock-and-key theory
Scientists use the lock-and-key theory to explain how enzymes work. It's quite simple:
- The enzyme acts like a lock
- The substance it works on (called the substrate) acts like a key
- The substrate fits perfectly into the enzyme's active site
- Once they join together, a reaction happens
- The products are then released, leaving the enzyme free to work again
Think of it like this: only the right key fits into the right lock. Only the right substrate fits into the right enzyme's active site.
If the shape of the active site changes, the enzyme becomes denatured. This means it stops working completely - like a broken lock that won't open.
Effect of temperature on enzymes
Temperature has a big effect on how well enzymes work:
- Low temperatures: Enzymes work slowly because molecules move slowly
- Higher temperatures: Enzymes work faster because molecules move faster and collide more often
- Too high temperatures: The enzyme gets denatured and stops working
There is an optimum temperature where the enzyme works best. This is usually around for human enzymes (body temperature).
What happens when it gets too hot?
- The enzyme's shape changes
- The active site changes shape
- The substrate can't fit anymore
- The enzyme is denatured and useless
Effect of pH on enzymes
pH measures how acidic or alkaline something is. Enzymes are very sensitive to pH changes:
Each enzyme has an optimum pH where it works best. If the pH is too high or too low, the enzyme's shape changes. When the shape changes, the active site changes and the substrate can't fit properly anymore. The enzyme becomes denatured.
Examples of optimum pH:
- Stomach enzymes work best in acidic conditions (low pH)
- Small intestine enzymes work best in alkaline conditions (high pH around 8)
Real-life example - bile and fat digestion
Worked Example: How Bile Helps Fat Digestion
Bile is made in the liver and helps digest fats. Here's the step-by-step process:
Step 1: Bile is alkaline (high pH)
Step 2: It neutralises acid from the stomach
Step 3: This creates the right pH conditions for lipase enzymes
Step 4: Lipase works best at pH (alkaline conditions)
Step 5: Bile also breaks fat into smaller droplets
Step 6: This gives lipase a bigger surface area to work on
Result: More fat gets digested faster!
Key Points to Remember:
- Enzymes are protein molecules with special shapes that help speed up reactions
- The lock-and-key theory explains how substrates fit perfectly into enzyme active sites
- Temperature affects enzyme activity - too hot and they get denatured
- pH affects enzyme shape - each enzyme has an optimum pH for best activity
- Denatured enzymes don't work because their active site shape has changed
- Real enzymes like lipase need the right pH conditions to digest fats properly