2.3.3Structure and Functions in Living Organisms: Part 1
Role of Enzymes — Edexcel International GCSE Science Double Award (Modular) - Biology Unit 1 Revision Notes & Topical Questions
5 min read 7 parts Lesson 7 of 32
Part 1 of 5Enzymes as biological catalysts
What an enzyme is
- What an enzyme is Enzymes are proteins made by living cells that speed up chemical reactions.
- They are described as biological catalysts.
- A catalyst is unchanged at the end of the reaction, so the same enzyme molecule can be used again and again.
Why living things need them
- Why living things need them The thousands of reactions in metabolism would be far too slow at body temperature without enzymes.
- A meal might take weeks to digest instead of a few hours.
- Enzymes therefore keep reaction rates high enough to sustain life.
Linked reactions
- Linked reactions In cells the product of one enzyme-controlled reaction is often the reactant for the next, so enzymes work in chains and pathways.
Part 2 of 5Specificity: the active site
Enzyme specificity
Active site
- Active site Each enzyme has a region called the active site, where the substrate binds.
- Its shape depends on the folded chain of amino acids that makes up the protein.
Complementary shape
- Complementary shape The active site is complementary in shape to its own substrate, in the way that a key fits a lock.
- Other molecules do not fit, so each enzyme catalyses only one reaction, or a few similar ones.
Enzyme-substrate complex
- Enzyme-substrate complex When the substrate enters the active site the two form an enzyme-substrate complex, and the reaction takes place.
Products leave
- Products leave The products have a different shape from the substrate, so they no longer fit and are released.
- The active site is then free to accept another substrate.
Part 3 of 5How an enzyme works
1
Enzyme and substrate molecules move randomly in the solution because of their kinetic energy.
2
A substrate molecule collides with the active site of an enzyme that has the matching shape.
3
The substrate binds to the active site and an enzyme-substrate complex forms. The reaction occurs, either breaking the substrate down or joining substrates together.
4
The product or products are released from the active site.
5
The enzyme is unchanged and goes on to catalyse the same reaction with another substrate molecule.
Part 4 of 5Two types of reaction
| Type | What happens | Example |
|---|---|---|
| Breakdown | One large substrate molecule is split into smaller products | Amylase breaks starch into maltose |
| Build-up | Smaller substrate molecules are joined into one larger product | Enzymes join glucose molecules to make starch or glycogen |
Part 5 of 5Linking the idea to rate
1More enzyme or substrate
- More enzyme or substrate Increasing the concentration of either one means more collisions between enzymes and substrate.
2More complexes form
- More complexes form More enzyme-substrate complexes are made every second.
3Faster reaction
- Faster reaction More product is formed in the same time, until all active sites are occupied.
Key terms
| Term | Meaning |
|---|---|
| Enzyme | A protein that acts as a biological catalyst, speeding up a reaction without being changed. |
| Catalyst | A substance that increases the rate of a reaction and is not used up. |
| Substrate | The molecule that an enzyme acts on. |
| Active site | The region of an enzyme whose shape is complementary to the substrate. |
| Enzyme-substrate complex | The temporary structure formed when a substrate is bound in the active site. |
| Specific | Describes an enzyme that only works on one substrate, because of the shape of its active site. |
Avoid these mistakes
1
Saying that enzymes are living. Enzymes are protein molecules; they are not alive and cannot be 'killed'.
2
Writing that enzymes are used up in the reaction. They are unchanged and can be reused.
3
Forgetting to state that the active site is complementary in shape to the substrate when explaining specificity.
4
Saying the enzyme 'fits' the substrate. It is the substrate that fits into the active site of the enzyme.
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