CIE iGCSE Biology-5.1 Enzymes- Study Notes- New Syllabus
CIE iGCSE Biology-5.1 Enzymes- Study Notes – New syllabus
CIE iGCSE Biology-5.1 Enzymes- Study Notes -CIE iGCSE Biology – per latest Syllabus.
Key Concepts:
Core
- Describe a catalyst as a substance that increases the rate of a chemical reaction and is not changed by the reaction
- Describe enzymes as proteins that are involved in all metabolic reactions, where they function as biological catalysts
- Describe why enzymes are important in all living organisms in terms of a reaction rate necessary to sustain life
- Describe enzyme action with reference to the shape of the active site of an enzyme being complementary to its substrate and the formation of products
- Investigate and describe the effect of changes in temperature and pH on enzyme activity with reference to optimum temperature and denaturation
Supplement
- Explain enzyme action with reference to: active site, enzyme-substrate complex, substrate and product
- Explain the specificity of enzymes in terms of the complementary shape and fit of the active site with the substrate
- Explain the effect of changes in temperature on enzyme activity in terms of kinetic energy, shape and fit, frequency of effective collisions and denaturation
- Explain the effect of changes in pH on enzyme activity in terms of shape and fit and denaturation
Enzymes
📘 What Is a Catalyst?
A catalyst is a substance that:
- Speeds up the rate of a chemical reaction
- Remains unchanged at the end of the reaction
Key Definition:
A catalyst increases the rate of a chemical reaction without being used up or changed itself.
🧠 Why Is This Important?
In living organisms, many reactions must occur quickly to sustain life. Without catalysts, most biological reactions would be far too slow to support survival.
🔍 Enzymes as Biological Catalysts
- Enzymes are special proteins that act as biological catalysts.
- They speed up essential processes like digestion, respiration, and DNA replication without being consumed in the reaction.
- At the end of the reaction, the enzyme remains unchanged and can be reused.
Example:
Hydrogen peroxide (H2O2) naturally breaks down into water and oxygen.
This reaction is very slow without a catalyst.
The enzyme catalase speeds it up greatly – and it remains unchanged after the reaction.
📌 Quick Recap
| Feature | Catalyst |
|---|---|
| Increases reaction rate | Yes |
| Used up in reaction | No |
| Changed by reaction | No |
| Can be reused | Yes |
Enzymes – Proteins That Control Metabolic Reactions
📘 What Are Enzymes?
Enzymes are special proteins produced by living cells. They speed up chemical reactions in the body without being changed – making them biological catalysts.
Key Definition:
An enzyme is a protein that acts as a biological catalyst, increasing the rate of a chemical reaction without being used up.
🔬 Enzymes in Metabolic Reactions
Metabolic reactions are all the chemical processes that keep an organism alive, such as:
- Breaking down food
- Releasing energy
- Building new molecules
- Removing waste products
Enzymes are essential in every one of these reactions.
⚡ Why Enzymes Are Vital
Without enzymes:
- Reactions would be too slow to sustain life
- Cells wouldn’t produce energy or proteins fast enough
- The body couldn’t function at normal temperatures
That’s why enzymes are essential – they allow life-sustaining reactions to happen quickly and efficiently at body temperature.
📌 Summary Table
| Feature | Explanation |
|---|---|
| Enzyme type | Protein |
| Function | Biological catalyst |
| Involved in | All metabolic reactions |
| Changed during reaction? | No – enzymes remain the same |
| Used up? | No – enzymes can be reused |
| Works best at… | Specific temperature and pH |
Tip:
- Enzymes are proteins
- They act as biological catalysts
- They speed up all metabolic reactions in the body
Why Enzymes Are Essential for Life
📘 Key Concept:
Enzymes are required in all living organisms because they speed up chemical reactions to rates fast enough to sustain life. Without enzymes, most reactions would be far too slow, and survival would not be possible.
🔍 The Problem Without Enzymes:
Inside cells, thousands of vital reactions occur every second, including:
- Digesting nutrients
- Releasing energy via respiration
- Synthesising proteins, DNA, and new cell structures
But these reactions on their own are too slow and would require high energy or heat to happen fast – which is incompatible with life.
The Role of Enzymes:
Enzymes act as biological catalysts that make these vital reactions fast enough to keep cells and organisms alive – all at normal body temperature (≈37°C).
Key Roles of Enzymes:
- Speed up digestion for nutrient absorption
- Enable continuous energy release from respiration
- Support fast growth, cell division, and repair
- Enable fast nerve and muscle responses
- Detoxify harmful byproducts like hydrogen peroxide
🔬 How Enzymes Work:
Enzymes lower the activation energy required for reactions. This lets reactions happen quickly at safe body temperatures.
They are also highly specific – each enzyme speeds up only one particular reaction. This ensures that processes occur:
- In the right cell
- At the right time
- With high efficiency
📌 Summary Table
| Without Enzymes | With Enzymes |
|---|---|
| Reactions too slow | Reactions fast enough to support life |
| High temperatures or energy needed | Work at normal body temperature (37°C) |
| Nutrients not broken down quickly | Digestion completed in minutes |
| Low energy supply from respiration | Energy released rapidly when needed |
| Slow cell division and repair | Normal growth and maintenance |
Final Tip
- Enzymes are essential because they increase the rate of reactions
- Without them, life-sustaining reactions would be far too slow
- Enzymes allow fast reactions at body temperature
How Enzymes Work – Enzyme Action and Active Site
📘 Core Concept:
Enzymes have a specially shaped active site that fits one specific molecule – called the substrate. This allows enzymes to catalyse only one reaction efficiently, using a mechanism called the lock-and-key model.
🔍 Step-by-Step: How Enzymes Act
- The Substrate: The molecule that the enzyme will act on (e.g. starch, protein).
- The Active Site: A specially shaped region on the enzyme where the substrate binds. It fits the substrate exactly.
- Enzyme Substrate Complex: When the substrate binds to the enzyme, they form a temporary combination for the reaction.
- Formation of Products: The enzyme either breaks down the substrate or helps build a new molecule. Products are released.
- Enzyme Is Unchanged: The enzyme is not altered in the process – it can be reused again and again.
🗝️ Lock-and-Key Model:
The substrate fits the active site of the enzyme like a key fits a lock. This ensures that enzymes are highly specific – each enzyme works with only one substrate type.
Example:
- Enzyme: Amylase
- Substrate: Starch
- Products: Simple sugars (e.g. maltose)
Amylase only acts on starch. It cannot digest proteins or fats – this shows enzyme specificity.
📌 Summary Table
| Term | Explanation |
|---|---|
| Substrate | Molecule that the enzyme acts upon |
| Active site | Region on enzyme that binds the substrate |
| Complementary shape | Substrate fits exactly into the active site |
| Enzyme–substrate complex | Temporary structure during the reaction |
| Product | New molecule(s) formed after the reaction |
| Enzyme after reaction | Unchanged – can catalyse more reactions |
Final Tip
- Enzymes have a specific shape
- The active site fits only one substrate
- The reaction forms a product, enzyme is not used up
- This is known as the lock-and-key model
Effect of Temperature and pH on Enzyme Activity
📘 Core Concept:
Enzyme activity is affected by temperature and pH. These factors can change the shape of the enzyme’s active site, affecting how well it works.
🔥 Effect of Temperature on Enzymes
- Low temperatures: Enzyme activity is slow due to fewer collisions.
- Rising temperature: More collisions → faster reactions.
- Optimum temperature: Maximum enzyme activity (about 37°C for humans).
- High temperatures: Enzyme denatures – active site changes shape → reaction stops.
Denaturation is permanent – the enzyme cannot work again.
🌈 Effect of pH on Enzymes
- pH scale: 0 (acidic) → 7 (neutral) → 14 (alkaline)
- Each enzyme has an optimum pH:
- Pepsin (stomach) – works best at pH 2
- Amylase (mouth) – works best at pH 7
- Too acidic or alkaline: Enzyme denatures → active site changes → reaction stops.
📌 Summary Table
| Factor | Low Level | Optimum Point | Too High / Too Extreme |
|---|---|---|---|
| Temperature | Slow enzyme activity | Max activity (~37°C) | Enzyme denatures |
| pH | Enzyme activity may be low | Each enzyme has a best pH | Denatured if too acidic or alkaline |
🧪 How to Investigate
- Prepare identical test tubes of enzyme + substrate (e.g. amylase + starch).
- Change only one factor (either temperature or pH).
- Use iodine test to monitor starch breakdown (colour change).
- Measure the time taken for the reaction to complete.
- Record which condition gives the fastest result → that’s the optimum.
Explaining Enzyme Action – Step by Step
Enzymes are biological catalysts. They speed up reactions by binding to a specific molecule (substrate) and turning it into a new substance (product) – all without being changed themselves.
🔬 Key Terms Explained
Substrate
- The substrate is the reactant – the specific molecule that the enzyme acts on.
- Example: In digestion, starch is a substrate broken down by amylase.
Active Site
- The active site is the region on the enzyme where the substrate fits. Its shape is complementary to the substrate – like a lock and key.
- Only the correct substrate can bind to an enzyme’s active site. This is why enzymes are specific.
Enzyme–Substrate Complex
- When the substrate binds to the active site, they form a temporary structure called the enzyme–substrate complex. This allows the enzyme to:
- Hold the substrate in the correct position
- Break it down or build it up into a product
Product
- Once the reaction is complete, the substrate is changed into a new substance – the product. It leaves the active site, and the enzyme is ready to catalyse another reaction.
Summary of Enzyme Action:
- Substrate fits into the enzyme’s active site
- They form an enzyme substrate complex
- A reaction occurs → substrate is changed
- A product is formed and released
- The enzyme is unchanged and reusable
📌 Real-Life Example Table
| Enzyme | Substrate | Product | Where It Happens |
|---|---|---|---|
| Amylase | Starch | Simple sugars | Mouth & small intestine |
| Protease | Protein | Amino acids | Stomach & small intestine |
| Lipase | Fats (lipids) | Fatty acids + Glycerol | Small intestine |
Enzyme Specificity – Lock-and-Key Fit
📘 Core Idea:
Enzymes are highly specific – each enzyme only works with one particular substrate. This is because the shape of the enzyme’s active site is exactly complementary to the shape of that one substrate.
🔬 Key Terms
Active Site: The special region on an enzyme where the substrate binds. It has a unique 3D shape designed to fit only one specific substrate.
Substrate: The molecule the enzyme acts on. It must fit exactly into the active site – like a key fitting into a lock.
🔑 Lock-and-Key Model
Enzyme specificity is explained using the lock-and-key model:
- Enzyme = Lock
- Substrate = Key
- The fit must be exact – or the reaction won’t occur
This is why each enzyme only catalyses one specific reaction.
📌 Why Is Specificity Important?
- Ensures that only the correct reactions happen inside cells
- Prevents enzymes from acting on the wrong molecule
- Keeps metabolic processes accurate and controlled
✅ Summary Table
| Feature | Explanation |
|---|---|
| Enzyme specificity | Each enzyme works with only one substrate |
| Active site | Has a specific shape complementary to its substrate |
| Lock-and-key model | Explains how enzymes recognize only their correct substrate |
| Result | Ensures correct, fast, and efficient chemical reactions |
Effect of Temperature on Enzyme Activity
📘 Core Idea:
Temperature affects how fast enzymes work by changing the kinetic energy of molecules, the frequency of collisions, and the shape of the enzyme’s active site. Too much heat can cause denaturation, permanently damaging the enzyme.
🔬 Step-by-Step Explanation
1. Low Temperatures (Below Optimum)
- Low kinetic energy
- Enzymes and substrates move slowly → fewer collisions
- Reaction rate is slow
- ⚠️ Enzyme is not damaged, just less active
2. Optimum Temperature
- Ideal kinetic energy → maximum effective collisions
- Active site fits perfectly
- Fastest reaction rate
- Optimum for human enzymes ≈ 37°C
3. High Temperatures (Above Optimum)
- Enzyme gains too much energy
- Shape of active site changes
- Substrate no longer fits → few or no enzyme-substrate complexes
- This is called denaturation
What Is Denaturation?
- The enzyme’s 3D structure is permanently altered
- The active site loses its specific shape
- The substrate can no longer bind
- The reaction stops
Note: Denaturation is irreversible – cooling won’t fix it.
📌 Summary Table
| Temperature | Effect on Enzyme Activity |
|---|---|
| Low | Low kinetic energy, fewer collisions, slow rate |
| Optimum | Best fit, most collisions, fastest reaction |
| High (above optimum) | Enzyme shape changes, fewer substrate fits |
| Very high | Denaturation – enzyme becomes permanently non-functional |
Effect of pH on Enzyme Activity
Enzymes work best at a specific pH level. Changes in pH affect the shape of the enzyme’s active site, making the substrate no longer fit – and may cause denaturation.
🔬 Step-by-Step Explanation
1. Each Enzyme Has an Optimum pH
The optimum pH is the level where the active site has the perfect shape for the substrate.
- Pepsin (stomach enzyme) → works best at pH 2
- Amylase (in saliva) → works best at pH 7
2. Change in pH = Change in Shape
If pH is too acidic or alkaline, it disrupts the enzyme’s bonds → active site changes shape.
- Substrate no longer fits
- Fewer enzyme substrate complexes
- Reaction rate slows down
3. Extreme pH = Denaturation
Very high or low pH levels can permanently damage enzyme structure.
- Active site is destroyed = denatured
- Substrate cannot bind
- Reaction stops
Note: Denaturation by pH is usually irreversible.
📌 Summary Table
| pH Level | Effect on Enzyme |
|---|---|
| Too low or too high | Active site shape changes slightly, reaction slows |
| Optimum pH | Perfect fit, maximum reaction rate |
| Extreme pH | Denaturation – active site is permanently damaged |