Nature of Enzyme Action Class 11 Biology Notes PDF | CBSE & NEET
Nature of Enzyme Action (Easy NEET Notes)
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| Nature of Enzyme Action: Step-by-step process showing enzyme-substrate complex formation, catalysis, and product release for CBSE Class 11 and NEET Biology. |
What is an Enzyme?
- Enzyme (E) = A biological catalyst that speeds up chemical reactions.
- Substrate (S) = The substance on which the enzyme acts.
- Product (P) = The final substance formed after the reaction.
Basic Enzyme Reaction
E + S → ES → EP → E + P
Meaning of each step:
- E + S = Enzyme meets the substrate.
- ES = Enzyme-Substrate Complex is formed.
- EP = Enzyme-Product Complex is formed after the reaction.
- E + P = Product is released and the enzyme remains unchanged.
NEET Point: The enzyme is not consumed during the reaction and can be used again.
Line-by-Line Explanation
Line 1:
"Each enzyme (E) has a substrate (S) binding site in its molecule."
Easy Meaning:
- Every enzyme has a special region called the active site.
- Only the correct substrate can fit into this active site.
Remember:
- Active site = Place where the substrate binds.
Line 2:
"A highly reactive enzyme-substrate complex (ES) is produced."
Easy Meaning:
- When the substrate enters the active site, an enzyme-substrate (ES) complex is formed.
- This is the first important step of the reaction.
NEET Fact:
- ES complex is highly reactive.
Line 3:
"This complex is short-lived."
Easy Meaning:
- The ES complex exists only for a very short time.
- Soon, the chemical reaction begins.
Line 4:
"It dissociates into its product(s) and the unchanged enzyme with an intermediate enzyme-product complex (EP)."
Easy Meaning:
- The substrate changes into the product.
- Before the product leaves, an enzyme-product (EP) complex is formed.
- Finally, the product is released.
- The enzyme remains unchanged.
Remember: ES → EP → E + P
Line 5:
"The formation of the ES complex is essential for catalysis."
Easy Meaning:
- Without forming the ES complex, the enzyme cannot perform the reaction.
- ES complex formation is compulsory for enzyme action.
NEET Question Point:
- Catalysis starts only after ES complex formation.
Catalytic Cycle of Enzyme Action
Step 1
"The substrate binds to the active site of the enzyme."
Easy Meaning:
- The substrate enters the enzyme's active site.
- It fits like a key into a lock.
Keyword:
- Active site
Step 2
"The binding of the substrate induces the enzyme to alter its shape, fitting more tightly around the substrate."
Easy Meaning:
- After binding, the enzyme changes its shape slightly.
- This helps it hold the substrate more tightly.
This is called:
- Induced Fit Model
NEET Point:
- The enzyme is flexible, not rigid.
Step 3
"The active site breaks the chemical bonds of the substrate and enzyme-product complex is formed."
Easy Meaning:
- The enzyme breaks old chemical bonds or helps form new ones.
- The substrate is converted into the product.
- Now the enzyme-product (EP) complex is formed.
Step 4
"The enzyme releases the products and is ready to bind another substrate."
Easy Meaning:
- The product leaves the enzyme.
- The enzyme becomes free again.
- It can start another reaction with a new substrate.
NEET Fact:
- One enzyme molecule can catalyse thousands of reactions.
Activation Energy
What is Activation Energy?
- It is the minimum energy required to start a chemical reaction.
Role of Enzyme
- Enzymes lower the activation energy.
- Because of this, the reaction becomes faster.
Important Point
-
Enzymes do not change:
- Initial energy of reactants
- Final energy of products
- Overall energy released (ΔG)
-
Enzymes only lower the activation energy.
Flow Chart
Enzyme + Substrate
↓
Enzyme-Substrate (ES) Complex
↓
Enzyme-Product (EP) Complex
↓
Enzyme + Product
NEET Important Points
- Enzyme = Biological catalyst.
- Substrate binds at the active site.
- Formation of ES complex is essential.
- ES complex is temporary (short-lived).
- Enzyme changes shape according to the Induced Fit Model.
- Enzyme converts substrate into product.
- Product is released.
- Enzyme remains unchanged and can be reused.
- Enzymes lower activation energy and increase reaction speed.
- Enzymes are substrate-specific (one enzyme acts on a specific substrate).
One-Line Revision (Exam Ready)
- E = Enzyme
- S = Substrate
- ES = Enzyme-Substrate Complex
- EP = Enzyme-Product Complex
- P = Product
- Active site = Binding site of substrate
- Induced fit = Enzyme changes shape after substrate binding
- Activation energy = Minimum energy needed to start a reaction
- Function of enzyme = Lowers activation energy and speeds up the reaction
- Enzyme remains unchanged after the reaction
- ES complex formation is essential for catalysis
NEET Mnemonic
"Bind → Fit → Break → Release → Repeat"
- Bind = Substrate binds to active site.
- Fit = Enzyme changes shape (induced fit).
- Break = Chemical reaction occurs; EP complex forms.
- Release = Product leaves the enzyme.
- Repeat = Enzyme is free to catalyse the next reaction.
CBSE Class 11 Biology – Enzymes (Nature of Enzyme Action)
Question Bank with Answers (CBSE Pattern)
A. Multiple Choice Questions (MCQs)
1. Enzymes are:
a) Hormones b) Catalysts c) Vitamins d) Minerals
Answer: b) Catalysts
2. The substrate binds to the enzyme at its:
a) Protein site b) Active site c) Product site d) Binding membrane
Answer: b) Active site
3. The enzyme-substrate complex is represented as:
a) EP b) SP c) ES d) PS
Answer: c) ES
4. The enzyme remains ______ after the reaction.
a) Destroyed b) Changed c) Unchanged d) Inactive
Answer: c) Unchanged
5. Formation of which complex is essential for enzyme action?
a) EP b) ES c) SP d) PE
Answer: b) ES
6. Enzymes increase the rate of reaction by:
a) Increasing activation energy b) Decreasing activation energy c) Increasing temperature d) Producing ATP
Answer: b) Decreasing activation energy
7. Which model explains the change in enzyme shape after substrate binding?
a) Fluid mosaic model b) Lock and key model c) Induced fit model d) Cell theory
Answer: c) Induced fit model
8. Product formed after enzyme action is represented by:
a) S b) E c) P d) ES
Answer: c) P
9. Which of the following is short-lived?
a) Product b) Enzyme c) Enzyme-substrate complex d) Active site
Answer: c) Enzyme-substrate complex
10. The correct sequence is:
a) E + P → ES b) E + S → ES → EP → E + P c) EP → ES → P d) E → P → ES
Answer: b) E + S → ES → EP → E + P
B. Very Short Answer Questions (1 Mark)
Q1. What is an enzyme?
Answer: An enzyme is a biological catalyst that speeds up chemical reactions without being consumed.
Q2. What is a substrate?
Answer: A substrate is the substance on which an enzyme acts.
Q3. Name the binding site of an enzyme.
Answer: Active site.
Q4. What is ES complex?
Answer: ES complex is the enzyme-substrate complex formed when a substrate binds to an enzyme.
Q5. Does the enzyme change after a reaction?
Answer: No, the enzyme remains unchanged.
Q6. What is activation energy?
Answer: It is the minimum energy required to start a chemical reaction.
Q7. Why are enzymes called biological catalysts?
Answer: Because they speed up biochemical reactions without being used up.
Q8. Which complex is formed first?
Answer: Enzyme-substrate (ES) complex.
C. Short Answer Questions (2 Marks)
Q1. Define enzyme-substrate complex.
Answer: The enzyme-substrate complex is a temporary complex formed when the substrate binds to the active site of the enzyme. It is essential for catalysis.
Q2. Why are enzymes substrate-specific?
Answer:
- Every enzyme has a specific active site.
- Only a particular substrate fits into the active site.
Q3. State two functions of enzymes.
Answer:
- Speed up biochemical reactions.
- Lower activation energy.
Q4. What happens after the product is formed?
Answer: The product is released from the enzyme, and the enzyme becomes free to catalyse another reaction.
D. Short Answer Questions (3 Marks)
Q1. Explain the catalytic cycle of enzyme action.
Answer:
- Substrate binds to the active site.
- Enzyme changes shape (induced fit).
- ES complex converts into EP complex.
- Product is released.
- Enzyme remains unchanged and can be reused.
Q2. Explain the role of activation energy.
Answer:
- Every reaction requires activation energy.
- Enzymes lower activation energy.
- Therefore, reactions occur faster.
Q3. Differentiate between substrate and product.
| Substrate | Product |
|---|---|
| Reactant | Final substance |
| Binds to enzyme | Released after reaction |
| Present before reaction | Formed after reaction |
E. Long Answer Questions (5 Marks)
Q1. Explain the nature of enzyme action.
Answer:
The action of enzymes occurs in the following steps:
- Substrate binds to the active site.
- Enzyme-substrate (ES) complex is formed.
- Enzyme changes its shape (induced fit).
- Chemical reaction converts substrate into product.
- Enzyme-product (EP) complex forms.
- Product is released.
- Enzyme remains unchanged and is reused.
Equation:
E + S → ES → EP → E + P
Enzymes lower activation energy and speed up reactions.
Q2. Explain activation energy with the help of enzymes.
Answer:
Activation energy is the minimum energy required to start a reaction.
Without enzyme:
- Activation energy is high.
- Reaction is slow.
With enzyme:
- Activation energy decreases.
- Reaction becomes fast.
Enzymes do not change the overall energy released.
F. Assertion and Reason Questions
Q1.
Assertion (A): Enzymes remain unchanged after the reaction.
Reason (R): Enzymes are biological catalysts.
Answer: ✅ Both A and R are true, and R is the correct explanation.
Q2.
Assertion (A): Formation of ES complex is necessary.
Reason (R): Product forms directly without substrate binding.
Answer: ❌ Assertion is true but Reason is false.
Q3.
Assertion (A): Enzymes lower activation energy.
Reason (R): Lower activation energy speeds up reactions.
Answer: ✅ Both true and R correctly explains A.
G. Fill in the Blanks
-
Enzymes are __________ catalysts. Answer: biological
-
The substrate binds at the __________ site. Answer: active
-
ES stands for __________. Answer: Enzyme-substrate complex
-
Enzymes lower __________ energy. Answer: activation
-
Enzyme remains __________ after the reaction. Answer: unchanged
-
Product is represented by the letter __________. Answer: P
-
EP stands for __________. Answer: Enzyme-product complex
-
Enzyme action begins with __________ binding. Answer: substrate
H. Case Study Questions
Case Study
An enzyme binds with a substrate to form an enzyme-substrate complex. The enzyme changes its shape to fit the substrate more tightly. The reaction occurs, products are formed, and the enzyme is released unchanged.
Questions
1. Which complex is formed first?
Answer: ES complex
2. Which model explains enzyme shape change?
Answer: Induced fit model
3. Why is enzyme reusable?
Answer: Because it remains unchanged after the reaction.
4. What is the role of the active site?
Answer: It binds the substrate.
I. Statement-Based Questions
State True or False
-
Enzymes are proteins. ✅ True
-
Enzymes increase activation energy. ❌ False
-
ES complex is permanent. ❌ False
-
Product leaves the enzyme after reaction. ✅ True
-
Enzymes are consumed during reactions. ❌ False
-
Active site binds substrate. ✅ True
J. Match the Following
| Column A | Column B |
|---|---|
| A. Enzyme | 1. Product |
| B. Substrate | 2. Biological catalyst |
| C. Active site | 3. Substance acted upon |
| D. Product | 4. Binding site |
Answer
A → 2
B → 3
C → 4
D → 1
K. Diagram-Based Question
Q. Write the enzyme action sequence.
Answer:
E + S → ES → EP → E + P
Where:
- E = Enzyme
- S = Substrate
- ES = Enzyme–Substrate Complex
- EP = Enzyme–Product Complex
- P = Product
L. Competency-Based Question
Q. A student says that enzymes get used up after one reaction. Do you agree? Give a reason.
Answer: No. Enzymes remain chemically unchanged after catalysing a reaction and can be reused to catalyse many more reactions.
Important CBSE Exam Points to Remember
- Enzymes are biological catalysts.
- The substrate binds to the active site.
- Formation of the enzyme–substrate (ES) complex is essential for catalysis.
- The induced fit model explains the slight change in enzyme shape after substrate binding.
- Enzymes lower activation energy, increasing the reaction rate.
- Enzymes are substrate-specific and remain unchanged after the reaction.
- Reaction sequence: E + S → ES → EP → E + P
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