Chemical Reactions and Enzymes – Easy Line-by-Line Notes (NEET Beginner Level)
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| Chemical reactions and enzymes explained with simple diagrams for CBSE Class 11 Biology students. |
1. What is a Chemical Reaction?
- A chemical reaction is a process in which old chemical bonds break and new bonds are formed.
- As a result, new substances (products) are produced.
- Example: Ba(OH)₂ + H₂SO₄ → BaSO₄ + 2H₂O
NEET Point: Chemical reaction = Breaking + Formation of bonds.
2. Types of Changes in Chemical Compounds
A. Physical Change
- Only the shape, size, or state changes.
- No chemical bonds are broken or formed.
- No new substance is produced.
Examples
- Ice → Water
- Water → Water vapour
- Cutting paper
- Breaking glass
Remember: Physical change = No new substance.
B. Chemical Change
- Old bonds break and new bonds form.
- A new substance is produced.
- This process is called a chemical reaction.
Examples
- Burning wood
- Rusting of iron
- Digestion of food
- Hydrolysis of starch into glucose
Remember: Chemical change = New substance formed.
3. Organic and Inorganic Chemical Reactions
Inorganic Reaction
Example: Ba(OH)₂ + H₂SO₄ → BaSO₄ + 2H₂O
- Does not involve carbon compounds.
- Called an inorganic reaction.
Organic Reaction
Example: Starch + Water → Glucose
- Involves carbon-containing compounds.
- Called an organic reaction.
4. What is Rate of Reaction?
- Rate of reaction means how fast products are formed.
Formula
Rate = Change in Product (P) / Change in Time (t)
Rate = ΔP / Δt
Simple Meaning
If more product is formed in less time → Fast reaction
If less product is formed in more time → Slow reaction
5. Velocity
- Velocity is also the rate of reaction when direction is specified.
NEET Note: Usually, "rate" and "velocity" are used similarly in enzyme reactions.
6. Factors Affecting Rate of Reaction
The reaction rate depends on several factors:
- Temperature
- Enzyme (Catalyst)
- Concentration
- pH
7. Effect of Temperature
General rule:
- Increase temperature by 10°C → Rate becomes 2 times (doubles).
- Decrease temperature by 10°C → Rate becomes half.
NEET Trick: +10°C = ×2 reaction rate
8. What is a Catalyst?
- A catalyst increases the speed of a reaction.
- It is not consumed during the reaction.
In living organisms, enzymes are biological catalysts.
9. What are Enzymes?
- Enzymes are biological catalysts.
- Mostly made of proteins.
- They increase the speed of biochemical reactions.
- Enzymes remain unchanged after the reaction.
Definition for NEET:
Enzymes are biological catalysts that increase the rate of biochemical reactions without being consumed.
10. Example of Enzyme Action
Reaction:
CO₂ + H₂O → H₂CO₃
Enzyme used:
Carbonic anhydrase
Without Enzyme
- Only about 200 molecules of carbonic acid are formed per hour.
- Reaction is very slow.
With Enzyme
- About 600,000 molecules are formed every second.
- Reaction becomes extremely fast.
Speed Increase
The enzyme increases the reaction speed by about
10 million (10⁷) times.
NEET Important Fact:
Carbonic anhydrase is one of the fastest enzymes.
11. Why are Enzymes Amazing?
- They work extremely fast.
- Each enzyme performs a specific reaction.
- Thousands of different enzymes are present in cells.
Remember:
One enzyme = One specific reaction
12. Metabolic Pathway
A metabolic pathway is a series of chemical reactions.
- Each step is controlled by one enzyme or an enzyme complex.
- The product of one reaction becomes the reactant for the next reaction.
Example
Glucose → Pyruvic acid
This conversion occurs through 10 enzyme-catalysed reactions.
13. Glucose Breakdown
Overall reaction:
C₆H₁₂O₆ + O₂ → 2 Pyruvic acid + 2H₂O
Glucose is gradually converted into pyruvic acid through many enzyme-controlled steps.
14. Different Products Under Different Conditions
A. Anaerobic Conditions (No Oxygen)
Occurs in skeletal muscles during heavy exercise.
Product:
Lactic acid
B. Aerobic Conditions (Oxygen Present)
Occurs under normal conditions.
Product:
Pyruvic acid (which later enters further aerobic respiration).
C. Fermentation in Yeast
Occurs without oxygen.
Product:
Ethanol (Alcohol) + CO₂
15. NEET Quick Revision Table
| Topic | Easy Point |
|---|---|
| Physical change | No bond breaking, no new substance |
| Chemical reaction | Bonds break and new bonds form |
| Rate of reaction | Product formed per unit time |
| Formula | Rate = ΔP/Δt |
| Catalyst | Speeds up reaction without being used |
| Enzyme | Biological catalyst |
| Carbonic anhydrase | Converts CO₂ + H₂O → H₂CO₃ |
| Without enzyme | 200 molecules/hour |
| With enzyme | 600,000 molecules/second |
| Enzyme speed | About 10 million times faster |
| Metabolic pathway | Series of enzyme-controlled reactions |
| Glucose → Pyruvate | 10 enzyme-catalysed steps |
| Anaerobic muscle | Lactic acid |
| Yeast fermentation | Ethanol |
| Aerobic condition | Pyruvic acid continues in respiration |
NEET One-Line Revision
- Physical change → No new substance.
- Chemical reaction → Bonds break and new bonds form.
- Enzymes are biological catalysts.
- Enzymes increase reaction speed without being consumed.
- Carbonic anhydrase is one of the fastest enzymes.
- Reaction rate usually doubles with every 10°C increase.
- A metabolic pathway is a series of enzyme-controlled reactions.
- Glucose is converted to pyruvate in 10 enzymatic steps.
- Anaerobic muscles produce lactic acid.
- Yeast fermentation produces ethanol.
CBSE Class 11 Biology
Chapter: Biomolecules – Chemical Reactions and Enzymes
Complete Question Bank with Answers
A. Multiple Choice Questions (MCQs)
1. A chemical reaction involves:
a) Change in shape only b) Breaking and formation of chemical bonds c) Change in colour only d) Change in size only
Answer: b) Breaking and formation of chemical bonds
2. Which of the following is a physical change?
a) Rusting of iron b) Burning of paper c) Ice melting into water d) Digestion of starch
Answer: c) Ice melting into water
3. Which of the following is an inorganic chemical reaction?
a) Photosynthesis b) Hydrolysis of starch c) Ba(OH)₂ + H₂SO₄ → BaSO₄ + 2H₂O d) Respiration
Answer: c)
4. The rate of reaction is:
a) Amount of reactant formed per unit time b) Amount of product formed per unit time c) Amount of catalyst formed d) None of these
Answer: b)
5. Biological catalysts are called:
a) Vitamins b) Hormones c) Enzymes d) Minerals
Answer: c)
6. Carbonic anhydrase catalyses the formation of:
a) Glucose b) Carbonic acid c) Lactic acid d) Ethanol
Answer: b)
7. Enzymes increase the rate of reaction by:
a) Consuming reactants b) Producing ATP c) Lowering activation energy d) Increasing products directly
Answer: c)
8. During anaerobic respiration in muscles, pyruvic acid is converted into:
a) Ethanol b) Carbon dioxide c) Lactic acid d) Water
Answer: c)
9. Fermentation in yeast produces:
a) Oxygen b) Ethanol c) Glucose d) Starch
Answer: b)
10. One enzyme generally catalyses:
a) Many unrelated reactions b) One specific reaction c) Only inorganic reactions d) Only physical changes
Answer: b)
B. Very Short Answer Questions (1 Mark)
1. What is a chemical reaction?
Answer: A process in which old chemical bonds break and new bonds are formed.
2. Define enzyme.
Answer: Enzymes are biological catalysts that speed up biochemical reactions.
3. Name the enzyme that converts CO₂ and water into carbonic acid.
Answer: Carbonic anhydrase.
4. What is meant by metabolic pathway?
Answer: A series of enzyme-catalysed reactions occurring in sequence.
5. Write the formula for rate of reaction.
Answer: Rate = ΔP/Δt
C. Short Answer Questions (2 Marks)
1. Differentiate between physical and chemical changes.
Answer:
Physical Change:
- No new substance formed.
- No bond breaking.
Chemical Change:
- New substance formed.
- Bonds break and new bonds are formed.
2. Why are enzymes called biological catalysts?
Answer:
- They increase the rate of biochemical reactions.
- They remain unchanged after the reaction.
3. Mention any two characteristics of enzymes.
Answer:
- Highly specific.
- Increase reaction rate.
4. What happens to glucose during anaerobic respiration in muscles?
Answer: Glucose is converted into pyruvic acid, which is then converted into lactic acid.
D. Long Answer Questions (3–5 Marks)
1. Explain the role of enzymes in living organisms.
Answer:
- Enzymes are biological catalysts.
- They speed up biochemical reactions.
- They lower activation energy.
- They are highly specific.
- They remain unchanged after the reaction.
- Without enzymes, metabolic reactions would occur very slowly.
2. Explain metabolic pathway with an example.
Answer: A metabolic pathway is a sequence of enzyme-controlled reactions.
Example: Glucose → Pyruvic acid
This conversion occurs through ten enzyme-catalysed reactions. Under different conditions, pyruvic acid forms lactic acid or ethanol.
3. Explain the action of carbonic anhydrase.
Answer: Carbonic anhydrase catalyses the reaction:
CO₂ + H₂O → H₂CO₃
Without the enzyme, only about 200 molecules are formed per hour. With the enzyme, about 600,000 molecules are formed every second, making the reaction nearly 10 million times faster.
E. Fill in the Blanks
-
Enzymes are __________ catalysts. Answer: Biological
-
Rate of reaction = __________ / Δt. Answer: ΔP
-
Carbonic anhydrase converts CO₂ and water into __________. Answer: Carbonic acid
-
Glucose is converted into __________ during glycolysis. Answer: Pyruvic acid
-
In yeast, fermentation produces __________. Answer: Ethanol
F. Assertion and Reason
1.
Assertion (A): Enzymes increase the rate of chemical reactions.
Reason (R): Enzymes act as biological catalysts.
Answer: Both A and R are true, and R is the correct explanation of A.
2.
Assertion (A): Physical changes involve bond breaking.
Reason (R): New substances are formed.
Answer: Both A and R are false.
3.
Assertion (A): Carbonic anhydrase is a fast-acting enzyme.
Reason (R): It speeds up the formation of carbonic acid.
Answer: Both A and R are true, and R is the correct explanation.
G. Match the Columns
| Column A | Column B |
|---|---|
| Physical change | Ice melting |
| Enzyme | Biological catalyst |
| Carbonic anhydrase | Carbonic acid |
| Anaerobic muscles | Lactic acid |
| Yeast | Ethanol |
Answers: 1 – Ice melting
2 – Biological catalyst
3 – Carbonic acid
4 – Lactic acid
5 – Ethanol
H. Statement-Based Questions
Statement I
Physical changes do not produce new substances.
Statement II
Chemical reactions involve breaking and formation of chemical bonds.
Options: A. Both statements are true. B. Both are false. C. Statement I is true but II is false. D. Statement I is false but II is true.
Answer: A
Statement I
Carbonic anhydrase slows down the reaction.
Statement II
Enzymes are biological catalysts.
Answer: Statement I is false; Statement II is true.
I. Case Study Questions
Case Study
A scientist studied the conversion of carbon dioxide and water into carbonic acid. Without enzyme only 200 molecules of product were formed in one hour. After adding carbonic anhydrase, about 600,000 molecules were formed every second.
Questions
1. Name the enzyme involved.
Answer: Carbonic anhydrase.
2. What is the function of this enzyme?
Answer: It speeds up the conversion of CO₂ and water into carbonic acid.
3. What type of catalyst is this?
Answer: Biological catalyst.
4. Approximately how much faster does the enzyme make the reaction?
Answer: About 10 million times faster.
J. Important Exam Questions
1 Mark
- Define enzyme.
- Define chemical reaction.
- What is a metabolic pathway?
- Write the rate formula.
- Name the fastest enzyme discussed.
2 Marks
- Differentiate between physical and chemical changes.
- Why are enzymes called catalysts?
- Mention two properties of enzymes.
3 Marks
- Explain enzyme specificity.
- Explain carbonic anhydrase reaction.
- Explain metabolic pathway.
5 Marks
- Explain enzymes and their importance in metabolism with suitable examples.
- Describe the conversion of glucose into pyruvic acid and explain its products under aerobic and anaerobic conditions.
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