9th Class Biology Notes 2026 — Chapter 7 – Federal Board
Class 9 Biology – Chapter 7: Metabolism
Board: Federal Board (FBISE)
Section I: Multiple Choice Questions
1. Which of the following statements is true about enzymes?
A) All enzymes are not proteins.
B) All enzymes are vitamins.
C) All enzymes are proteins.
D) All proteins are enzymes.
Correct Answer: C) All enzymes are proteins.
2. The lock-and-key hypothesis of enzyme action supports that:
A) Active sites are flexible.
B) Active sites are rigid.
C) Active site efficiency increases.
D) Active sites can change their shape.
Correct Answer: B) Active sites are rigid.
3. Which graph shows how temperature affects the rate of an enzyme-controlled reaction?
A) Graph A
B) Graph B
C) Graph C
D) Graph D
Correct Answer: Diagram required. The graph options were not included in the supplied material.
4. What is true about cofactors?
A) They break hydrogen bonds in proteins.
B) They help facilitate enzyme activity.
C) They increase activation energy.
D) They are composed of proteins.
Correct Answer: B) They help facilitate enzyme activity.
5. A change in pH can alter the active site by affecting the:
A) Ionization of amino acids
B) Shape of substrate
C) Ionization of cofactor
D) Ionization of coenzyme
Correct Answer: A) Ionization of amino acids
6. The catalytic region of an enzyme recognizes and binds the substrate and carries out the reaction. This region is called the:
A) Cofactor
B) Activator
C) Inhibitor
D) Active site
Correct Answer: D) Active site
7. The graph shows the activity of three digestive enzymes at different pH levels. Which statement is correct?
A) Enzyme X and Y are both active at pH 7.
B) Enzyme X and Z are both active at pH 4.
C) Enzyme Y and Z are both active at pH 4.
D) Enzyme Y and Z are both active at pH 7.
Correct Answer: Diagram required. The graph was not included in the supplied material.
8. The diagram shows an amylase molecule catalysing the breakdown of a starch molecule. Which are the labelled parts P, Q, R, and S?
A) P: Enzyme, Q: Product, R: Substrate, S: Active site
B) R: Enzyme, S: Product, P: Substrate, Q: Active site
C) S: Enzyme, P: Product, Q: Substrate, R: Active site
D) S: Enzyme, R: Product, P: Substrate, Q: Active site
Correct Answer: Diagram required. The labelled diagram was not included in the supplied material.
9. Glycolysis is the breakdown of:
A) Fructose
B) Glucose
C) Lactose
D) Maltose
Correct Answer: B) Glucose
10. The mechanism of ATP synthesis is:
A) Phosphorylation
B) Photosynthesis
C) Respiration
D) Glucose
Correct Answer: A) Phosphorylation
11. In aerobic respiration, pyruvic acid changes into:
A) Glucose
B) Fructose
C) Acetyl CoA
D) Citric acid
Correct Answer: C) Acetyl CoA
12. Which of the following uses oxygen as the final acceptor?
A) Glycolysis
B) Electron transport chain
C) Krebs cycle
D) Photosynthesis
Correct Answer: B) Electron transport chain
13. Which of the following produces carbon dioxide?
A) Krebs cycle
B) Electron transport chain
C) Glycolysis
D) Photosynthesis
Correct Answer: A) Krebs cycle
14. What are the products of the light reactions in photosynthesis?
A) ATP, NADPH, and oxygen
B) ATP and NADP
C) ATP, PGA, and oxygen
D) PGA and oxygen
Correct Answer: A) ATP, NADPH, and oxygen
15. The diagram shows an overview of photosynthesis. Which numbered molecule is an organic biomolecule?
A) I
B) II
C) III
D) IV
Correct Answer: Diagram required. The numbered diagram was not included in the supplied material.
16. Which feature helps plants make most of their food through photosynthesis?
A) Broad and flat leaves
B) Spiny leaves
C) Yellow leaves
D) Curled leaves
Correct Answer: A) Broad and flat leaves
17. The diagram shows pondweed in a test tube filled with water. Which conditions would cause the plant to produce more bubbles?
A) Dissolved CO₂: Present, Light: Bright, Temperature: Cool
B) Dissolved CO₂: Present, Light: Bright, Temperature: Warm
C) Dissolved CO₂: Present, Light: Dim, Temperature: Cool
D) Dissolved CO₂: Absent, Light: Dim, Temperature: Warm
Correct Answer: B) Dissolved CO₂: Present, Light: Bright, Temperature: Warm
18. Which of the following processes is used by plants to produce oxygen during photosynthesis?
A) Intake of water
B) Intake of CO₂
C) Photolysis of water
D) Calvin cycle
Correct Answer: C) Photolysis of water
19. The diagram shows an overview of cellular respiration. Which labelled part represents NADH?
A) X only
B) Y only
C) Z only
D) Both Y and Z
Correct Answer: Diagram required. The labelled diagram was not included in the supplied material.
20. A child left a carton on the lawn for two days. When the carton was picked up, the grass underneath had turned yellow. What caused the grass to change colour?
A) Lack of oxygen
B) Lack of carbon dioxide
C) Lack of light
D) Lack of water
Correct Answer: C) Lack of light
Section II: Short Answer Questions
1. Why are enzymes called biological catalysts?
Enzymes are biological catalysts, mostly made of proteins, that accelerate biochemical reactions without being consumed themselves.
They speed up metabolic processes inside living organisms under normal biological conditions. Therefore, they are called biological catalysts.
2. Name the factors affecting enzyme activity.
The main factors affecting enzyme activity are:
Temperature
pH
Substrate concentration
Enzyme concentration
Inhibitors
Activators
These factors influence the rate of enzyme-catalysed reactions.
3. At what pH do pepsin and trypsin enzymes act best?
Pepsin: Functions best in a strongly acidic medium, with an optimum pH of approximately 2.
Trypsin: Functions most efficiently in an alkaline medium, with an optimum pH of approximately 8.
4. What happens to an enzyme when it is frozen below 0°C?
Freezing an enzyme below \(0^\circ C\) generally does not permanently damage or denature its structure. Its catalytic activity temporarily decreases or stops because molecular movement and kinetic energy are reduced.
When the enzyme is thawed, it can regain its activity.
5. Which protein-digesting enzyme functions in an acidic medium?
Pepsin is the main protein-digesting enzyme present in the stomach. It functions effectively in an acidic medium.
Hydrochloric acid (HCl) in gastric juice maintains the acidic environment required for pepsin activity.
6. How does an enzyme lower activation energy?
Enzymes lower activation energy by bringing substrate molecules into the correct position and helping to weaken particular chemical bonds.
They stabilize the transition state and reduce the energy required for a reaction to occur. As a result, biochemical reactions take place more rapidly.
7. Why are enzymes specific, and why can’t each enzyme speed up many different reactions?
Enzymes are highly specific because their active sites have particular shapes and chemical properties that match specific substrates.
Only suitable substrates can bind to the active site and form an enzyme-substrate complex. Therefore, an enzyme usually catalyses a particular reaction or a group of closely related reactions.
8. Why is a small quantity of enzyme enough to catalyse a large number of substrate molecules into products?
Enzymes are not consumed during the reactions they catalyse. After a product is released, the enzyme remains available to bind with another substrate molecule.
In this way, a small quantity of enzyme can catalyse a large number of substrate molecules repeatedly.
9. According to the induced-fit model, the active site is flexible. Does this mean that any substrate can attach to this flexible active site? If not, explain.
No, any substrate cannot attach to a flexible active site.
The flexibility of the active site allows it to adjust its shape around a suitable substrate. However, the substrate must still have complementary chemical properties and a suitable structure to bind with the enzyme.
Therefore, enzymes remain highly specific even according to the induced-fit model.
10. Sketch the structure of ATP.
ATP (Adenosine Triphosphate) consists of three main components:
Adenine: A nitrogenous base.
Ribose: A five-carbon sugar.
Three phosphate groups: A chain of three phosphate groups attached to ribose.
The structure can be represented as:
The phosphate groups are involved in the storage and transfer of energy in cells.
11. Write the equations of the following processes:
(a) Photosynthesis
(b) Fermentation (Alcoholic)
(c) Aerobic Respiration
12. Name the products of anaerobic respiration in muscle cells.
The main product of anaerobic respiration in human muscle cells is lactic acid (lactate). A small amount of ATP energy is also produced.
13. Why is a part of photosynthesis called the dark reaction?
The dark reaction, also known as the Calvin cycle, is called the dark reaction because its biochemical steps do not directly require light energy.
However, it usually takes place during daylight because it depends on ATP and NADPH produced during the light reactions.
14. How are photosynthesis and aerobic respiration interlinked with each other?
Photosynthesis and aerobic respiration are complementary processes.
Photosynthesis produces glucose and oxygen, which are used in cellular respiration.
Aerobic respiration releases carbon dioxide and water, which plants use during photosynthesis.
Thus, the products of one process serve as the raw materials for the other.
Section III: Extensive Answer Questions
1. Define enzymes and describe their characteristics and specificity.
Definition
Enzymes are organic catalysts, mostly globular proteins, synthesized by living cells. They increase the rate of biochemical reactions without undergoing any permanent change themselves.
Characteristics of Enzymes
High Efficiency: Enzymes are highly efficient and can accelerate biochemical reactions millions of times.
Specificity: Each enzyme generally catalyses a specific reaction or acts on particular substrates.
Sensitivity: Enzyme activity is affected by temperature, pH, and substrate concentration.
Reusability: Enzymes remain unchanged at the end of a reaction and can be used repeatedly.
Biological Function: They regulate and accelerate metabolic reactions in living organisms.
Specificity of Enzymes
Two models explain how enzymes recognize their substrates:
Lock-and-Key Model: The active site has a relatively rigid shape that matches a specific substrate, just as a key fits into a particular lock.
Induced-Fit Model: The active site is flexible and adjusts its shape slightly when a suitable substrate binds to it.
Both models explain the specific nature of enzyme-substrate interactions.
2. What is activation energy? Explain with reference to enzymes.
Definition
Activation energy is the minimum amount of energy required to start a chemical reaction by breaking existing bonds and initiating the formation of new bonds.
Role of Enzymes
Chemical reactions require activation energy to proceed. Without enzymes, many biochemical reactions would occur too slowly under normal biological conditions.
Enzymes lower the activation energy by:
Forming temporary enzyme-substrate complexes.
Bringing reactant molecules closer together.
Positioning substrates correctly for the reaction.
Helping weaken particular chemical bonds.
Stabilizing the transition state.
As a result, metabolic reactions can occur rapidly at normal body temperatures, such as approximately \(37^\circ C\), without requiring excessive heat.
3. What happens to enzymes when you increase or decrease the following factors?
(a) Temperature
As temperature increases, the movement of molecules becomes faster, increasing enzyme activity up to an optimum temperature, commonly around \(37^\circ C\)–\(40^\circ C\) for many human enzymes.
Beyond the optimum temperature, excessive heat disrupts the bonds maintaining the enzyme’s structure. The enzyme becomes denatured and loses its activity.
At low temperatures, enzyme activity decreases because molecular movement slows down. However, low temperatures generally do not cause permanent damage.
(b) pH
Every enzyme works within a particular pH range and has an optimum pH at which it functions most efficiently.
A change in pH can affect the ionization of amino acids at the active site. This may alter the active site’s shape and reduce enzyme activity.
Extreme changes in pH can cause denaturation.
(c) Substrate Concentration
When substrate concentration increases, the rate of an enzyme-catalysed reaction initially increases because more substrate molecules are available to bind with active sites.
Eventually, all available active sites become occupied. The enzyme reaches its maximum reaction rate, known as \(V_{\max}\).
After this point, adding more substrate does not significantly increase the reaction rate unless more enzyme is added.
4. Only the related key can open the lock. How is this fact true for enzymes? Explain with examples.
This concept is explained by Emil Fischer’s Lock-and-Key Hypothesis (1894).
According to this model, the active site of an enzyme has a specific shape that matches its substrate. Just as only the correct key can fit into a particular lock, only a suitable substrate can bind to an enzyme’s active site.
Example:
The enzyme sucrase has an active site that is specifically suited to bind with sucrose. It catalyses the breakdown of sucrose into glucose and fructose.
Other sugars, such as lactose and maltose, do not fit the active site in the same way. Therefore, sucrase does not catalyse their breakdown.
This explains the specificity of enzymes.
5. Describe the structure and uses of ATP.
ATP stands for Adenosine Triphosphate. It is known as the energy currency of the cell because it stores and transfers energy for cellular activities.
Structure of ATP
ATP consists of three main components:
Adenine: A nitrogenous base.
Ribose: A five-carbon pentose sugar.
Phosphate Groups: Three phosphate groups arranged in a chain and attached to ribose.
Adenine and ribose together form adenosine. When three phosphate groups are attached, the complete molecule is called adenosine triphosphate (ATP).
Uses of ATP
ATP provides energy for many important cellular processes, including:
Active Transport: Provides energy for the movement of substances across cell membranes against their concentration gradients.
Muscle Contraction: Supplies energy for muscle fibres to contract.
Nerve Function: Supports energy-requiring processes involved in nerve impulse transmission.
Synthesis of Molecules: Provides energy for the formation of proteins and other complex molecules.
6. Describe and sketch the light reactions of photosynthesis.
Location
The light-dependent reactions take place on the thylakoid membranes inside chloroplasts.
Process
Chlorophyll molecules absorb sunlight and become excited.
The excited electrons move through an electron transport chain.
Light energy splits water molecules into oxygen, hydrogen ions, and electrons. This process is called photolysis.
The movement of electrons helps generate ATP through a process called photophosphorylation.
NADP⁺ accepts electrons and hydrogen to form NADPH.
Oxygen is released as a by-product.
Main Products
ATP
NADPH
Oxygen
Simplified Flow Diagram
7. Describe and sketch the dark reactions of photosynthesis.
Location
The dark reactions, also known as the Calvin cycle, take place in the stroma of the chloroplast.
Process
Carbon dioxide enters the Calvin cycle.
Carbon dioxide is fixed into organic molecules through a series of enzyme-controlled reactions.
ATP and NADPH produced during the light reactions provide the energy and reducing power required for these reactions.
The carbon compounds are reduced to form G3P, a three-carbon sugar.
Some G3P molecules are used to produce glucose and other organic compounds, while the remaining molecules help regenerate the starting carbon acceptor.
Although these reactions are called dark reactions, they do not directly require light. However, they usually occur during daylight because they depend on ATP and NADPH from the light reactions.
Simplified Flow Diagram
8. Explain the factors affecting the rate of photosynthesis.
The rate of photosynthesis is affected by several environmental factors.
1. Light Intensity
Increasing light intensity increases the rate of photosynthesis up to a certain saturation point. Beyond this point, the rate becomes nearly constant because other factors become limiting.
2. Carbon Dioxide Concentration
An increase in carbon dioxide concentration increases the rate of photosynthesis and sugar production up to a certain limit, provided other conditions remain suitable.
3. Temperature
Photosynthesis depends on enzyme-controlled reactions.
Very low temperatures slow down these reactions.
Suitable temperatures increase enzyme activity.
Excessively high temperatures can denature photosynthetic enzymes and reduce the rate of photosynthesis.
4. Water Availability
Water is an essential raw material for photosynthesis. When water is insufficient, plants may close their stomata to reduce water loss. This restricts the entry of carbon dioxide and decreases the rate of photosynthesis.
9. Compare respiration and photosynthesis with examples.
Photosynthesis | Respiration |
|---|---|
It is an anabolic process. | It is a catabolic process. |
It builds organic molecules such as glucose. | It breaks down glucose to release energy. |
It takes place in chloroplasts of photosynthetic cells. | It takes place in all living cells. |
It uses carbon dioxide and water. | Aerobic respiration uses glucose and oxygen. |
It converts light energy into chemical energy. | It releases energy in the form of ATP. |
It releases oxygen. | It produces carbon dioxide and water during aerobic respiration. |
Equation of Photosynthesis
Equation of Aerobic Respiration
Relationship Between the Two Processes
Photosynthesis and respiration are complementary processes. Photosynthesis produces glucose and oxygen, which are used in aerobic respiration. Respiration produces carbon dioxide and water, which can be used again during photosynthesis.
Together, these processes help maintain the balance of carbon dioxide and oxygen in the environment.
10. What are the advantages and significance of anaerobic respiration and fermentation in daily life?
Anaerobic respiration and fermentation allow cells and microorganisms to obtain energy without using oxygen. They are important in living organisms and have several applications in daily life.
1. Energy Production
Anaerobic respiration allows organisms to produce ATP when oxygen is unavailable or insufficient.
For example, human muscle cells can use anaerobic pathways during intense exercise when the oxygen supply cannot meet energy demands.
2. Baking and Brewing
Yeast carries out alcoholic fermentation, producing carbon dioxide and ethanol.
In baking, carbon dioxide causes bread dough to rise.
In brewing, ethanol is produced during the fermentation of sugars.
3. Dairy Industry
Lactic acid bacteria ferment milk sugars and produce lactic acid.
This process is used in the preparation of yogurt, cheese, and other fermented dairy products.
4. Survival in Low-Oxygen Conditions
Some microorganisms can obtain energy through anaerobic processes in environments where oxygen is absent or limited, such as waterlogged soils.
Thus, anaerobic respiration and fermentation are important for energy production, food preparation, and various industrial processes.
Notes by Sir Dilawar
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