9th Class Biology Chapter 6 Notes cover the topic of Enzymes, an important chapter in the matric Biology syllabus. This chapter explains metabolism, enzyme structure, characteristics, and the factors that affect enzyme action.
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These 9th Class Biology Chapter 6 Notes are prepared according to the Punjab Board syllabus and are useful for quick revision, exam preparation, and test papers.
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Metabolism and Its Types
Metabolism is the set of biochemical reactions that occur in living organisms to maintain life. The word metabolism comes from a Greek word meaning “change.” The concept of metabolism was first introduced by Ibn-e-Nafees.
Metabolism allows organisms to grow, reproduce, maintain their structures, and respond to their environment. It has two main types:
Anabolism
Anabolism includes biochemical reactions in which larger molecules are synthesized. Energy is utilized in this process. Examples include:
- Photosynthesis
- Assimilation
Catabolism
Catabolism includes biochemical reactions in which larger molecules are broken down. Energy is released in this process. Examples include:
- Respiration
- Digestion of food
Role of Enzymes in Metabolism
During metabolism, chemicals are transformed from one form to another by enzymes. Enzymes are crucial to metabolism because they act as biocatalysts and speed up and regulate metabolic pathways.
What are Enzymes?
Enzymes are proteins that catalyze (speed up) biochemical reactions without being changed during the reaction. The term “enzyme” was first used by German physiologist Winhelm Kuhne in 1878.
Key Terms in Enzyme Action
- Substrate – the molecule on which an enzyme acts
- Product – the molecule formed after the enzyme converts the substrate
- Activation energy – the minimum energy required to start a reaction
Enzymes lower the activation energy barrier, allowing reactions to proceed at a faster rate.
Ways Enzymes Lower Activation Energy
- Altering the shape of the substrate
- Disrupting the charge distribution on substrates
- Bringing substrates into the correct orientation to react
Characteristics of Enzymes
This section of the 9th Class Biology Chapter 6 Notes explains the important characteristics of enzymes:
- All enzymes are proteins – made of amino acids
- Increase reaction rate – millions of times faster than uncatalyzed reactions
- Not consumed by reaction – enzymes remain unchanged after the reaction
- Enzymes are specific – each enzyme works on a specific substrate
- Active sites – the small region of the enzyme that binds the substrate
- Enzyme activity can be regulated – by inhibitors and activators
- Enzymes work in a specific order – forming metabolic pathways
Cofactors, Coenzymes, and Prosthetic Groups
Some enzymes need extra non-protein components called cofactors to function properly:
- Cofactors – can be inorganic (metal ions) or organic (flavin, heme)
- Prosthetic groups – organic cofactors tightly bound to the enzyme
- Co-enzymes – organic cofactors loosely attached to the enzyme (e.g., riboflavin, thiamine, folic acid)
Uses of Enzymes in Industry
Enzymes are widely used in different industries for fast chemical reactions:
- Food Industry – enzymes break starch into simple sugars for bread and buns
- Brewing Industry – enzymes break starch and proteins for fermentation
- Paper Industry – enzymes break starch to lower viscosity, aiding paper production
- Biological Detergents – protease enzymes remove protein stains; amylase enzymes remove starch residues
Factors Affecting Enzyme Action
This part of the 9th Class Biology Chapter 6 Notes covers the major factors that influence enzyme activity, a topic frequently asked in exams.
1. Temperature
Increasing temperature speeds up enzyme-catalyzed reactions, but only up to a certain point called the optimum temperature.
- Every enzyme has its own optimum temperature
- Human enzymes work best at 37°C
- Beyond optimum temperature, enzymes lose their shape
Denaturation of Enzymes
When temperature rises well above the optimum, heat energy increases the vibration of atoms in the enzyme, causing it to lose its globular structure. This is called denaturation, and it can block enzyme activity completely.
2. Substrate Concentration
Increasing substrate concentration increases the reaction rate, but only until all active sites are occupied.
Saturation of Active Sites
When all active sites are occupied at high substrate concentration, adding more substrate does not increase the reaction rate. This state is called saturation of active sites.
3. pH (Optimum pH)
Enzymes work at their maximum rate at a specific pH called the optimum pH. A slight change in pH can slow down or completely block enzyme activity.
Examples:
- Pepsin (works in stomach) – active in acidic medium (low pH)
- Trypsin (works in small intestine) – active in alkaline medium (high pH)
Mechanism of Enzyme Action
When an enzyme attaches to a substrate, a temporary enzyme-substrate (ES) complex is formed. The enzyme catalyzes the reaction, converting the substrate into product. The ES complex then breaks, releasing the enzyme and product.
Lock and Key Model
Proposed by German chemist Emil Fischer in 1894, this model suggests that the enzyme and substrate have specific shapes that fit exactly into one another, like a lock and key. This model explains enzyme specificity.
Induced Fit Model
Proposed by American biologist Daniel Koshland in 1958, this model is more widely accepted than the lock and key model. According to this model, the active site is not rigid — it changes shape (is “molded”) to fit the substrate properly.
Specificity of Enzymes
There are over 2,000 known enzymes, and each is involved in one specific chemical reaction. Enzymes are also substrate specific.
Examples of Enzyme Specificity
- Protease – breaks peptide bonds in proteins, does not work on starch
- Amylase – breaks down starch
- Lipase – acts only on lipids, digesting them into fatty acids and glycerol
The specificity of enzymes is determined by the geometric shape of their active sites, which fit only with specific substrates.
FAQs
Q1. What topics are covered in 9th Class Biology Chapter 6 Notes?
These notes cover metabolism, enzyme structure, characteristics of enzymes, factors affecting enzyme action, and the lock and key and induced fit models, as per the Punjab Board syllabus.
Q2. What is the difference between anabolism and catabolism?
Anabolism is the synthesis of larger molecules and requires energy, like photosynthesis. Catabolism is the breakdown of larger molecules and releases energy, like respiration and digestion.
Q3. What factors affect enzyme action?
Three main factors affect enzyme action: temperature, substrate concentration, and pH. Each enzyme has an optimum temperature and pH where it works at maximum efficiency, and beyond these limits, activity decreases.
Q4. What is the difference between the lock and key model and the induced fit model?
The lock and key model says enzyme and substrate shapes fit perfectly, like a lock and key. The induced fit model says the active site changes shape slightly to mold around the substrate for a better fit.
Q5. Why do enzymes stop working at high temperatures?
At high temperatures, heat energy increases atomic vibrations in the enzyme, causing it to lose its globular shape. This process, called denaturation, blocks the enzyme’s ability to bind with its substrate.
Q6. Why should I study 9th Class Biology Chapter 6 Notes before exams?
These notes simplify complex topics like enzyme mechanisms and factors affecting enzyme activity into easy-to-understand points, helping students revise faster and score better in their board exams.