Explore Chapter 7: Enzymes from the Class 9th Biology New Syllabus of the Lahore Board. This detailed post includes a thorough explanation of enzyme characteristics, the lock-and-key model vs. the induced-fit model, factors affecting enzyme activity, pH, temperature effects, competitive and non-competitive inhibition, and much more. Get exam-prepared with solved MCQs, short answers, and detailed explanations. Ideal for students looking to master this chapter for top performance in exams.”
1. Primarily, all enzymes are:
- Options:
a) Nucleic acids
b) Proteins
c) Carbohydrates
d) Lipids - Answer: b) Proteins
- Explanation: Enzymes are biological catalysts made up of proteins. They accelerate chemical reactions in the body.
- Tip: Remember, most enzymes end in “-ase,” and they are protein-based.
2. Which best defines an enzyme?
- Options:
a) A chemical that breaks down food.
b) A hormone that regulates metabolism.
c) A protein that speeds up reactions.
d) A molecule that stores energy. - Answer: c) A protein that speeds up reactions.
- Explanation: Enzymes lower the activation energy required for a chemical reaction, thereby speeding it up without being consumed in the process.
- Tip: Focus on the key term “speed up reactions” in the question.
3. What can happen if an enzyme is exposed to a temperature that is higher than its optimal temperature?
- Options:
a) Enzyme activity rate will increase.
b) Enzyme’s shape will change, potentially reducing its activity.
c) Enzyme will speed up the reaction and remain stable.
d) Enzyme will become a substrate itself. - Answer: b) Enzyme’s shape will change, potentially reducing its activity.
- Explanation: High temperatures can denature enzymes, causing them to lose their shape and function.
- Tip: Recall that enzymes have an optimal temperature range for activity.
4. Enzymes are specific in their action because:
- Options:
a) Their active sites fit specific substrates.
b) They are always proteins.
c) They are consumed in reactions.
d) They work only at high temperatures. - Answer: a) Their active sites fit specific substrates.
- Explanation: Enzyme specificity arises from the “lock-and-key” model, where the active site of an enzyme binds only to specific substrates.
- Tip: Visualize the lock-and-key analogy for enzyme specificity.
5. Prosthetic groups are:
- Options:
a) Required by all enzymes.
b) Proteins in nature.
c) Loosely attached with enzymes.
d) Tightly bound to enzymes. - Answer: d) Tightly bound to enzymes.
- Explanation: Prosthetic groups are non-protein molecules that are permanently attached to enzymes and assist in their catalytic activity.
- Tip: Remember, “tightly bound” differentiates prosthetic groups from coenzymes.
6. How does increasing temperature affect enzyme activity?
- Options:
a) Increases activity to a point.
b) Always decreases activity.
c) Makes enzymes non-functional.
d) No effect on enzyme. - Answer: a) Increases activity to a point.
- Explanation: Enzymes work optimally within a temperature range. Beyond this range, the activity decreases due to denaturation.
- Tip: Recall the bell-shaped curve of enzyme activity vs. temperature.
7. How does a competitive inhibitor affect enzyme action?
- Options:
a) Attaches with the substrate.
b) Changes enzyme shape.
c) Attaches and blocks the active site.
d) Blocks the cofactors. - Answer: c) Attaches and blocks the active site.
- Explanation: Competitive inhibitors compete with the substrate by binding to the enzyme’s active site, preventing substrate interaction.
- Tip: Remember, “competitive” means direct competition for the active site.
8. An enzyme works best at a pH of 7.4. It is placed in an acidic solution with a pH of 4.0. How will this affect the enzyme?
- Options:
a) The active site will be modified, reducing substrate binding.
b) Enzyme activity will increase.
c) Enzyme will become a substrate.
d) No change will occur. - Answer: a) The active site will be modified, reducing substrate binding.
- Explanation: A pH far from the enzyme’s optimal range can alter its shape, affecting its functionality.
- Tip: Recall that extreme pH values can denature enzymes.
9. What is TRUE according to the induced-fit model of enzyme action?
- Options:
a) Enzyme’s active site changes shape to bind the substrate.
b) Substrate must fit the enzyme perfectly before binding.
c) No shape changes occur during binding.
d) Enzyme is inactivated during the process. - Answer: a) Enzyme’s active site changes shape to bind the substrate.
- Explanation: The induced-fit model suggests that the enzyme undergoes a conformational change to better accommodate the substrate.
- Tip: Contrast this with the rigid “lock-and-key” model.
10. What is true about the optimum pH values of the following enzymes of the digestive system?
- Options:
a) Pepsin works at low pH while trypsin works at high pH.
b) Both work at high pH.
c) Both work at low pH.
d) Pepsin works at high pH while trypsin works at low pH. - Answer: a) Pepsin works at low pH while trypsin works at high pH.
- Explanation: Pepsin operates in the acidic environment of the stomach (low pH), while trypsin functions in the alkaline environment of the small intestine (high pH).
- Tip: Recall the specific environments where these enzymes are active.
B. Short Answer Questions
- Define metabolism. Differentiate between catabolism and anabolism.
- Answer: Metabolism refers to all the chemical processes in the body that maintain life.
- Catabolism: Breakdown of molecules to release energy (e.g., digestion).
- Anabolism: Synthesis of molecules, requiring energy (e.g., protein synthesis).
- Answer: Metabolism refers to all the chemical processes in the body that maintain life.
- Which type of metabolism demands input of energy? Give an example.
- Answer: Anabolism requires energy input. Example: DNA synthesis.
- Define an enzyme. What is its role in metabolism?
- Answer: An enzyme is a protein catalyst that speeds up chemical reactions in the body.
- Role in metabolism: Enzymes lower the activation energy required for metabolic reactions, making them efficient.
- Answer: An enzyme is a protein catalyst that speeds up chemical reactions in the body.
- What is the active site of an enzyme? State its importance in enzyme specificity.
- Answer: The active site is the region of an enzyme where the substrate binds and the reaction occurs.
- Importance: It ensures that only specific substrates fit, maintaining reaction specificity.
- Answer: The active site is the region of an enzyme where the substrate binds and the reaction occurs.
- Provide an example of a specific enzyme-substrate pair.
- Answer: Enzyme: Amylase, Substrate: Starch.
- How does pH affect enzyme activity?
- Answer: Deviations from the optimal pH can denature the enzyme, altering its structure and reducing activity.
- Provide two examples of enzymes that operate optimally at specific pH.
- Answer:
- Pepsin (pH 1.5–2)
- Trypsin (pH 8)
- Answer:
- What do you mean by optimum temperature and pH?
- Answer: Optimum temperature and pH are the conditions where an enzyme exhibits maximum activity.
- Which type of enzyme inhibitors inhibit the enzymes without attaching to the active site?
- Answer: Non-competitive inhibitors.
- Differentiate between competitive and non-competitive inhibition.
- Answer:
- Competitive inhibition: Inhibitor binds to the active site, blocking substrate binding.
- Non-competitive inhibition: Inhibitor binds elsewhere, altering the enzyme’s shape and reducing activity.
- Answer:
Detailed Answer
1. Describe the characteristics of enzymes.
- Biological Catalysts: Enzymes are proteins that catalyze biochemical reactions, increasing their speed without being consumed or permanently altered.
- Specificity: Each enzyme is highly specific to its substrate due to the unique shape of its active site, often described using the “lock-and-key” or “induced-fit” model.
- Reusable: Enzymes are not consumed during reactions and can be used repeatedly for the same type of reaction.
- Temperature Sensitivity: Enzymes work best within a narrow temperature range. High temperatures can denature them, while low temperatures slow down molecular motion, reducing activity.
- pH Sensitivity: Each enzyme has an optimal pH at which it functions most effectively. Extreme pH values can alter the enzyme’s structure and reduce its activity.
- Regulation: Enzymes can be regulated by activators (which increase activity) or inhibitors (which decrease activity).
- Cofactors and Coenzymes: Some enzymes require non-protein molecules (like metal ions or organic molecules) to function. These are called cofactors and coenzymes, respectively.
2. Describe how temperature extremes can inhibit enzyme activity and lead to enzyme denaturation.
- At High Temperatures:
- Enzymes are proteins, and high temperatures disrupt their hydrogen bonds, ionic bonds, and other interactions maintaining their structure.
- This denaturation leads to the unfolding of the enzyme, rendering it non-functional because the active site loses its specific shape.
- Example: Denaturation of human enzymes typically occurs above 40°C.
- At Low Temperatures:
- Molecular motion decreases significantly at lower temperatures, resulting in fewer collisions between enzymes and substrates.
- The enzyme-substrate complex formation slows down, leading to reduced reaction rates.
- Effects of Prolonged Temperature Extremes:
- High temperatures can cause permanent denaturation, while low temperatures often cause reversible inactivation.
- Optimal enzyme activity is observed within a specific temperature range, typically between 35°C and 40°C for most human enzymes.
- Practical Implications:
- Heat-sensitive enzymes are used in industries where precise temperature control is required, such as in brewing or pharmaceuticals.
3. How does pH affect enzyme activity?
- Optimal pH:
- Each enzyme functions best at a specific pH, called its “optimal pH.” For example, pepsin in the stomach works best at a pH of 1.5–2, while trypsin in the intestine works best at pH 8.
- This pH aligns with the enzyme’s natural environment.
- Effect on Ionization:
- pH changes affect the ionization of amino acids at the enzyme’s active site and the substrate.
- If the active site loses its correct charge distribution, it may fail to bind the substrate.
- Denaturation at Extreme pH:
- Both highly acidic and highly alkaline environments can disrupt the enzyme’s tertiary and quaternary structures by breaking hydrogen bonds and ionic interactions.
- This structural alteration prevents the enzyme from functioning effectively.
- Reversible vs. Irreversible Effects:
- Minor pH deviations may cause reversible changes in activity, but extreme pH shifts can permanently denature the enzyme.
- Example: Salivary amylase operates around neutral pH (7), but becomes inactive in the acidic environment of the stomach.
4. Briefly describe the factors that affect enzyme activity.
- Temperature:
- Enzyme activity increases with temperature until it reaches the optimum point, beyond which activity decreases due to denaturation.
- Example: Human enzymes have an optimal temperature around 37°C.
- pH:
- Each enzyme has a specific pH range for optimal activity. Deviations from this range reduce enzyme efficiency or lead to denaturation.
- Example: Digestive enzymes like pepsin (acidic) and trypsin (alkaline) have different pH optima.
- Substrate Concentration:
- Enzyme activity increases with substrate concentration until all active sites are saturated. Beyond this point, increasing substrate concentration has no further effect.
- Enzyme Concentration:
- Increasing enzyme concentration increases the rate of reaction, provided the substrate is in excess.
- Inhibitors:
- Competitive inhibitors: Bind to the active site, blocking substrate access.
- Non-competitive inhibitors: Bind elsewhere on the enzyme, altering its shape and reducing functionality.
- Cofactors and Coenzymes:
- These molecules are essential for the activity of some enzymes.
- Example: Metal ions like Mg²⁺ are cofactors, while vitamins like B6 act as coenzymes.
- Environmental Conditions:
- Factors like ionic strength, salinity, and pressure can also influence enzyme activity.
5. Compare the Lock-and-Key model and Induced-Fit model of enzyme action.
- Lock-and-Key Model:
- Proposed by Emil Fischer, it suggests that the enzyme’s active site is rigid and fits only specific substrates, like a key fits into a lock.
- Strength: Explains enzyme specificity well.
- Limitation: Fails to explain flexibility and conformational changes in enzymes.
- Induced-Fit Model:
- Proposed by Daniel Koshland, this model suggests that the enzyme’s active site is flexible and molds itself to fit the substrate.
- Strength: Explains the dynamic nature of enzyme-substrate interactions and the formation of the enzyme-substrate complex.
- Limitation: Slightly more complex to conceptualize compared to the lock-and-key model.
- Comparison:
- The lock-and-key model emphasizes rigidity and specificity, while the induced-fit model accounts for enzyme flexibility and adaptability.
- Induced-fit is considered more accurate and widely accepted today due to evidence from structural biology.
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