Chapter 7: Metabolism

Select the correct answer:

1. Which is true about enzyme?

A) all enzymes are not protein

B) all enzymes are vitamins

C) all enzymes are proteins

D) all enzymes are enzyme

Answer: C) all enzymes are proteins (Note: While ribozymes are RNA, the vast majority of enzymes are proteins. Option C is the standard answer in most textbooks.)

2. 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 site can change its shape

Answer: B) active sites are rigid

Explanation: The lock and key model proposes that the active site is a rigid structure that is perfectly complementary to the substrate. The induced fit model proposes flexibility.

3. Which graph shows how temperature affects the rate of an enzyme-controlled reaction? (Graphs A, B, C, D showing different temperature-rate relationships)

Answer: D (Graph showing an increase in rate up to an optimum temperature, followed by a sharp decrease due to denaturation.)

4. What is true about cofactors?

A) break hydrogen bond in proteins

B) help facilitate enzyme activity

C) increase activation energy

D) are composed of proteins

Answer: B) help facilitate enzyme activity

Explanation: Cofactors are non-protein molecules or ions that assist enzymes in catalyzing reactions. They can help stabilize the enzyme, participate in catalysis, or carry chemical groups.

5. 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

Answer: A) ionization of amino acids

Explanation: Changes in pH alter the ionization state of amino acid side chains in the active site, disrupting ionic bonds and hydrogen bonds, which can change the shape of the active site and affect enzyme activity.

6. The catalytic region of enzyme recognizes and binds the substrate and carries the reaction. This region is called as:

A) cofactor

B) activator

C) inhibitor

D) active site

Answer: D) active site

Explanation: The active site is the specific region of an enzyme where the substrate binds and the chemical reaction occurs.

7. The graph shows the activity of three digestive enzymes at different pH levels. Which statement is incorrect? (Graph showing activity curves for enzymes X, Y, Z at different pH)

A) Enzyme X and Y are both active at pH 7

B) Enzyme X and Z are both active at pH 2

C) Enzyme Y and Z are both active at pH 7

D) Enzyme Y and Z are both active at pH 4

Answer: B) Enzyme X and Z are both active at pH 2 (This is likely incorrect if X is a stomach enzyme (pepsin, optimum pH 2) and Z is an intestinal enzyme (trypsin, optimum pH 8).)

8. The diagram shows an enzyme molecule catalyzing the breakdown of a starch molecule. Which are the labelled parts P, Q, R and S?

Answer: C (S = Enzyme, P = Product 1, Q = Substrate, R = Active site)

9. Glycolysis is the breakdown of

A) fructose

B) glucose

C) lactose

D) maltose

Answer: B) glucose

Explanation: Glycolysis is the metabolic pathway that converts glucose into pyruvate, producing ATP and NADH.

10. The mechanism of ATP synthesis is

A) phosphorylation

B) photosynthesis

C) respiration

D) glucose

Answer: A) phosphorylation

Explanation: ATP synthesis involves the addition of a phosphate group to ADP, which is called phosphorylation. This can occur through substrate-level phosphorylation or oxidative phosphorylation.

11. In aerobic respiration pyruvic acid changes to

A) glucose

B) fructose

C) Acetyl CoA

D) citric acid

Answer: C) Acetyl CoA

Explanation: In aerobic respiration, pyruvic acid (pyruvate) is converted to Acetyl CoA before entering the Krebs cycle.

12. Which of these uses oxygen as the final acceptor?

A) glycolysis

B) electron transport chain

C) Krebs cycle

D) photosynthesis

Answer: B) electron transport chain

Explanation: In the electron transport chain, oxygen acts as the final electron acceptor, combining with electrons and protons to form water.

13. Which of these produces carbon dioxide?

A) Krebs cycle

B) electron transport chain

C) glycolysis

D) photosynthesis

Answer: A) Krebs cycle

Explanation: Carbon dioxide is released during the Krebs cycle (citric acid cycle) during the decarboxylation of isocitrate and alpha- ketoglutarate.

14. What are the products of light reactions in photosynthesis?

A) ATP, NADPH and oxygen

B) ATP and NADP

C) ATP, PGA and oxygen

D) PGA and oxygen

Answer: A) ATP, NADPH and oxygen

15. The diagram shows an overview of photosynthesis. Which numbered molecules are organic biomolecule? (Diagram of chloroplast with numbered molecules)

A) I

B) II

C) III

D) IV

Answer: C or D (Organic molecules like glucose would be the product of the Calvin cycle.)

16. Which feature helps plants to make most food by the process of photosynthesis?

A) broad and flat leaves

B) spiny leaves

C) yellow leaves

D) curled leaves

Answer: A) broad and flat leaves

Explanation: Broad, flat leaves provide a large surface area for absorbing sunlight, which is essential for photosynthesis.

17. The diagram shows a pond weed in a test tube filled with water. Which conditions would cause the plant to produce more bubbles?

Answer: B (Present CO₂, Bright light, Warm temperature provides optimal conditions for photosynthesis.)

18. Which of the following processes is used by plants to make oxygen during the process of photosynthesis?

A) intake of water

B) intake of CO₂

C) photolysis of water

D) Calvin cycle

Answer: C) photolysis of water

Explanation: Oxygen is produced during the light-dependent reactions when water is split (photolysis) to provide electrons and protons.

19. The diagram shows overview of cellular respiration. Which labelled part represents NADH? (Diagram of cellular respiration with labeled parts X, Y, Z)

A) X only

B) Y only

C) Z only

D) both Y and Z

Answer: (NADH is produced in glycolysis and the Krebs cycle.)

20. A child left a carton on the lawn for two days. When the carton was picked up, the grass under it 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

Answer: C) lack of light

Explanation: Without light, chlorophyll breaks down, and the grass turns yellow because photosynthesis cannot occur.

1. Why enzymes are called biological catalyst?

Answer: Enzymes are called biological catalysts because they speed up chemical reactions in living organisms without being consumed or permanently altered in the process. They lower the activation energy required for reactions to occur, allowing metabolic processes to proceed at rates sufficient for life. Like inorganic catalysts, they are required in small amounts and do not affect the equilibrium of a reaction. However, enzymes are highly specific and are typically proteins. They are essential for virtually all cellular processes, including digestion, metabolism, and DNA replication.

2. Name the factors affecting enzyme activity.

Answer: The factors affecting enzyme activity include:

Temperature: Enzymes have an optimum temperature; activity increases up to this point, then decreases due to denaturation. pH: Each enzyme has an optimum pH; deviations can alter the active site and reduce activity. Substrate Concentration: Activity increases with substrate concentration until the enzyme becomes saturated (Vmax). Enzyme Concentration: Activity is directly proportional to enzyme concentration when substrate is in excess. Inhibitors: Competitive and non-competitive inhibitors decrease enzyme activity. Cofactors and Coenzymes: Required for the activity of some enzymes.

3. At what pH pepsin and trypsin enzymes act the best?

Answer: Pepsin acts best at a very acidic pH, typically around pH 2.0, which is the pH of the stomach. It is a digestive enzyme that breaks down proteins into peptides. Trypsin acts best at a slightly alkaline pH, typically around pH 8.0, which is the pH of the small intestine. It is a pancreatic enzyme that continues protein digestion. These different pH optima reflect the different environments in which these enzymes function.

4. What happens to an enzyme when it is frozen below 0°C?

Answer: When an enzyme is frozen below 0°C, it becomes inactive but is generally not denatured. The low temperature reduces the kinetic energy of the enzyme and substrate molecules, slowing down or stopping the reaction. However, the enzyme's three- dimensional structure is typically preserved. If the enzyme is thawed, its activity is usually restored. This is different from high temperatures, which cause irreversible denaturation. Freezing is often used to preserve enzymes for long-term storage.

5. Which protein digesting enzyme functions in acidic medium?

Answer: Pepsin is a protein-digesting enzyme that functions in an acidic medium. It is secreted by the chief cells of the stomach as an inactive precursor called pepsinogen. Pepsinogen is activated to pepsin by hydrochloric acid (HCl) in the stomach. Pepsin breaks down proteins into smaller peptides. Its optimum pH is around 2.0. Other protein-digesting enzymes, like trypsin and chymotrypsin, function in the alkaline environment of the small intestine.

6. How enzyme lowers the activation energy?

Answer: Enzymes lower the activation energy of a reaction by providing an alternative reaction pathway with a lower energy barrier. They achieve this through several mechanisms:

Bringing Substrates Together: Enzymes bind substrates in the correct orientation and proximity. Straining Bonds: They distort the substrate's bonds, making them easier to break. Providing Microenvironment: The active site can provide a specific pH or non-polar environment that favors the reaction. Stabilizing Transition State: They stabilize the transition state of the reaction. By lowering the activation energy, a greater fraction of molecules can react at a given temperature, increasing the reaction rate.

7. Why are enzyme specific and why can't each one speed up many different reactions?

Answer: Enzymes are specific because their active site has a unique three-dimensional shape and chemical properties that are complementary to a specific substrate (or a group of closely related substrates). The substrate binds to the active site like a key fits a lock (or through induced fit). This specificity is determined by the amino acid sequence of the enzyme. Because the active site is tailored to a specific substrate, it cannot accommodate many different molecules. Thus, each enzyme typically catalyzes only one type of reaction or a small set of related reactions. This specificity ensures that metabolic pathways are tightly regulated.

8. Why small quantity of enzyme is enough for catalysing large number of substrate molecules into products?

Answer: A small quantity of enzyme is enough because enzymes are not consumed or permanently altered during the reaction. After catalyzing the conversion of a substrate to a product, the enzyme is released unchanged and can bind to another substrate molecule. This allows a single enzyme molecule to participate in the reaction cycle many times. The number of substrate molecules converted per enzyme molecule per unit time is called the turnover number, which can be very high (e.g., catalase can convert millions of molecules per second). Therefore, only a small amount of enzyme is needed.

9. According to induced fit model, the active site is flexible. Does it mean that any substrate can attach with this flexible active site? If not, then explain.

Answer: No, the flexibility of the active site in the induced fit model does not mean that any substrate can attach. The active site is flexible, but it still has a specific shape and chemical properties that are complementary to a particular substrate. When the correct substrate binds, the active site changes shape (induced fit) to better fit the substrate and catalyze the reaction. Other molecules that do not have the correct shape or chemical properties cannot bind effectively. The specificity is still maintained; the flexibility just allows for a tighter fit and better catalysis.

10. Sketch the structure of ATP.

Answer: (Diagram Description) ATP (Adenosine Triphosphate) consists of:

Adenine: A nitrogenous base. Ribose: A five-carbon sugar. Three Phosphate Groups: Attached in a chain to the 5' carbon of the ribose. The bonds between the phosphate groups are phosphoanhydride bonds, which are high-energy bonds. The terminal phosphate group is often removed to release energy, forming ADP (adenosine diphosphate) and inorganic phosphate (Pi).

11. Write the equation of: (a) photosynthesis (b) fermentation (c) aerobic respiration.

Answer:

(a) Photosynthesis: 6CO₂ + 12H₂O + light energy [source symbol unavailable] C₆H₁₂O₆ + 6O₂ + 6H₂O

(b) Fermentation (Alcoholic): C₆H₁₂O₆ [source symbol unavailable] 2C₂H₅OH + 2CO₂ + energy (ATP)

(c) Aerobic Respiration: C₆H₁₂O₆ + 6O₂ [source symbol unavailable] 6CO₂ + 6H₂O + energy (ATP)

12. Name the products of anaerobic respiration in muscle cells.

Answer: The products of anaerobic respiration in muscle cells are lactic acid and a small amount of ATP. During intense exercise, when oxygen supply is limited, glucose is broken down into lactic acid. This process is called lactic acid fermentation. The accumulation of lactic acid causes muscle fatigue and cramps. The ATP produced provides energy for short bursts of activity.

13. Why a part of photosynthesis is called dark reactions.

Answer: A part of photosynthesis is called dark reactions (or light-independent reactions) because it does not directly require light energy to proceed. The dark reactions, also known as the Calvin cycle, use the ATP and NADPH produced during the light- dependent reactions to fix carbon dioxide into glucose. While they do not directly use light, they are dependent on the products of the light reactions and typically occur in the stroma of the chloroplast. They can occur in the presence or absence of light.

14. How photosynthesis and aerobic respiration are interlinked with each other?

Answer: Photosynthesis and aerobic respiration are interlinked because they are essentially reverse processes. Photosynthesis uses carbon dioxide and water to produce glucose and oxygen, while aerobic respiration uses glucose and oxygen to produce carbon

dioxide, water, and ATP. The products of one process are the reactants of the other. They form a cycle that maintains the balance of oxygen and carbon dioxide in the atmosphere. They are also interdependent in terms of energy flow in ecosystems.

1. Define enzyme and describe their characteristics and specifications.

Answer: Enzyme Definition: Enzymes are biological catalysts, primarily proteins, that speed up chemical reactions in living organisms by lowering the activation energy required for the reaction. They are highly specific and are not consumed or permanently altered during the reaction.

Characteristics of Enzymes:

1. Protein Nature: Almost all enzymes are globular proteins (with the exception of ribozymes).

2. Catalytic Efficiency: They are highly efficient, increasing reaction rates by factors of 10⁶ to 10¹².

3. Specificity: They are highly specific to their substrates (lock and key or induced fit models).

4. Reversibility: They catalyze reversible reactions and do not alter the equilibrium.

5. pH Sensitivity: Each enzyme has an optimal pH.

6. Temperature Sensitivity: Each enzyme has an optimal temperature; high temperatures cause denaturation.

7. Cofactor Requirement: Many require cofactors (metal ions) or coenzymes (organic molecules).

8. Regulation: Activity can be regulated by inhibitors, activators, and feedback mechanisms.

9. Turnover Number: The number of substrate molecules converted per enzyme molecule per unit time.

10. Location: They are often compartmentalized within cells.

Specifications:

Enzymes are named by adding the suffix "-ase" to the substrate name (e.g., lactase) or the reaction type (e.g., dehydrogenase). They have an active site where substrate binds. They lower activation energy.

In summary, enzymes are essential biological catalysts with specific characteristics that make them vital for all metabolic processes.

2. What is energy of activation? Explain with reference to enzyme.

Answer: Energy of Activation (Ea): The energy of activation is the minimum amount of energy that reactant molecules must possess in order to undergo a chemical reaction and form products. It represents the energy barrier that must be overcome for the reaction to proceed. Even for exergonic (spontaneous) reactions, an initial input of energy is needed to reach the transition state.

Reference to Enzymes: Enzymes lower the activation energy of a reaction. They do this by providing an alternative reaction pathway with a lower energy barrier. The enzyme binds to the substrate, forming an enzyme-substrate complex. This interaction stabilizes the transition state and weakens the bonds in the substrate, making it easier for the reaction to occur.

Mechanism:

Enzymes bring substrates together in the correct orientation. They strain the chemical bonds in the substrate. They provide a favorable microenvironment (e.g., specific pH) for the reaction. They may participate directly in catalysis by forming temporary covalent bonds.

Importance: By lowering the activation energy, enzymes allow reactions to proceed at a much faster rate at physiological temperatures. Without

enzymes, many metabolic reactions would be too slow to sustain life.

In summary, the energy of activation is the energy barrier for a reaction, and enzymes lower this barrier to speed up biochemical reactions.

3. What happens to enzymes when you increase or decrease: (a) Temperature (b) pH (c) Substrate concentration.

Answer:

(a) Temperature:

Increase: As temperature increases, the rate of reaction increases up to an optimum temperature (e.g., 37°C for human enzymes). This is because higher temperatures increase the kinetic energy of molecules, leading to more frequent and effective collisions. Above the optimum, the enzyme denatures (loses its 3D shape), and the rate decreases sharply. Decrease: Lower temperatures decrease kinetic energy and collision frequency, slowing the reaction. However, the enzyme is not denatured and can regain activity when warmed.

(b) pH:

Increase or Decrease from Optimum: Each enzyme has an optimal pH. Deviations from this pH alter the ionization of amino acid side chains in the active site. This disrupts ionic bonds and hydrogen bonds, changing the shape of the active site and reducing the enzyme's ability to bind the substrate. Extreme pH values can cause irreversible denaturation.

(c) Substrate Concentration:

Increase: At low substrate concentrations, the rate of reaction is directly proportional to substrate concentration. As substrate concentration increases, more active sites are occupied, and the rate increases. At a very high substrate concentration, all active sites are saturated (Vmax), and further increase in substrate does not increase the rate. Decrease: Lower substrate concentration reduces the frequency of collisions between enzyme and substrate, decreasing the reaction rate.

4. Only the related key can open the lock. How this fact is true for enzyme? Explain with examples.

Answer: The statement "Only the related key can open the lock" is a metaphor for enzyme specificity, described by the lock and key model proposed by Emil Fischer.

Lock and Key Model:

The enzyme (lock) has a rigid active site with a specific shape. The substrate (key) has a complementary shape. Only the substrate with the correct shape can fit into the active site, forming an enzyme-substrate complex. This ensures that each enzyme catalyzes only a specific reaction.

Examples:

1. Lactase: The enzyme lactase specifically breaks down lactose (milk sugar) into glucose and galactose. It cannot break down sucrose or maltose because their shapes do not fit the active site of lactase.

2. Amylase: Salivary amylase specifically breaks down starch into maltose. It cannot digest proteins or lipids.

3. Urease: The enzyme urease specifically breaks down urea into ammonia and carbon dioxide. It cannot act on other substrates.

Induced Fit Model:

The lock and key model has been refined by the induced fit model, which proposes that the active site is flexible and changes shape upon substrate binding to better fit the substrate and catalyze the reaction.

In summary, enzyme specificity is analogous to a lock and key, where only the specific substrate can bind to the enzyme's active site and undergo reaction.

5. Describe the structure and uses of ATP.

Answer: Structure of ATP (Adenosine Triphosphate): ATP is a nucleotide composed of three main parts:

1. Adenine: A nitrogenous base (purine).

2. Ribose: A five-carbon sugar.

3. Three Phosphate Groups: Attached in a chain to the 5' carbon of the ribose.

The bonds between the phosphate groups are phosphoanhydride bonds, which are high-energy bonds. The terminal phosphate group is often removed to release energy, forming ADP (adenosine diphosphate) and inorganic phosphate (Pi).

Uses of ATP: ATP is the universal energy currency of the cell. It provides energy for:

1. Muscle Contraction: ATP powers the sliding of actin and myosin filaments.

2. Active Transport: ATP powers ion pumps (e.g., sodium-potassium pump).

3. Protein Synthesis: ATP is required for translation.

4. DNA Replication: ATP is required for replication.

5. Nerve Impulse Transmission: ATP powers ion pumps in neurons.

6. Photosynthesis: ATP is produced in light reactions and used in the Calvin cycle.

7. Cell Division: ATP is required for spindle formation and chromosome movement.

In summary, ATP is a high-energy molecule that stores and transfers energy for all cellular processes. Its structure allows it to release energy rapidly when needed.

6. Describe and sketch light reactions of photosynthesis.

Answer: Light Reactions of Photosynthesis:

The light reactions occur in the thylakoid membranes of the chloroplast. They require light energy to produce ATP and NADPH, and release oxygen as a byproduct.

Steps:

1. Light Absorption: Pigments (chlorophyll a, chlorophyll b, carotenoids) in the antenna complex absorb light energy and transfer it to the reaction center.

2. Excitation of PSII: In Photosystem II (P680), light energy excites an electron, which is transferred to the primary electron acceptor.

3. Photolysis of Water: The electron hole in P680 is filled by an electron from water. Water is split (photolysis) into oxygen, protons (H⁺), and electrons. Oxygen is released as a byproduct.

4. Electron Transport Chain: The excited electron from PSII is passed through a series of electron carriers (plastoquinone, cytochrome b6f, plastocyanin), generating a proton gradient.

5. Excitation of PSI: In Photosystem I (P700), light energy excites an electron, which is transferred to ferredoxin.

6. NADPH Production: The electron from ferredoxin reduces NADP⁺ to NADPH.

7. ATP Synthesis: The proton gradient drives ATP synthase to produce ATP (photophosphorylation).

Sketch Description: (A diagram would show the thylakoid membrane with PSII, PSI, electron transport chain, ATP synthase, and the flow of electrons from water to NADP⁺, with proton gradient and ATP/NADPH production.)

In summary, light reactions convert light energy into chemical energy (ATP and NADPH) and produce oxygen.

7. Describe and sketch dark reactions of photosynthesis.

Answer: Dark Reactions (Calvin Cycle): The dark reactions, also known as the Calvin cycle or light-independent reactions, occur in the stroma of the chloroplast. They use ATP and NADPH from the light reactions to fix carbon dioxide into glucose.

Steps:

1. Carbon Fixation: CO₂ is fixed by the enzyme RuBisCO onto ribulose-1,5-bisphosphate (RuBP), forming an unstable 6- carbon intermediate that splits into two molecules of 3-phosphoglycerate (3-PGA).

2. Reduction: Each 3-PGA is phosphorylated by ATP and reduced by NADPH to glyceraldehyde-3-phosphate (G3P).

3. Regeneration of RuBP: Most of the G3P is used to regenerate RuBP, requiring ATP. One G3P molecule is used to synthesize glucose.

Sketch Description: (A diagram would show the Calvin cycle with three phases: Carbon fixation, Reduction, and Regeneration. It would show CO₂ entering, ATP and NADPH being used, and G3P being produced.)

In summary, dark reactions use ATP and NADPH to fix CO₂ into glucose. They are called dark reactions because they do not directly require light.

8. Explain the factors affecting rate of photosynthesis.

Answer: The rate of photosynthesis is affected by several environmental factors:

1. Light Intensity: As light intensity increases, the rate of photosynthesis increases up to a saturation point. Beyond this point, other factors become limiting.

2. Carbon Dioxide Concentration: As CO₂ concentration increases, the rate of photosynthesis increases up to a saturation point. CO₂ is often the limiting factor in nature.

3. Temperature: The rate of photosynthesis increases with temperature up to an optimum (usually 25-35°C). Above this, enzymes denature, and the rate decreases.

4. Water Availability: Water is a reactant in photosynthesis. Water stress can reduce the rate by causing stomatal closure.

5. Chlorophyll Concentration: The amount of chlorophyll affects light absorption.

6. Mineral Nutrients: Nutrients like nitrogen, magnesium, and iron are essential for chlorophyll synthesis.

In summary, light, CO₂, temperature, water, and nutrients all affect the rate of photosynthesis. The factor in shortest supply (limiting factor) determines the rate.

9. Compare respiration and photosynthesis with examples.

Answer:

In summary, photosynthesis and respiration are complementary processes that cycle matter and energy in ecosystems.

10. What are the advantages and significance of anaerobic respiration and fermentation in your daily life?

Answer: Advantages and Significance of Anaerobic Respiration and Fermentation:

1. Energy Production Without Oxygen: Allows organisms to produce ATP when oxygen is limited (e.g., during intense exercise in muscle cells).

2. Food and Beverage Industry: Fermentation is used to produce:

Bread: Yeast fermentation produces CO₂, which makes dough rise. Alcoholic Beverages: Yeast fermentation produces ethanol (beer, wine). Yogurt and Cheese: Lactic acid bacteria ferment lactose to lactic acid. Vinegar: Acetic acid bacteria ferment ethanol to acetic acid.

3. Industrial Products: Fermentation is used to produce chemicals like citric acid, glycerol, and antibiotics.

4. Waste Management: Anaerobic digestion of organic waste produces biogas (methane).

5. Survival in Extreme Environments: Some organisms (e.g., deep-sea bacteria) rely on anaerobic respiration.

In summary, anaerobic respiration and fermentation are essential for energy production, food production, industry, and waste management. They have significant applications in daily life.