Exercise | Chapter 4 : Bioenergetics | Class 11 Biology Notes (FBISE, Best for Exams)

 

Exercise Chapter 4: Bioenergetics Class 11

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MCQs with answers from Chapter 4 : Bioenergetics (Federal Board, 11th Class): 

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MCQs – Bioenergetics

(i) Removal of the source of carbon dioxide from photosynthesizing chloroplasts results in rapid changes in the concentration of certain chemicals. Which one of the following represents the correct combination of concentration changes?

ATP	RuBP	PGA
☑ A) decreases	decreases	increases
☐ B) decreases	increases	no change
☐ C) increases	increases	decreases
☐ D) increases	no change	decreases

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(ii) What are the products of the light reactions in photosynthesis?

☐ A) ATP and NADP
☑ B) ATP, NADPH₂, and oxygen
☐ C) ATP, PGA, and NADH₂
☐ D) ATP, PGA, and oxygen

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(iii) During the light-dependent stage of photosynthesis, the photoactivated pigment removes an electron from the hydroxylation derived from the water molecule. The fate of the free hydroxyl radical is that it:

☑ A) Is broken down into oxygen and a free radical of hydrogen
☐ B) Is used to raise the activation level of chlorophyll by donating a positive charge
☐ C) Is used to produce adenosine triphosphate from adenosine diphosphate
☐ D) Reduces carbon dioxide to sugar

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(iv) Carbon dioxide labeled with ¹⁴C has been used to identify the intermediate compounds in the Calvin cycle, the light-independent stage in photosynthesis. Which compound would be the first to contain the ¹⁴C?

☐ A) Glucose
☑ B) PGA
☐ C) RuBP
☐ D) Starch

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(v) The rate of photosynthesis of a freshwater plant is measured using five spectral colors. Which sequence of colors would give an increasing photosynthetic response?

Smallest → Largest response
☐ A) Blue → Green → Yellow → Orange → Red
☐ B) Green → Yellow → Orange → Red → Blue
☐ C) Red → Orange → Yellow → Green → Blue
☑ D) Yellow → Green → Orange → Blue → Red

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(vi) During dark reactions, the three carbon atoms of 3-PGA are derived from

☐ A) RuBP only
☐ B) CO₂ only
☑ C) RuBP + CO₂
☐ D) RuBP + CO₂ + PEP

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(vii) Chlorophyll is soluble in:

☐ A) Water
☑ B) Organic solvent
☐ C) Water and organic solvent
☐ D) Not in any solvent

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(viii) Photorespiration takes place only in:

☐ A) Root
☐ B) Mitochondria
☐ C) Green parts of the plant
☑ D) All cells of the plant

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(ix) In C₄ plants, fixation of CO₂ occurs in:

☐ A) Palisade tissue
☐ B) Cortex of stem
☑ C) Spongy mesophyll and bundle sheath
☐ D) Phloem tissue

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(x) ATP synthesis during light reactions is:

☐ A) Oxidative
☐ B) Photolysis
☐ C) Substrate phosphorylation
☑ D) Photophosphorylation

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(xi) In C₃ plants, the first stable product of photosynthesis during the dark reaction is:

☐ A) PGA
☑ B) G3P
☐ C) RuBP
☐ D) Oxaloacetate

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(xii) The diagram shows the Krebs cycle. At what numbered stages does decarboxylation take place?

☐ A) 1 and 2
☑ B) 1, 2, and 3
☐ C) 1, 3, and 4
☐ D) 1, 2, 3, and 4

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short questions:

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1. What is the electromagnetic spectrum?

The electromagnetic spectrum is the range of all types of electromagnetic radiation, categorized by wavelength and frequency. It includes gamma rays, X-rays, ultraviolet (UV) rays, visible light, infrared (IR) rays, microwaves, and radio waves. Visible light, which is used in photosynthesis, ranges from 380 nm (violet) to 750 nm (red).

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2. Explain the ‘action spectrum’ of photosynthesis.

The action spectrum shows the relative effectiveness of different wavelengths of light in driving photosynthesis. It is determined by measuring oxygen evolution or CO₂ uptake at various wavelengths. It closely resembles the absorption spectrum of chlorophyll, peaking in the blue (~430 nm) and red (~680 nm) regions, with a smaller peak in the green region due to accessory pigments.

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3. What are the types of chlorophyll?

• Chlorophyll a: Primary pigment, essential for photosynthesis, absorbs blue-violet and red light.
• Chlorophyll b: Accessory pigment, absorbs blue and red-orange light, transfers energy to chlorophyll a.
• Chlorophyll c and d: Found in certain algae and cyanobacteria, absorb different light spectra.

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4. What is the importance of carotene?

Carotene is an accessory pigment that:

• Absorbs light in the blue-green region and transfers energy to chlorophyll.
• Acts as an antioxidant, protecting chlorophyll from photooxidative damage.
• Plays a role in photoprotection by dissipating excess light energy.

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5. Describe the ‘absorption spectrum’ in photosynthesis.

The absorption spectrum represents the specific wavelengths of light absorbed by pigments. Chlorophyll a and b primarily absorb blue and red light, while carotenoids absorb blue-green light, enhancing the range of usable light for photosynthesis.

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6. What is a photosystem? Explain.

A photosystem is a protein-pigment complex in the thylakoid membrane that captures light energy for photosynthesis. It consists of:

• Antenna complex: Collects and transfers photons to the reaction center.
• Reaction center: Contains chlorophyll a, where light energy excites electrons, initiating electron transport.
  Two types:
• Photosystem I (PSI, P700): Absorbs at 700 nm, involved in cyclic and non-cyclic photophosphorylation.
• Photosystem II (PSII, P680): Absorbs at 680 nm, initiates photolysis of water.

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7. What is the role of carbon dioxide in photosynthesis?

CO₂ is the carbon source for glucose synthesis in the Calvin cycle. It combines with RuBP (ribulose bisphosphate) in the first step of the cycle, catalyzed by RuBisCO, forming 3-PGA, which is further processed into glucose.

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8. How was it confirmed that ‘plants split water as a source of hydrogen, releasing hydrogen as a byproduct’?

This was confirmed by Ruben and Kamen using heavy isotope-labeled oxygen (¹⁸O). When plants were given H₂¹⁸O, the released oxygen was labeled, proving that O₂ comes from water, not CO₂.

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9. What is the importance of G3P?

G3P (Glyceraldehyde-3-Phosphate) is a key intermediate in the Calvin cycle. It:

• Forms glucose, sucrose, and starch.
• Serves as a precursor for fatty acids and amino acids.
• Can be converted into ribulose-5-phosphate, regenerating RuBP.

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10. What is the effect of temperature on the activities of RuBisCO?

• Optimal temperature: 25–30°C for maximum activity.
• Higher temperatures (>35°C): Increases oxygenase activity, leading to photorespiration.
• Lower temperatures (<15°C): Reduces enzymatic efficiency and slows the Calvin cycle.

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11. What are the disadvantages of photorespiration?

• Wastes energy: Uses ATP and NADPH without producing glucose.
• Reduces efficiency: Competes with CO₂ fixation in RuBisCO.
• Results in carbon loss: Some carbon is lost as CO₂ instead of being used for sugar synthesis.

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12. How has photorespiration evolved?

Photorespiration evolved as a consequence of RuBisCO’s dual affinity for CO₂ and O₂. When atmospheric O₂ levels increased, RuBisCO’s oxygenase activity became significant. C₄ and CAM plants evolved mechanisms to minimize photorespiration by concentrating CO₂ near RuBisCO.

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13. Write the differences between:

(a) Chlorophyll a vs. Chlorophyll b

Feature	Chlorophyll a	Chlorophyll b
Function	Primary pigment	Accessory pigment
Absorption	Blue-violet and red	Blue and red-orange
Role	Directly involved in light reactions	Transfers energy to chlorophyll a

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(b) Carotene vs. Xanthophyll

Feature	Carotene	Xanthophyll
Color	Orange	Yellow
Function	Light absorption and antioxidant	Light absorption and photoprotection

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(c) Action spectrum vs. Absorption spectrum

Feature	Action Spectrum	Absorption Spectrum
Definition	Shows effectiveness of wavelengths in photosynthesis	Shows wavelengths absorbed by pigments

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(d) Absorption spectrum of Chlorophyll a vs. Chlorophyll b

Feature	Chlorophyll a	Chlorophyll b
Peak absorption	430 nm (blue) & 680 nm (red)	453 nm (blue) & 642 nm (red)

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(e) Antenna complex vs. Reaction center

Feature	Antenna Complex	Reaction Center
Function	Captures and transfers photons	Converts light into chemical energy

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(f) Photosystem I vs. Photosystem II

Feature	Photosystem I	Photosystem II
Absorption peak	700 nm (P700)	680 nm (P680)
Function	Produces NADPH	Splits water, generates O₂

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(g) Light-dependent vs. Light-independent reactions

Feature	Light-dependent	Light-independent
Location	Thylakoid membrane	Stroma
Products	ATP, NADPH, O₂	Glucose

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(h) Oxidative phosphorylation vs. Photophosphorylation

Feature	Oxidative	Photophosphorylation
Energy Source	Electron transport chain	Light energy

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(i) Cyclic vs. Non-cyclic Photophosphorylation

Feature	Cyclic	Non-cyclic
Products	ATP only	ATP + NADPH + O₂

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(j) C₄ vs. C₃ Carbon Fixation

Feature	C₄	C₃
First Product	4-carbon (Oxaloacetate)	3-carbon (PGA)
Photorespiration	Reduced	High

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(k) Lactic acid vs. Alcoholic fermentation

Feature	Lactic Acid	Alcoholic
End product	Lactate	Ethanol + CO₂

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(l) Calvin cycle vs. Krebs cycle

Feature	Calvin Cycle	Krebs Cycle
Function	Fixes CO₂	Releases CO₂

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(m) Oxidative phosphorylation vs. Substrate-level phosphorylation

Feature	Oxidative	Substrate-level
ATP Source	Electron transport chain	Direct enzyme-mediated phosphorylation

15. What is photosynthesis? Explain the role of light in photosynthesis.

Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy, producing glucose and oxygen from carbon dioxide and water. The general equation is:

$$6CO_2 + 6H_2O + Light \rightarrow C_6H_{12}O_6 + 6O_2$$

Role of Light in Photosynthesis:

• Light provides the energy required to drive the photochemical reactions in the light-dependent phase.
• It excites electrons in chlorophyll molecules, initiating the electron transport chain (ETC).
• It leads to photolysis of water, producing protons, electrons, and oxygen.
• Light energy helps in the synthesis of ATP and NADPH, which are used in the light-independent phase (Calvin cycle).

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16. Describe the structure of chlorophyll.

Chlorophyll is a magnesium-containing porphyrin pigment essential for photosynthesis.

Structure:

• Tetrapyrrole ring (porphyrin head): Similar to hemoglobin, it has a central Mg²⁺ ion that absorbs light.
• Phytol tail: A long hydrocarbon tail that anchors chlorophyll into the thylakoid membrane.
• Different types exist, such as chlorophyll-a (primary pigment) and chlorophyll-b (accessory pigment).

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17. Write a note on the photosynthetic pigment carotene.

Carotenes are accessory pigments in photosynthesis.

• They belong to the carotenoid family and are responsible for orange/red coloration.
• They absorb light in the blue-violet range and transfer energy to chlorophyll-a.
• They protect chlorophyll from photooxidation by dissipating excess light energy.

Example: β-carotene, a precursor of Vitamin A.

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18. Explain the arrangement of photosystems.

Photosystems are light-harvesting complexes located in the thylakoid membrane of chloroplasts.

Two types exist:

1. Photosystem I (PSI, P700):

   • Absorbs light at 700 nm.
   • Involved in cyclic and non-cyclic photophosphorylation.
   • Produces NADPH.

2. Photosystem II (PSII, P680):

   • Absorbs light at 680 nm.
   • Participates in photolysis of water (splitting H₂O into O₂, H⁺, and e⁻).
   • Initiates the electron transport chain.

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19. Describe the role of water in photosynthesis.

Water is essential in photosynthesis for:

• Electron supply: Water undergoes photolysis in PSII, donating electrons to replace those lost by chlorophyll.
• Oxygen production: Splitting of water releases O₂, which is released into the atmosphere.
• Proton gradient: H⁺ ions contribute to the formation of ATP via chemiosmosis.

$$2H_2O \rightarrow 4H^+ + 4e^- + O_2$$

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20. Describe the mechanism of photosynthesis.

Photosynthesis occurs in two phases:

1. Light-dependent reactions (Thylakoid membrane):

   • Light energy excites electrons in PSII.
   • Electrons travel through the ETC, producing ATP and NADPH.
   • Water undergoes photolysis, releasing O₂.

2. Light-independent reactions (Calvin cycle in Stroma):

   • ATP and NADPH from light reactions drive the fixation of CO₂.
   • RuBisCO enzyme catalyzes the conversion of CO₂ into glucose.

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21. Explain in detail the light-dependent phase of photosynthesis.

The light-dependent phase occurs in the thylakoid membrane and involves:

1. Excitation of electrons:
   • PSII absorbs light (P680), exciting electrons to a higher state.
2. Electron Transport Chain (ETC):
   • Electrons move through plastoquinone (PQ), cytochrome complex, plastocyanin (PC).
   • Energy from electrons pumps H⁺ ions into the thylakoid lumen, creating a proton gradient.
3. ATP synthesis:
   • Protons move through ATP synthase, generating ATP (photophosphorylation).
4. PSI activation:
   • Electrons from PSII reach PSI (P700) and get excited again.
   • These electrons reduce NADP⁺ to NADPH.
5. Water photolysis:
   • Splits into O₂, H⁺, and electrons, replenishing PSII.

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22. Explain in detail the light-independent phase of photosynthesis.

The light-independent phase (Calvin cycle) occurs in the stroma and fixes CO₂ into glucose.

Three phases:

1. Carbon Fixation:
   • RuBisCO enzyme incorporates CO₂ into RuBP, forming PGA (3-phosphoglycerate).
2. Reduction:
   • PGA is converted into G3P (glyceraldehyde-3-phosphate) using ATP & NADPH.
3. Regeneration of RuBP:
   • Some G3P forms glucose, while the rest regenerates RuBP using ATP.

Net equation:

$$6CO_2 + 18ATP + 12NADPH \rightarrow C_6H_{12}O_6 + 18ADP + 12NADP^+$$

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23. Describe cyclic photophosphorylation.

Cyclic photophosphorylation occurs in PSI only, producing ATP but no NADPH or O₂.

• Excited electrons from PSI (P700) travel through the ETC and return to PSI.
• Electron carriers involved: Ferredoxin (Fd) → Cytochrome b6f → Plastocyanin (PC).
• Used in conditions of high ATP demand or low NADP⁺ availability.

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24. Describe the Calvin cycle.

The Calvin cycle (C₃ pathway) occurs in the stroma and is responsible for CO₂ fixation.

Three stages:

1. Carboxylation:
   • RuBisCO enzyme catalyzes the reaction of CO₂ with RuBP, forming PGA.
2. Reduction:
   • PGA → G3P using ATP & NADPH.
3. Regeneration:
   • G3P regenerates RuBP with the help of ATP.

Net gain:

• 6 CO₂ molecules form 1 glucose (C₆H₁₂O₆).
• Uses 18 ATP and 12 NADPH.

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25. Describe the kinds of cellular respiration.

Cellular respiration is the process of oxidizing glucose to produce ATP. It is of two types:

1. Aerobic Respiration:

   • Requires oxygen.
   • Glucose is completely oxidized into CO₂ and H₂O.
   • Yields 36-38 ATP per glucose.
   • Occurs in cytoplasm (glycolysis) & mitochondria (Krebs cycle & ETC).

2. Anaerobic Respiration:

   • Occurs in absence of oxygen.
   • Glucose is incompletely oxidized into lactic acid (animals) or ethanol & CO₂ (yeasts).
   • Yields only 2 ATP per glucose.
   • Occurs in cytoplasm.

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26. Give an account of ‘Glycolysis’.

Glycolysis (Embden-Meyerhof pathway) is the first step of glucose metabolism occurring in the cytoplasm.

• Converts 1 glucose (C₆H₁₂O₆) into 2 pyruvate (C₃H₄O₃).
• Net gain: 2 ATP & 2 NADH.

Phases:

1. Energy Investment Phase:
   • Glucose is phosphorylated (uses 2 ATP) → Fructose-1,6-bisphosphate.
2. Cleavage Phase:
   • Splits into 2 molecules of G3P (glyceraldehyde-3-phosphate).
3. Energy Payoff Phase:
   • G3P oxidized to pyruvate, yielding 4 ATP (net gain: 2 ATP) & 2 NADH.

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27. Explain oxidation of pyruvate.

After glycolysis, pyruvate (C₃H₄O₃) is converted into Acetyl-CoA in the mitochondrial matrix by pyruvate dehydrogenase complex (PDC).

Reaction:

$$2 Pyruvate + 2 NAD⁺ + 2 CoA → 2 Acetyl-CoA + 2 NADH + 2 CO₂$$

• CO₂ is released (decarboxylation).
• NADH is generated.
• Acetyl-CoA enters the Krebs cycle.

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28. Explain Krebs cycle.

The Krebs cycle (TCA cycle, Citric Acid Cycle) occurs in the mitochondrial matrix and fully oxidizes Acetyl-CoA into CO₂.

Steps:

1. Acetyl-CoA (C₂) + Oxaloacetate (C₄) → Citrate (C₆).
2. Citrate undergoes decarboxylation, forming CO₂.
3. NADH, FADH₂, and ATP/GTP are produced.
4. Oxaloacetate is regenerated.

Net Yield (per glucose, i.e., 2 cycles):

• 6 NADH, 2 FADH₂, 2 ATP/GTP, 4 CO₂.

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29. Explain electron transport chain (ETC).

ETC occurs in the inner mitochondrial membrane and uses electrons from NADH & FADH₂ to generate ATP.

Steps:

1. Complex I (NADH dehydrogenase): Transfers electrons from NADH → Ubiquinone (Q).
2. Complex II (Succinate dehydrogenase): Transfers electrons from FADH₂ → Q.
3. Complex III (Cytochrome bc1): Transfers electrons from Q → Cytochrome c.
4. Complex IV (Cytochrome c oxidase): Transfers electrons to O₂, forming H₂O.

Proton Gradient:

• Electrons transfer energy to pump H⁺ into the intermembrane space.
• This drives ATP synthesis via chemiosmosis.

ATP Yield:

• 3 ATP per NADH, 2 ATP per FADH₂.

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30. Explain chemiosmosis and oxidative phosphorylation.

• Chemiosmosis: Protons move back into the matrix through ATP synthase, generating ATP.
• Oxidative Phosphorylation: ATP synthesis using the energy of electrons transported through the ETC.

Final ATP Yield (Per Glucose):

• Glycolysis: 2 ATP, 2 NADH.
• Krebs cycle: 2 ATP, 6 NADH, 2 FADH₂.
• ETC: 34 ATP.
• Total: 36-38 ATP.

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31. Describe substrate-level phosphorylation.

• ATP is generated directly by transferring a phosphate group from a high-energy substrate.
• Occurs in Glycolysis & Krebs cycle.

Examples:

• Glycolysis: Phosphoenolpyruvate (PEP) → Pyruvate + ATP.
• Krebs cycle: Succinyl-CoA → Succinate + ATP.

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32. Give an account of photorespiration in plants.

Photorespiration occurs when RuBisCO binds O₂ instead of CO₂, leading to:

• Wastage of ATP & NADPH.
• Release of CO₂ without sugar production.

Steps:

1. O₂ binds to RuBisCO → Forms 2-phosphoglycolate (C₂) instead of PGA.
2. Peroxisome & mitochondria metabolize glycolate, but ATP is lost.
3. CO₂ is released, reversing carbon fixation.

Occurs in C₃ plants (e.g., wheat, rice, soybean) under high temperature & low CO₂.

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33. Explain that C₄ photosynthesis is an adaptation to the problem in photorespiration.

C₄ plants (e.g., maize, sugarcane) evolved to minimize photorespiration by using a spatial separation of CO₂ fixation.

C₄ Pathway:

1. CO₂ fixation in mesophyll cells:
   • PEP carboxylase fixes CO₂ into oxaloacetate (C₄) → Converted to malate.
2. Transport to bundle sheath cells:
   • Malate releases CO₂ near RuBisCO, ensuring high CO₂:O₂ ratio.
3. Calvin cycle occurs normally, reducing photorespiration.

Advantages of C₄ Plants:

• More efficient in hot, dry conditions.
• Reduces water loss & increases CO₂ availability.

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