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Respiration in Plants

NCERT Class 11 · Biology Based on NCERT Class 11 Biology textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explains how plants, like all living organisms, obtain energy from food through the process of cellular respiration. It covers the stepwise breakdown of glucose to release energy, first in the cytoplasm (glycolysis) and then, in the presence of oxygen, within the mitochondria (Krebs' cycle and electron transport system). The one big idea is that respiration is not a single combustion but a controlled, enzyme-catalysed oxidation of organic molecules, with energy trapped as ATP, the energy currency of the cell.
Key Concepts (14)
Respiration
The breaking of C-C bonds of complex compounds through oxidation within the cells, leading to release of considerable amount of energy.
e.g. When we eat food, the glucose in it is broken down in our cells to release energy for activities like running or thinking.
Respiratory substrates
The compounds that are oxidised during the process of respiration to release energy.
e.g. Carbohydrates like glucose are the most common respiratory substrate, but proteins and fats can also be used under certain conditions.
ATP (Adenosine Triphosphate)
The energy currency of the cell; the energy released by oxidation in respiration is used to synthesise ATP, which is broken down whenever energy needs to be utilised.
e.g. When a muscle cell needs energy to contract, it breaks down ATP into ADP and phosphate, releasing energy.
Glycolysis
The partial oxidation of glucose to form two molecules of pyruvic acid, occurring in the cytoplasm of the cell and present in all living organisms.
e.g. In anaerobic bacteria, glycolysis is the only process of respiration to obtain energy.
EMP pathway
The scheme of glycolysis given by Gustav Embden, Otto Meyerhof, and J. Parnas.
e.g. This is another name for the glycolytic pathway, often used in textbooks.
Fermentation
The incomplete oxidation of glucose under anaerobic conditions, where pyruvic acid is converted to CO2 and ethanol (in yeast) or to lactic acid (in some bacteria and animal muscles).
e.g. Yeast cells ferment glucose to produce alcohol and CO2, which is used in making bread and alcoholic beverages.
Aerobic respiration
The process that leads to a complete oxidation of organic substances in the presence of oxygen, releasing CO2, water and a large amount of energy.
e.g. Most higher organisms, including humans and plants, use aerobic respiration to obtain maximum energy from glucose.
Krebs' cycle (Tricarboxylic Acid Cycle / TCA cycle)
A cyclic pathway in the mitochondrial matrix where acetyl CoA is completely oxidised to CO2, with the production of NADH, FADH2, and ATP.
e.g. After glycolysis, pyruvate is converted to acetyl CoA, which then enters the Krebs' cycle for further breakdown.
Electron Transport System (ETS)
The metabolic pathway present in the inner mitochondrial membrane through which electrons pass from one carrier to another, ultimately reducing O2 to H2O and driving ATP synthesis.
e.g. NADH and FADH2 from glycolysis and Krebs' cycle donate electrons to the ETS, which powers the production of most of the cell's ATP.
Oxidative phosphorylation
The process of ATP synthesis coupled to the electron transport system, where the energy of oxidation-reduction is used to create a proton gradient for ATP production.
e.g. Unlike photophosphorylation which uses light energy, oxidative phosphorylation uses the energy released during electron transfer in respiration.
Respiratory Quotient (RQ)
The ratio of the volume of CO2 evolved to the volume of O2 consumed in respiration.
e.g. For carbohydrates, RQ is 1; for fats, it is less than 1 (e.g., 0.7 for tripalmitin).
Amphibolic pathway
A metabolic pathway that is involved in both catabolism (breakdown) and anabolism (synthesis).
e.g. The respiratory pathway is amphibolic because it breaks down glucose for energy but also provides intermediates like acetyl CoA for fatty acid synthesis.
Substrate level phosphorylation
The synthesis of ATP directly from ADP and a phosphorylated intermediate during a metabolic reaction, without the involvement of the electron transport chain.
e.g. In glycolysis, ATP is produced by substrate level phosphorylation during the conversion of BPGA to PGA and PEP to pyruvic acid.
Chemiosmotic hypothesis
The mechanism of membrane-linked ATP synthesis where a proton gradient across the inner mitochondrial membrane drives ATP production via ATP synthase.
e.g. The energy from electron transport pumps protons into the intermembrane space, and their flow back through ATP synthase generates ATP.
Formulas & Equations (5)
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy
Reads as: Glucose plus six molecules of oxygen yields six molecules of carbon dioxide plus six molecules of water plus energy
Variables: C6H12O6 = glucose; O2 = oxygen; CO2 = carbon dioxide; H2O = water
Use when: Use this equation to represent the overall summary of aerobic respiration or complete combustion of glucose.
⚠ Students forget to balance the equation or write the energy term incorrectly (e.g., writing 'ATP' instead of 'Energy').
Pyruvic acid + CoA + NAD+ → Acetyl CoA + CO2 + NADH + H+
Reads as: Pyruvic acid plus coenzyme A plus NAD plus yields acetyl coenzyme A plus carbon dioxide plus NADH plus H plus
Variables: Pyruvic acid = 3-carbon compound; CoA = coenzyme A; NAD+ = nicotinamide adenine dinucleotide (oxidised); Acetyl CoA = 2-carbon compound; NADH = reduced form; H+ = hydrogen ion
Use when: Use this to describe the oxidative decarboxylation of pyruvate before it enters the Krebs' cycle.
⚠ Students omit the coenzymes (NAD+ and CoA) or forget that this step produces CO2 and NADH.
Pyruvic acid + 4NAD+ + FAD+ + 2H2O + ADP + Pi → 3CO2 + 4NADH + H+ + FADH2 + ATP
Reads as: Pyruvic acid plus four NAD plus plus FAD plus plus two water plus ADP plus inorganic phosphate yields three carbon dioxide plus four NADH plus H plus plus FADH2 plus ATP
Variables: NAD+ = nicotinamide adenine dinucleotide (oxidised); FAD+ = flavin adenine dinucleotide (oxidised); ADP = adenosine diphosphate; Pi = inorganic phosphate; NADH = reduced NAD; FADH2 = reduced FAD; ATP = adenosine triphosphate
Use when: Use this summary equation for the complete oxidation of one molecule of pyruvate via the Krebs' cycle.
⚠ Students confuse this with the glucose summary equation or forget that this is per pyruvate, not per glucose.
RQ = Volume of CO2 evolved / Volume of O2 consumed
Reads as: Respiratory quotient equals volume of carbon dioxide evolved divided by volume of oxygen consumed
Variables: RQ = respiratory quotient (dimensionless); CO2 = carbon dioxide; O2 = oxygen
Use when: Use this to calculate the respiratory quotient for different substrates (e.g., carbohydrates, fats, proteins).
⚠ Students invert the ratio (write O2/CO2) or forget that RQ for fats is less than 1.
2(C51H98O6) + 145O2 → 102CO2 + 98H2O + energy
Reads as: Two molecules of tripalmitin plus 145 molecules of oxygen yields 102 molecules of carbon dioxide plus 98 molecules of water plus energy
Variables: C51H98O6 = tripalmitin (a fatty acid); O2 = oxygen; CO2 = carbon dioxide; H2O = water
Use when: Use this equation to calculate the RQ for fats, which is 102/145 = 0.7.
⚠ Students forget to balance the equation or incorrectly calculate the RQ value.
How It Works
Glycolysis (EMP Pathway)
  1. Glucose (6C) is phosphorylated to glucose-6-phosphate using ATP, catalysed by hexokinase.
  2. Glucose-6-phosphate is isomerised to fructose-6-phosphate.
  3. Fructose-6-phosphate is phosphorylated to fructose-1,6-bisphosphate using another ATP.
  4. Fructose-1,6-bisphosphate is split into two 3-carbon molecules: dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL).
  5. PGAL is oxidised and phosphorylated to 1,3-bisphosphoglycerate (BPGA), producing NADH + H+.
  6. BPGA transfers a phosphate to ADP to form ATP and 3-phosphoglyceric acid (PGA).
  7. PGA is converted to 2-phosphoglycerate, then to phosphoenolpyruvate (PEP).
  8. PEP transfers a phosphate to ADP to form ATP and pyruvic acid.
💡 Net gain of 2 ATP and 2 NADH per glucose molecule; glycolysis occurs in the cytoplasm and does not require oxygen.
Fermentation (Anaerobic Respiration)
  1. Pyruvic acid from glycolysis is decarboxylated to acetaldehyde by pyruvic acid decarboxylase, releasing CO2.
  2. Acetaldehyde is reduced to ethanol by alcohol dehydrogenase, using NADH + H+ (which is reoxidised to NAD+).
  3. Alternatively, in lactic acid fermentation, pyruvic acid is directly reduced to lactic acid by lactate dehydrogenase, using NADH + H+.
💡 Only 2 ATP are produced per glucose; NAD+ is regenerated to allow glycolysis to continue; the process is hazardous (acid or alcohol produced).
Aerobic Respiration: Pyruvate Decarboxylation
  1. Pyruvate is transported from the cytoplasm into the mitochondrial matrix.
  2. Pyruvate undergoes oxidative decarboxylation catalysed by pyruvic dehydrogenase, requiring NAD+ and Coenzyme A.
  3. The products are acetyl CoA, CO2, and NADH + H+.
💡 This step links glycolysis to the Krebs' cycle; it produces 2 NADH per glucose (since 2 pyruvates are formed).
Krebs' Cycle (TCA Cycle)
  1. Acetyl CoA (2C) condenses with oxaloacetic acid (4C) and water to form citric acid (6C), catalysed by citrate synthase; CoA is released.
  2. Citrate is isomerised to isocitrate.
  3. Isocitrate is oxidatively decarboxylated to α-ketoglutaric acid (5C), producing NADH + H+ and CO2.
  4. α-ketoglutaric acid is oxidatively decarboxylated to succinyl-CoA (4C), producing NADH + H+ and CO2.
  5. Succinyl-CoA is converted to succinic acid, producing GTP (which is converted to ATP) via substrate level phosphorylation.
  6. Succinic acid is oxidised to fumaric acid, producing FADH2.
  7. Fumaric acid is converted to malic acid.
  8. Malic acid is oxidised to oxaloacetic acid, producing NADH + H+.
💡 Per turn of the cycle: 3 NADH, 1 FADH2, 1 ATP (or GTP), and 2 CO2 are produced. The cycle occurs in the mitochondrial matrix and requires continuous replenishment of oxaloacetic acid.
Electron Transport System (ETS) and Oxidative Phosphorylation
  1. NADH from the matrix is oxidised by NADH dehydrogenase (Complex I), transferring electrons to ubiquinone.
  2. FADH2 (from succinate oxidation) transfers electrons to ubiquinone via Complex II.
  3. Reduced ubiquinone (ubiquinol) transfers electrons to cytochrome bc1 complex (Complex III).
  4. Cytochrome c (a mobile carrier) transfers electrons from Complex III to cytochrome c oxidase (Complex IV).
  5. Complex IV (containing cytochromes a and a3) transfers electrons to molecular O2, the final electron acceptor, forming H2O.
  6. As electrons pass through the chain, protons are pumped from the matrix to the intermembrane space, creating a proton gradient.
  7. Protons flow back into the matrix through ATP synthase (Complex V), driving the synthesis of ATP from ADP and Pi (oxidative phosphorylation).
💡 Oxidation of 1 NADH yields 3 ATP; oxidation of 1 FADH2 yields 2 ATP. Oxygen is the final hydrogen acceptor and is vital for driving the process.
Exam Traps — Don't Make These Mistakes
⚠ Confusing 'Respiration' with 'Breathing'
✗ Respiration is the process of inhaling oxygen and exhaling carbon dioxide.
→ Breathing (gaseous exchange) is a physical process, while cellular respiration is a biochemical process of energy release from food. The chapter clearly defines respiration as the breaking of C-C bonds by oxidation.
✓ Respiration is the cellular process of breaking down organic molecules (like glucose) through oxidation to release energy, which is trapped as ATP.
⚠ Forgetting that Glycolysis Occurs in the Cytoplasm
✗ Glycolysis takes place in the mitochondria.
→ The chapter explicitly states that glycolysis occurs in the cytoplasm of the cell, while the Krebs' cycle and ETS occur in the mitochondria.
✓ Glycolysis occurs in the cytoplasm, where glucose is partially oxidised to pyruvic acid.
⚠ Mistaking the Net ATP Yield in Aerobic Respiration as Always 38
✗ Aerobic respiration of one glucose molecule yields exactly 38 ATP molecules.
→ The chapter states that 38 ATP is a theoretical maximum based on assumptions (e.g., no intermediates used elsewhere, only glucose respired). In a living system, the actual yield is lower due to various factors.
✓ The theoretical net gain of ATP during aerobic respiration of one glucose molecule is 38 ATP, but this is an ideal calculation; actual yield varies in living cells.
⚠ Confusing the Role of Oxygen in Fermentation vs. Aerobic Respiration
✗ Fermentation requires oxygen to produce alcohol.
→ Fermentation is an anaerobic process; it occurs in the absence of oxygen. The chapter states that fermentation takes place under anaerobic conditions.
✓ Fermentation occurs under anaerobic conditions (without oxygen), where pyruvic acid is converted to ethanol or lactic acid.
⚠ Incorrectly Stating the RQ for Fats
✗ The respiratory quotient for fats is 1.
→ The chapter provides the calculation for tripalmitin, showing RQ = 102/145 = 0.7, which is less than 1. For carbohydrates, RQ is 1.
✓ The respiratory quotient for fats is less than 1 (e.g., 0.7 for tripalmitin), because fats require more oxygen for complete oxidation relative to CO2 produced.
⚠ Thinking the Respiratory Pathway is Only Catabolic
✗ The respiratory pathway is a catabolic pathway because it breaks down glucose.
→ The chapter explains that the respiratory pathway is amphibolic because it is involved in both catabolism (breakdown) and anabolism (synthesis), e.g., acetyl CoA can be used to synthesise fatty acids.
✓ The respiratory pathway is an amphibolic pathway because it functions in both breakdown (catabolism) and synthesis (anabolism) of biomolecules.
Compare & Contrast
Oxygen requirement and energy yield
Aerobic Respiration
Fermentation (Anaerobic Respiration)
Aerobic respiration requires oxygen and yields a large amount of energy (38 ATP per glucose), while fermentation occurs without oxygen and yields only 2 ATP per glucose.
Location and products
Glycolysis
Krebs' Cycle
Glycolysis occurs in the cytoplasm and produces pyruvic acid, NADH, and a net of 2 ATP; Krebs' cycle occurs in the mitochondrial matrix and produces CO2, NADH, FADH2, and 1 ATP per turn.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 Short Answer (3 marks) on glycolysis or Krebs' cycle, 1 Long Answer (5 marks) on ETS or comparison of respiration types
Most asked: Glycolysis steps and net ATP yield · Krebs' cycle steps and products · Electron Transport System and oxidative phosphorylation · Comparison of aerobic and anaerobic respiration · Respiratory Quotient calculations · Amphibolic nature of the respiratory pathway
Don't skip: Describe the electron transport system and explain how ATP is synthesised during oxidative phosphorylation.; Explain the amphibolic nature of the respiratory pathway with examples.; Calculate the net gain of ATP from one glucose molecule during aerobic respiration, stating the assumptions made.
One-Line Revision (read aloud before the exam)
  1. Respiration is the oxidation of C-C bonds in organic compounds to release energy, which is trapped as ATP.
  2. ATP is the energy currency of the cell, used for all energy-requiring processes.
  3. Glycolysis occurs in the cytoplasm and partially oxidises glucose to two molecules of pyruvic acid.
  4. The EMP pathway is another name for glycolysis, named after Embden, Meyerhof, and Parnas.
  5. Fermentation is an anaerobic process that yields only 2 ATP per glucose and produces ethanol or lactic acid.
  6. Aerobic respiration requires oxygen and yields a theoretical maximum of 38 ATP per glucose.
  7. Pyruvate is converted to acetyl CoA in the mitochondrial matrix before entering the Krebs' cycle.
  8. The Krebs' cycle occurs in the mitochondrial matrix and produces CO2, NADH, FADH2, and ATP.
  9. The electron transport system is located on the inner mitochondrial membrane and uses O2 as the final electron acceptor.
  10. Oxidative phosphorylation couples electron transport to ATP synthesis via a proton gradient.
  11. The respiratory quotient (RQ) is the ratio of CO2 evolved to O2 consumed; it is 1 for carbohydrates and less than 1 for fats.
  12. The respiratory pathway is amphibolic because it is involved in both catabolism and anabolism.
  13. Plants have no specialised respiratory organs; they exchange gases through stomata and lenticels.
  14. NADH and FADH2 are oxidised in the ETS to produce 3 ATP and 2 ATP per molecule, respectively.
  15. In fermentation, NAD+ is regenerated from NADH to allow glycolysis to continue.

MCQs — 10 Questions with Answers

Q1. Which of the following statements about plant respiration is CORRECT?

  • A. Plants respire only during the day when photosynthesis occurs
  • B. Plants have specialized respiratory organs like lungs to exchange gases
  • C. All living cells in a plant, including non-green cells, require oxygen for respiration ✓
  • D. Respiration in plants produces energy that is directly used for metabolic processes

Answer: C — All plant cells (green and non-green) respire continuously; non-green cells depend entirely on translocated food for oxidation.

Q2. The enzyme that catalyzes the first step of glycolysis, converting glucose to glucose-6-phosphate, is:

  • A. Invertase
  • B. Hexokinase ✓
  • C. Isomerase
  • D. Phosphatase

Answer: B — Hexokinase phosphorylates glucose to glucose-6-phosphate, which is the critical first committed step of glycolysis.

Q3. Why can plants survive without specialized respiratory organs like lungs?

  • A. Plants do not respire because they produce oxygen during photosynthesis
  • B. Each living cell in a plant is located close to the surface, allowing direct gas diffusion; additionally, gases are not transported between organs ✓
  • C. Plants have thick cell walls that prevent gas exchange
  • D. Stomata and lenticels directly supply oxygen to all internal cells

Answer: B — Plants lack centralized gas transport; individual cells near the surface exchange gases directly through stomata/lenticels, and photosynthetic cells produce their own O₂.

Q4. During the complete oxidation of one glucose molecule (C₆H₁₂O₆), how many carbon dioxide molecules are released?

  • A. 2 CO₂ molecules
  • B. 3 CO₂ molecules
  • C. 6 CO₂ molecules ✓
  • D. 12 CO₂ molecules

Answer: C — The combustion equation C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O shows that all 6 carbons in glucose are oxidized to 6 CO₂ molecules.

Q5. Which of the following is a key advantage of oxidising glucose in multiple enzymatic steps rather than in a single combustion reaction?

  • A. More carbon dioxide is produced per glucose molecule
  • B. Energy release can be coupled with ATP synthesis, preventing energy loss as heat ✓
  • C. Oxygen is not required for the process
  • D. Pyruvate is formed instead of water and carbon dioxide

Answer: B — Step-wise oxidation with enzyme control allows energy to be released in amounts suitable for ATP synthesis; single combustion would release all energy as heat.

Q6. Assertion: Glycolysis occurs in the cytoplasm of all living cells. Reason: Glycolysis is an anaerobic process that requires no oxygen and is therefore present in both aerobic and anaerobic organisms.

  • A. Both Assertion and Reason are true, and Reason is the correct explanation ✓
  • B. Both Assertion and Reason are true, but Reason is not the correct explanation
  • C. Assertion is true, but Reason is false
  • D. Both Assertion and Reason are false

Answer: A — Glycolysis (EMP pathway) occurs in the cytoplasm universally and is anaerobic, making it the common pathway in all organisms regardless of O₂ availability.

Q7. In plant cells, sucrose derived from photosynthesis must be converted before entering the glycolytic pathway. Which TWO enzymes are involved in this conversion?

  • A. Invertase and Phosphatase
  • B. Invertase and Hexokinase ✓
  • C. Isomerase and Hexokinase
  • D. Kinase and Dehydrogenase

Answer: B — Invertase hydrolyzes sucrose to glucose and fructose; hexokinase then phosphorylates glucose to glucose-6-phosphate, the committed glycolytic substrate.

Q8. Which statement about respiratory substrates in plants is INCORRECT?

  • A. Carbohydrates are the primary respiratory substrates in plants
  • B. Proteins and fats can serve as respiratory substrates under certain conditions
  • C. Glucose is derived only from photosynthesis and never from storage carbohydrates ✓
  • D. Organic acids can be oxidized as respiratory substrates in some plants

Answer: C — Glucose in plant cells comes from both photosynthesis (forming sucrose) and breakdown of storage carbohydrates (starch), not only from photosynthesis.

Q9. A scientist observes that a plant cell in the root (non-photosynthetic tissue) is respiring. How is this cell obtaining glucose for respiration if it cannot photosynthesize?

  • A. Root cells synthesize glucose from minerals absorbed from soil
  • B. Glucose is translocated from photosynthetic leaves through the phloem to non-green tissues ✓
  • C. Root cells absorb glucose directly from soil organic matter
  • D. Root cells only respire during the day when sugars are transported downward

Answer: B — Non-photosynthetic plant tissues depend on translocation of photosynthetically-produced sugars (sucrose in phloem) from leaves to supply glucose for respiration.

Q10. Why is ATP described as the 'energy currency' of the cell rather than glucose being used directly for cellular processes?

  • A. ATP is produced in smaller quantities and therefore more economical than glucose
  • B. ATP is a high-energy compound that can be rapidly synthesized and broken down to release energy in controlled, usable amounts at specific cellular locations ✓
  • C. Glucose is too large a molecule to enter cells
  • D. ATP contains more total energy per molecule than glucose

Answer: B — ATP serves as a universal, readily-available energy shuttle — it is synthesized where energy is released and hydrolyzed where energy is needed, providing controlled energy delivery.

Flashcards

What is cellular respiration?

Breakdown of food materials inside cells through oxidation to release energy trapped as ATP.

Why do plants need respiration if they photosynthesize?

Only green cells photosynthesise; non-green cells and organs need respiration to oxidise food for energy.

Define glycolysis.

Partial oxidation of glucose to two molecules of pyruvic acid occurring in the cytoplasm without oxygen.

What is the role of ATP in cells?

ATP acts as the energy currency of the cell, storing and providing energy for all life processes.

How do plants exchange gases without lungs?

Plants use stomata in leaves and lenticels in stems for gaseous exchange; each cell is close to the surface.

What are respiratory substrates?

Compounds oxidised during respiration; usually carbohydrates, but also proteins, fats, and organic acids in plants.

Write the complete combustion equation for glucose.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (released mostly as heat without enzymatic control).

What is the EMP pathway?

The glycolytic pathway named after Embden, Meyerhof, and Parnas, occurring in the cytoplasm of all living cells.

Why is glucose oxidised in many steps instead of one?

Step-wise oxidation allows energy release to be coupled with ATP synthesis instead of being lost as heat.

Distinguish between aerobic and anaerobic organisms.

Aerobic organisms require oxygen for respiration; anaerobes (facultative or obligate) can survive without oxygen using fermentation or partial glycolysis.

Important Board Questions

Define respiration. Why is it essential for all living cells, including green plant cells? [2 marks]

Define respiration as stepwise oxidation of food to release energy trapped as ATP. Explain that even green cells have non-photosynthetic parts (roots, non-green leaves, vascular tissue) that require translocated glucose for respiration.

Explain why plants can manage without specialized respiratory organs like lungs, whereas animals require them. Support your answer with at least two structural and physiological reasons. [5 marks]

Reason 1: Each plant cell is located close to surface (stomata in leaves, lenticels in stems); short diffusion distance. Reason 2: Low respiration rates in plants compared to animals. Reason 3: Loose parenchyma packing creates interconnected air spaces. Reason 4: Photosynthetic cells produce their own O₂; gases not transported between organs. Compare with animals' high metabolic demands and centralized gas transport.

Using the combustion equation for glucose, explain why cells oxidize glucose in multiple enzymatic steps within the cytoplasm and mitochondria rather than in a single reaction. What would be the consequence if glucose were completely oxidized in one step? [6 marks]

Complete equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy. Show that single-step combustion releases ~2880 kJ/mol mostly as heat. Explain that step-wise oxidation (glycolysis → Krebs cycle → electron transport) releases energy in small packets (~30.5 kJ per ATP), allowing coupling to ATP synthesis. One-step reaction would waste energy as heat and leave cells unable to harness it for biosynthesis and life processes.

Assertion-Reason Questions for Respiration in Plants

Q1. Assertion (A):

Plants require oxygen for aerobic respiration and must have systems in place to ensure its availability.

Reason (R):

Plants possess specialised respiratory organs similar to animals, such as lungs and gills, to facilitate oxygen uptake.

Answer: C — A is true but R is false — The assertion is true—plants do require O₂ for respiration. However, the reason is false because the chapter explicitly states plants have NO specialised respiratory organs. Instead, they use stomata and lenticels for gas exchange. This tests whether students understand the difference between what is true (assertion) and why it's true (reason).

Q2. Assertion (A):

Glycolysis is a metabolic pathway present in all living organisms and is the only respiratory process that occurs in anaerobic organisms.

Reason (R):

Glycolysis does not require oxygen and involves the partial oxidation of glucose to pyruvic acid in the cytoplasm.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both statements are true, and the reason correctly explains why glycolysis is the only process available to anaerobic organisms—it is oxygen-independent. The chapter confirms glycolysis occurs in all organisms and is the EMP pathway in the cytoplasm.

Q3. Assertion (A):

In photosynthesising cells during daylight, oxygen availability is not a limiting factor for respiration.

Reason (R):

Photosynthesising cells contain high concentrations of dissolved oxygen due to stomatal uptake from the atmosphere.

Answer: C — A is true but R is false — The assertion is true—the chapter states that when cells photosynthesise, oxygen is released within the cell, eliminating limitation. However, the reason is false because it attributes oxygen presence to atmospheric uptake via stomata, not recognizing that oxygen is produced within the cell during photosynthesis.

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Case-Based Questions (CBQ) for Respiration in Plants

Case Study:

A farmer in Punjab noticed that his rice paddy fields, which were deliberately flooded for crop growth, showed stunted root development and poor plant vigor compared to well-drained fields. Laboratory analysis revealed that roots in waterlogged soil exhibited higher respiration rates but accumulated less ATP per unit biomass. Enzyme assays showed increased glycolytic enzyme activity but decreased activity of enzymes involved in aerobic pathways. When the farmer introduced aeration (using pumps), root growth recovered within two weeks and respiration enzyme profiles normalized.

Q1. Why would roots in waterlogged, oxygen-poor soil show higher respiration rates despite producing less ATP?

  • A. Roots switch to anaerobic glycolysis, which requires more glucose molecules to be consumed to obtain the same ATP as aerobic respiration ✓
  • B. Waterlogged conditions denature enzymes, forcing the plant to produce more respiratory enzymes
  • C. High water content increases the diffusion rate of glucose into root cells
  • D. Anaerobic respiration is faster than aerobic respiration in all conditions

Answer: A

Q2. The increased glycolytic enzyme activity in waterlogged roots indicates which metabolic adaptation?

  • A. A shift from aerobic respiration to anaerobic glycolysis due to oxygen limitation ✓
  • B. An increase in photosynthetic capacity to produce more glucose
  • C. Enhanced mitochondrial function to compensate for low ATP production
  • D. Accumulation of toxic metabolic byproducts requiring higher enzyme levels

Answer: A

Q3. Based on the chapter's explanation of respiratory pathways, which of the following molecules would likely accumulate in waterlogged roots due to anaerobic conditions?

  • A. CO₂ and H₂O from complete oxidation of glucose
  • B. Pyruvic acid that is converted to ethanol or lactic acid in fermentation ✓
  • C. Glucose-6-phosphate from the entry of additional glucose into glycolysis
  • D. ATP from the increased breakdown of glucose

Answer: B

Q4. When aeration was introduced and root enzyme profiles normalized, the most likely change in root metabolism would be:

  • A. Increase in both glycolytic and aerobic enzymes simultaneously to maximize ATP production
  • B. Complete cessation of glycolysis and exclusive reliance on anaerobic fermentation
  • C. Decrease in glycolytic enzyme activity and increase in aerobic respiration enzymes, restoring efficient ATP production ✓
  • D. No change in enzyme levels because they are constitutively expressed regardless of oxygen availability

Answer: C

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