Chapter Introduction
Respiration in Plants is Chapter 14 of the NCERT Class 11 Biology textbook and belongs to the Plant Physiology unit. This chapter covers cellular respiration — the process by which cells break down organic molecules to release energy in the form of ATP. It covers glycolysis, fermentation, the Krebs cycle, the electron transport chain, oxidative phosphorylation, and the respiratory quotient.
This chapter is the metabolic counterpart to Chapter 13 (Photosynthesis) and is critical for NEET and CBSE. Questions on ATP yield, the Krebs cycle, and the amphibolic nature of respiration appear frequently.
Key Definitions
| Term | Definition |
|---|---|
| Cellular respiration | The enzymatic breakdown of organic food (especially glucose) to release energy as ATP |
| Glycolysis | The partial oxidation of glucose to pyruvate in the cytoplasm; also called the Embden-Meyerhof pathway |
| Fermentation | Anaerobic breakdown of pyruvate to ethanol or lactic acid; yields only 2 ATP per glucose |
| Aerobic respiration | Complete oxidation of glucose to CO₂ and H₂O in the presence of O₂; yields 36–38 ATP |
| Krebs cycle (TCA cycle) | The cyclic pathway in the mitochondrial matrix that oxidises acetyl-CoA to CO₂, producing NADH, FADH₂, and ATP |
| Electron transport chain (ETC) | A series of electron carriers on the inner mitochondrial membrane that transfer electrons to O₂, releasing energy for ATP synthesis |
| Oxidative phosphorylation | Synthesis of ATP using energy from the proton gradient created by ETC |
| Chemiosmosis | ATP synthesis driven by a proton gradient across a membrane (inner mitochondrial membrane) |
| ATP synthase | The enzyme complex (F₀–F₁) that synthesises ATP as protons flow through it |
| Respiratory quotient (RQ) | Ratio of CO₂ produced to O₂ consumed: RQ = CO₂/O₂ |
| Amphibolic pathway | A metabolic pathway that functions in both anabolism and catabolism |
| Acetyl-CoA | The 2-carbon compound that enters the Krebs cycle; formed from pyruvate by oxidative decarboxylation |
| Decarboxylation | Removal of a carbon atom as CO₂ from an organic compound |
Detailed Concepts
Glycolysis (Embden-Meyerhof Pathway)
- Occurs in the cytoplasm; does not require oxygen.
- Breaks one molecule of glucose (6C) into two molecules of pyruvate (3C each).
- Involves 10 enzyme-catalysed steps.
Glycolysis summary:
| Step | Reaction | ATP/NADH |
|---|---|---|
| 1 | Glucose → Glucose-6-phosphate | Uses 1 ATP (hexokinase) |
| 2 | Glucose-6-phosphate → Fructose-6-phosphate | Isomerisation |
| 3 | Fructose-6-phosphate → Fructose-1,6-bisphosphate | Uses 1 ATP (PFK-1; rate-limiting step) |
| 4 | Fructose-1,6-bisphosphate → DHAP + G3P | Aldolase cleaves 6C into two 3C |
| 5 | DHAP → G3P | Triose phosphate isomerase |
| 6 | 2 × G3P → 2 × 1,3-bisphosphoglycerate | Produces 2 NADH (per glucose) |
| 7 | 2 × 1,3-BPG → 2 × 3-PGA | Produces 2 ATP (substrate-level phosphorylation) |
| 8 | 2 × 3-PGA → 2 × 2-PGA | Isomerisation |
| 9 | 2 × 2-PGA → 2 × PEP | Enolase; water removed |
| 10 | 2 × PEP → 2 × Pyruvate | Produces 2 ATP (pyruvate kinase) |
Net yield per glucose:
- 2 Pyruvate
- 2 ATP (net: 4 produced – 2 used)
- 2 NADH
- Phosphofructokinase (PFK-1) is the rate-limiting enzyme of glycolysis.
Fermentation (Anaerobic Respiration)
- When oxygen is unavailable, pyruvate is fermented instead of entering mitochondria.
- Net yield: 2 ATP per glucose (much less than aerobic respiration).
Types of fermentation:
| Feature | Alcoholic Fermentation | Lactic Acid Fermentation |
|---|---|---|
| Organism | Yeast (Saccharomyces) | Muscle cells (during strenuous exercise), some bacteria (Lactobacillus) |
| Pyruvate → | Acetaldehyde → Ethanol | Lactate |
| CO₂ released | Yes (pyruvate → acetaldehyde + CO₂) | No |
| Enzymes | Pyruvate decarboxylase, alcohol dehydrogenase | Lactate dehydrogenase |
| NAD⁺ regenerated | Yes (by reduction of acetaldehyde to ethanol) | Yes (by reduction of pyruvate to lactate) |
- In both types, the purpose of fermentation is to regenerate NAD⁺ so glycolysis can continue.
- Yeast poison themselves when ethanol concentration exceeds ~13% (basis of wine production).
Aerobic Respiration
Pyruvate Oxidation (Link Reaction)
- Pyruvate (3C) enters the mitochondria and undergoes oxidative decarboxylation.
- Catalysed by pyruvate dehydrogenase complex.
- Pyruvate + CoA + NAD⁺ → Acetyl-CoA (2C) + CO₂ + NADH
- This occurs twice per glucose (2 pyruvate → 2 acetyl-CoA).
- Yield: 2 NADH per glucose.
Krebs Cycle (TCA Cycle / Citric Acid Cycle)
- Discovered by Hans Krebs (1937; Nobel Prize 1953).
- Occurs in the mitochondrial matrix.
- Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C).
Krebs cycle steps and yield (per acetyl-CoA; multiply by 2 for per glucose):
| Step | Reaction | Product |
|---|---|---|
| 1 | Acetyl-CoA + Oxaloacetate → Citrate | Condensation (citrate synthase) |
| 2 | Citrate → Isocitrate | Isomerisation (aconitase) |
| 3 | Isocitrate → α-Ketoglutarate + CO₂ | 1 NADH (isocitrate dehydrogenase) |
| 4 | α-Ketoglutarate → Succinyl-CoA + CO₂ | 1 NADH (α-ketoglutarate dehydrogenase) |
| 5 | Succinyl-CoA → Succinate | 1 ATP (substrate-level phosphorylation; succinyl-CoA synthetase) |
| 6 | Succinate → Fumarate | 1 FADH₂ (succinate dehydrogenase, membrane-bound) |
| 7 | Fumarate → Malate | Hydration (fumarase) |
| 8 | Malate → Oxaloacetate | 1 NADH (malate dehydrogenase) |
Per acetyl-CoA: 3 NADH + 1 FADH₂ + 1 ATP + 2 CO₂ Per glucose (2 acetyl-CoA): 6 NADH + 2 FADH₂ + 2 ATP + 4 CO₂
- The Krebs cycle is called the amphibolic pathway because it is both catabolic (breaking down acetyl-CoA) and anabolic (providing intermediates for synthesis of amino acids, fatty acids, etc.).
- Anaplerotic reactions replenish cycle intermediates when they are drawn off for biosynthesis.
Electron Transport Chain (ETC) and Oxidative Phosphorylation
- Occurs on the inner mitochondrial membrane (cristae).
- Electrons from NADH and FADH₂ are passed through a series of electron carriers with increasing reduction potential.
ETC complexes:
| Complex | Name | Function |
|---|---|---|
| I | NADH dehydrogenase (NADH → FMN → Fe-S) | Accepts electrons from NADH, pumps protons |
| II | Succinate dehydrogenase (FADH₂ → Fe-S) | Accepts electrons from FADH₂ (from Krebs cycle); does NOT pump protons |
| III | Cytochrome bc₁ complex (Cyt b, Fe-S, Cyt c₁) | Transfers electrons to cytochrome c; pumps protons |
| IV | Cytochrome c oxidase (Cyt a, Cyt a₃) | Transfers electrons to O₂; O₂ + 4H⁺ + 4e⁻ → 2H₂O; pumps protons |
- Ubiquinone (CoQ) is a mobile electron carrier between Complexes I/II and III.
- Cytochrome c is a mobile carrier between Complexes III and IV.
- FADH₂ enters at Complex II (bypasses Complex I), so it yields fewer ATP than NADH.
Oxidative phosphorylation and chemiosmosis: 1. As electrons flow through Complexes I, III, and IV, protons are pumped from the matrix to the intermembrane space. 2. This creates a proton gradient (higher [H⁺] in intermembrane space). 3. Protons flow back through ATP synthase (F₀–F₁ complex). 4. F₀ is the transmembrane proton channel; F₁ is the catalytic head that synthesises ATP. 5. The proton motive force drives ATP synthesis.
ATP yield from oxidative phosphorylation:
| Source | Per Molecule | Per Glucose |
|---|---|---|
| Glycolysis net ATP | 2 ATP | 2 ATP |
| Glycolysis NADH | ≈ 2–3 ATP each | 4–6 ATP* |
| Pyruvate oxidation NADH | 3 ATP each | 6 ATP |
| Krebs cycle ATP | 1 ATP each | 2 ATP |
| Krebs cycle NADH | 3 ATP each | 18 ATP |
| Krebs cycle FADH₂ | 2 ATP each | 4 ATP |
| Total | 36–38 ATP |
Note: Cytoplasmic NADH from glycolysis must be shuttled into the mitochondria. The malate-aspartate shuttle yields 3 ATP per NADH; the glycerol-phosphate shuttle yields only 2 ATP per NADH.
- Final electron acceptor: O₂ (oxygen).
- Cyanide inhibits cytochrome c oxidase (Complex IV), blocking the ETC — this is why cyanide is lethal.
Respiratory Quotient (RQ)
RQ = Volume of CO₂ produced / Volume of O₂ consumed
RQ values for different substrates:
| Substrate | RQ Value | Explanation |
|---|---|---|
| Carbohydrates | 1.0 | Equal amounts of CO₂ and O₂ (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O) |
| Fats | ≈ 0.7 | More O₂ needed per CO₂ produced (fats are more reduced) |
| Proteins | ≈ 0.9 | Intermediate between carbs and fats |
| Organic acids (e.g. malic acid) | > 1.0 (≈ 1.3) | More CO₂ released than O₂ consumed (already partially oxidised) |
| Starving/germinating seeds (using fat) | < 1.0 | Fat is the respiratory substrate |
- RQ indicates which substrate is being used for respiration.
- In CAM plants at night, RQ can approach 0 because CO₂ is fixed rather than released.
Amphibolic Nature of Respiration
- Respiration is catabolic (breaks down glucose to release energy).
- But intermediates of the Krebs cycle are anabolic precursors:
- Oxaloacetate → aspartate, amino acids
- α-Ketoglutarate → glutamate, amino acids
- Succinyl-CoA → porphyrins (haem, chlorophyll)
- Acetyl-CoA → fatty acids, sterols
- This dual role makes respiration (especially the Krebs cycle) an amphibolic pathway.
Important Diagrams to Practise
1. Glycolysis pathway — all 10 steps with enzyme names, intermediates, and ATP/NADH at each step. 2. Krebs cycle — circular diagram with all 8 intermediates, CO₂ release points, and NADH/FADH₂/ATP production. 3. Electron transport chain — Complexes I–IV on the inner mitochondrial membrane with electron flow, proton pumping, and O₂ as final acceptor. 4. ATP synthase (F₀–F₁) — proton channel + catalytic head showing chemiosmotic ATP synthesis. 5. Overview of aerobic respiration — glucose → pyruvate (cytoplasm) → acetyl-CoA → Krebs (matrix) → ETC (inner membrane) → ATP.
Common Mistakes to Avoid
1. Confusing substrate-level phosphorylation and oxidative phosphorylation — substrate-level phosphorylation directly transfers a phosphate group to ADP (glycolysis step 7/10, Krebs step 5); oxidative phosphorylation uses the proton gradient from ETC. 2. Counting 2 ATP from glycolysis as 4 — gross production is 4 ATP, but 2 are used in the investment phase; net is 2 ATP. 3. Saying FADH₂ yields the same ATP as NADH — NADH yields ≈ 3 ATP; FADH₂ yields ≈ 2 ATP because it enters ETC at Complex II (skips Complex I). 4. Forgetting that glycolysis occurs in the cytoplasm — glycolysis is not in the mitochondria; only the Krebs cycle and ETC are mitochondrial. 5. Confusing alcoholic and lactic acid fermentation — yeast produces ethanol + CO₂; muscle cells produce lactate with no CO₂ release. 6. Saying RQ for fats is > 1 — RQ for fats is approximately 0.7 because they are more reduced and need more O₂ per carbon than carbohydrates. 7. Thinking plants do not respire in light — plants respire all the time; in daylight, photosynthesis rate exceeds respiration rate, giving the appearance of no respiration.
Exam Tips
- For VSA (1 mark): Know the definitions of RQ, oxidative phosphorylation, chemiosmosis, and amphibolic pathway. Know RQ values for carbohydrates (1.0), fats (0.7), and organic acids (>1.0).
- For SA (2–3 marks): Compare aerobic vs anaerobic respiration, or explain the link reaction, or calculate RQ for a given substrate.
- For LA (5 marks): Describe the Krebs cycle with all steps, or explain the ETC and oxidative phosphorylation with a diagram. A question on "Why is respiration called amphibolic?" is frequently asked.
- Numerical question: "How many ATP are produced from one molecule of FADH₂?" (Answer: 2, because it enters at Complex II).
- NCERT-tested lines: "FADH₂ oxidations yield less energy than NADH" and "The respiratory pathway is an amphibolic pathway."
- The difference between total ATP (36–38) often appears as a short answer — be precise about the shuttle system explanation.
How to Use This Guide
This guide is structured for layered revision:
- First pass: Read Chapter Introduction and Key Definitions to orient yourself.
- Second pass: Study Detailed Concepts section by section, matching with your NCERT textbook.
- Third pass: Use the tables and Common Mistakes for quick comparison and self-testing.
- Final pass: Use Quick Revision and Exam-Day Recall for last-minute review.
Additional Detail: Glycolysis — All 10 Steps with Enzymes
| Step | Substrate | Product | Enzyme | ATP/NADH |
|---|---|---|---|---|
| 1 | Glucose | Glucose-6-phosphate | Hexokinase | Uses 1 ATP |
| 2 | Glucose-6-phosphate | Fructose-6-phosphate | Phosphoglucoisomerase | — |
| 3 | Fructose-6-phosphate | Fructose-1,6-bisphosphate | Phosphofructokinase-1 (PFK-1) | Uses 1 ATP |
| 4 | Fructose-1,6-bisphosphate | DHAP + G3P | Aldolase | — |
| 5 | DHAP | G3P | Triose phosphate isomerase | — |
| 6 | 2 × G3P | 2 × 1,3-bisphosphoglycerate | G3P dehydrogenase | Produces 2 NADH |
| 7 | 2 × 1,3-BPG | 2 × 3-PGA | Phosphoglycerate kinase | Produces 2 ATP |
| 8 | 2 × 3-PGA | 2 × 2-PGA | Phosphoglyceromutase | — |
| 9 | 2 × 2-PGA | 2 × PEP | Enolase | — (removes H₂O) |
| 10 | 2 × PEP | 2 × Pyruvate | Pyruvate kinase | Produces 2 ATP |
- Steps 1 and 3 are the investment phase (2 ATP consumed).
- Steps 7 and 10 are the payoff phase (4 ATP produced).
- Step 3 (PFK-1) is the committed, rate-limiting step of glycolysis.
- Step 6 is the only oxidation step in glycolysis (NAD⁺ reduced to NADH).
Additional Detail: Plants and Respiration — Special Features
- Unlike animals, plants do not have specialised respiratory organs — every plant cell respire independently.
- Plants have lower respiratory rates than animals because they are less metabolically active per unit mass.
- In plants, respiration occurs through lenticels (on stems) and stomata (on leaves) for gas exchange, but respiration itself happens in every living cell.
- When photosynthesis rate exceeds respiration rate (in daylight), the net effect is O₂ release and CO₂ absorption — but respiration does not stop.
- The compensation point is the light intensity at which photosynthesis rate equals respiration rate (net gas exchange = 0).
- In germinating seeds, respiration rate is very high because stored food (starch, fats) is being broken down for growth energy.
- Respiration in roots is critical for active mineral uptake — root cells need ATP for active transport of ions.
Additional Detail: Aerobic vs Anaerobic Respiration
| Feature | Aerobic Respiration | Anaerobic Respiration (Fermentation) |
|---|---|---|
| Oxygen required | Yes | No |
| Final electron acceptor | O₂ | Organic molecule (acetaldehyde or pyruvate) |
| End products | CO₂ + H₂O | Ethanol + CO₂ (yeast) or Lactate (muscle) |
| ATP yield per glucose | 36–38 | 2 |
| Site | Cytoplasm (glycolysis) + Mitochondria | Cytoplasm only |
| Krebs cycle | Present | Absent |
| ETC | Present | Absent |
| Complete oxidation? | Yes | No (energy left in ethanol/lactate) |
| Efficiency | ~40% (of energy in glucose captured as ATP) | ~2% |
NCERT Reading Method
1. Read the chapter once to understand the full flow: glycolysis → fermentation → aerobic respiration (link reaction → Krebs → ETC) → RQ → amphibolic nature. 2. Mark all definitions (glycolysis, oxidative phosphorylation, chemiosmosis, RQ, amphibolic pathway) and every ATP yield (2 from glycolysis, 36–38 total aerobic, 2 from fermentation). 3. Pay close attention to the Krebs cycle intermediates and products — this is the most frequently tested pathway. 4. Solve in-text and exercise questions without copying — especially ATP yield calculations and RQ problems. 5. Compare your answers with NCERT language and correct missing keywords. 6. Reattempt wrong questions after one day.
Answer-Writing Tips
- Use NCERT keywords in definitions — "The respiratory pathway is an amphibolic pathway" and "FADH₂ oxidations yield less energy than NADH."
- Draw the Krebs cycle as a circular diagram with all 8 intermediates and product labels — this is the most important diagram in the chapter.
- For ATP yield questions, always show the breakdown by stage (glycolysis, link reaction, Krebs, ETC) rather than just the final number.
- For RQ questions, write the balanced equation first, then compute the ratio — do not guess.
- In 5-mark answers on the ETC, describe each complex and emphasise that O₂ is the final electron acceptor.
- For "Why is respiration amphibolic?" questions, give at least three specific examples of Krebs cycle intermediates used in biosynthesis.
Practice Checklist
- Solve all NCERT exercise questions from this chapter — especially ATP yield calculations and RQ problems.
- Prepare five one-mark facts: glycolysis location (cytoplasm), PFK-1 is rate-limiting, Krebs cycle discoverer, RQ for carbs (1.0), final ETC electron acceptor (O₂).
- Write one three-mark answer on "Differentiate between aerobic and anaerobic respiration."
- Write one five-mark answer on "Describe the Krebs cycle with a diagram" or "Explain the ETC and oxidative phosphorylation."
- Calculate the ATP yield from one molecule of FADH₂ (answer: 2 ATP) and one molecule of NADH (answer: 3 ATP) — know why they differ.
- Update your mistake notebook before moving to the next chapter.
Quick Revision Plan
| Time | Task |
|---|---|
| 15 minutes | Read marked NCERT lines — focus on glycolysis steps, Krebs cycle, ETC complexes, and RQ values |
| 20 minutes | Revise this guide, redrawing the Krebs cycle and ETC diagrams |
| 25 minutes | Solve textbook or sample questions — especially ATP yield and RQ numericals |
| 10 minutes | Correct mistakes and note weak areas |
Quick Revision
| Topic | One-Line Recall |
|---|---|
| Glycolysis | Cytoplasm; glucose → 2 pyruvate; net 2 ATP + 2 NADH; PFK-1 is rate-limiting |
| Fermentation | Anaerobic; alcoholic (yeast: ethanol + CO₂) or lactic acid (muscle: lactate); net 2 ATP |
| Link reaction | Pyruvate → Acetyl-CoA + CO₂ + NADH (mitochondria; pyruvate dehydrogenase) |
| Krebs cycle | Matrix; acetyl-CoA + oxaloacetate → citrate → 8 steps → oxaloacetate; 3 NADH + 1 FADH₂ + 1 ATP per turn |
| ETC | Inner membrane; Complexes I–IV; electrons → O₂; protons pumped across membrane |
| Oxidative phosphorylation | Proton gradient → ATP synthase → ATP; NADH ≈ 3 ATP; FADH₂ ≈ 2 ATP |
| Total ATP | 36–38 per glucose (aerobic); 2 per glucose (anaerobic) |
| RQ | CO₂/O₂; carbs = 1.0, fats ≈ 0.7, proteins ≈ 0.9, organic acids > 1.0 |
| Amphibolic | Krebs cycle is both catabolic (breaks down) and anabolic (provides precursors) |
| Substrate-level phosphorylation | Direct phosphate transfer to ADP (glycolysis steps 7, 10; Krebs step 5) |
| Cyanide poisoning | Inhibits Complex IV (cytochrome c oxidase), blocks ETC — lethal |
Exam-Day Recall
Before the exam, revise Respiration in Plants using your marked NCERT pages, the Krebs cycle diagram, and the ATP yield breakdown. If you can trace glucose through glycolysis, link reaction, Krebs cycle, and ETC with ATP counts at each stage, and explain why the Krebs cycle is amphibolic aloud in five minutes, your basics are ready.