Respiration in Plants
'Respiration is the bridge between the energy stored in food and the energy needed for cellular work.' — Biochemistry
1. Chapter Overview
RESPIRATION is the process by which cells BREAK DOWN organic molecules (mainly glucose) to RELEASE ENERGY in the form of ATP. This chapter covers the COMPLETE pathway of AEROBIC respiration — GLYCOLYSIS, KREBS CYCLE (TCA cycle), and the ELECTRON TRANSPORT SYSTEM (ETS) — along with ANAEROBIC respiration (fermentation). It also discusses the RESPIRATORY QUOTIENT (RQ) and how respiration DIFFERS between plants and animals.
2. Do Plants Respire?
- YES — plants respire ALL THE TIME (24 hours, day AND night)
- During the day: Photosynthesis DOMINATES (net O₂ release)
- At night: Only respiration occurs (net CO₂ release)
- Plants do NOT have specialised respiratory organs — gas exchange occurs through DIFFUSION (stomata, lenticels, root hairs)
3. Overall Equation
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 36-38 ATP + heat
4. Glycolysis (EMP Pathway)
Location
- CYTOPLASM of the cell
Steps
- Energy investment phase: Glucose (6C) → 2 molecules of G3P (3C)
- Uses 2 ATP (phosphorylation)
- Energy payoff phase: G3P → PYRUVATE (3C)
- Produces 4 ATP + 2 NADH
Net Yield of Glycolysis
| Input | Output |
|---|---|
| 1 Glucose | 2 Pyruvate |
| 2 ATP (invested) | 4 ATP (produced) |
| — | Net: 2 ATP |
| — | 2 NADH |
| — | 2 H₂O |
Fate of Pyruvate
| Condition | Fate |
|---|---|
| Aerobic | Enters KREBS CYCLE (after conversion to Acetyl-CoA) |
| Anaerobic (yeast) | ETHANOL + CO₂ (alcoholic fermentation) |
| Anaerobic (muscle/animals) | LACTIC ACID (lactic acid fermentation) |
5. Fermentation (Anaerobic Respiration)
Alcoholic Fermentation (Yeast)
- Pyruvate → Acetaldehyde → ETHANOL + CO₂
- NADH → NAD⁺ (regenerates NAD⁺ needed for glycolysis)
Lactic Acid Fermentation (Bacteria, Animal muscle)
- Pyruvate → LACTIC ACID
- NADH → NAD⁺
Comparison: Aerobic vs Anaerobic
| Feature | Aerobic | Anaerobic |
|---|---|---|
| O₂ required | YES | NO |
| ATP yield | 36-38 ATP per glucose | 2 ATP per glucose |
| End products | CO₂ + H₂O | Ethanol/CO₂ or Lactic acid |
| Location | Cytoplasm + Mitochondria | Cytoplasm only |
| Efficiency | HIGH | LOW |
6. Aerobic Respiration (Mitochondrial)
Step 1: Pyruvate Dehydrogenase Complex (Link Reaction)
- Location: Mitochondrial MATRIX
- Pyruvate + CoA + NAD⁺ → Acetyl-CoA + CO₂ + NADH
- Irreversible step — PYRUVATE COMMITTED to Krebs cycle
Step 2: Krebs Cycle (TCA Cycle / Citric Acid Cycle)
Location: Mitochondrial MATRIX
Steps:
- Acetyl-CoA (2C) + Oxaloacetate (4C) → Citrate (6C)
- Series of DECARBOXYLATIONS and OXIDATIONS
- Regeneration of OXALOACETATE (4C)
Yield per turn (per Acetyl-CoA):
| Product | Number |
|---|---|
| NADH | 3 |
| FADH₂ | 1 |
| ATP (GTP) | 1 |
| CO₂ | 2 |
For 1 glucose (2 turns): 6 NADH + 2 FADH₂ + 2 ATP + 4 CO₂
Step 3: Electron Transport System (ETS)
Location: Inner MITOCHONDRIAL membrane
Components:
- NADH dehydrogenase (Complex I) — transfers e⁻ from NADH → CoQ
- Succinate dehydrogenase (Complex II) — transfers e⁻ from FADH₂
- Cytochrome bc₁ complex (Complex III)
- Cytochrome c (mobile carrier)
- Cytochrome c oxidase (Complex IV) — transfers e⁻ → O₂
Final electron acceptor: OXYGEN (O₂) → H₂O
7. Oxidative Phosphorylation (Chemiosmotic Theory)
Mechanism (Peter Mitchell)
- Electron transport PUMPS H⁺ from matrix to INTERMEMBRANE SPACE
- Creates a PROTON GRADIENT (electrochemical gradient)
- H⁺ flows BACK through ATP synthase (complex V)
- Energy released → ATP SYNTHESIS
ATP Yield Summary (Per Glucose)
| Step | ATP from NADH | ATP from FADH₂ | Total ATP |
|---|---|---|---|
| Glycolysis (net) | 2 NADH × 2.5 = 5 | 0 | 5 + 2 (substrate level) |
| Link reaction | 2 NADH × 2.5 = 5 | 0 | 5 |
| Krebs cycle | 6 NADH × 2.5 = 15 | 2 FADH₂ × 1.5 = 3 | 15 + 3 + 2 (substrate level) |
| TOTAL | ~36-38 ATP |
8. Respiratory Quotient (RQ)
- RQ = Volume of CO₂ released / Volume of O₂ consumed
- Carbohydrates (glucose): RQ = 6CO₂/6O₂ = 1.0
- Fats (tripalmitin): RQ ≈ 0.7 (less O₂ per CO₂ — more reduced)
- Proteins: RQ ≈ 0.8-0.9
- Organic acids: RQ > 1 (oxalic acid — more CO₂ per O₂)
9. Common Mistakes
- Glycolysis occurs in CYTOPLASM, not mitochondria: Many students think ALL respiration is in mitochondria
- Krebs cycle produces GTP, which is CONVERTED to ATP: The 'ATP' from Krebs is actually GTP → ATP
- O₂ is the FINAL electron acceptor, NOT the direct source of ATP: O₂ accepts e⁻ at the END of ETS, making H₂O
- Plants respire 24/7 — they do NOT photosynthesise at night: In the dark, only respiration occurs
- NADH from glycolysis enters mitochondria via SHUTTLE SYSTEMS: G3P shuttle or Malate-Aspartate shuttle
10. CBSE Exam Focus
- Glycolysis — steps and yield (5-mark)
- Krebs cycle — steps (3/5-mark)
- Electron transport system and oxidative phosphorylation (5-mark)
- Aerobic vs anaerobic respiration — comparison (3-mark)
- Respiratory Quotient — calculation (3-mark)
- Fate of pyruvate — 3 possibilities (3-mark)
11. Self-Test (5+ Q&A)
Q1: Where does glycolysis occur and what are its products? A: CYTOPLASM. Products: 2 Pyruvate, 2 ATP (net), 2 NADH, 2 H₂O.
Q2: What is the role of O₂ in aerobic respiration? A: O₂ is the FINAL ELECTRON ACCEPTOR at the END of the electron transport system. It combines with e⁻ and H⁺ to form WATER. Without O₂, the ETS would STOP.
Q3: How many ATP molecules are produced from one glucose molecule during aerobic respiration? A: ~36-38 ATP (glycolysis: 2 + 5; link reaction: 5; Krebs: 2 + 15 + 3 = 36-38). The exact number depends on the shuttle system.
Q4: What are the end products of alcoholic and lactic acid fermentation? A: Alcoholic: ETHANOL + CO₂ + 2 ATP (per glucose). Lactic acid: LACTIC ACID + 2 ATP.
Q5: Calculate the RQ for respiration of tripalmitin (a fat): 2C₅₁H₁₀₄O₆ + 145O₂ → 102CO₂ + 104H₂O. A: RQ = CO₂/O₂ = 102/145 = 0.70 (typical for FATS).
12. Conclusion
Respiration is the energy-RELEASING process that POWERS all cellular activities. Glycolysis (in cytoplasm) partially breaks down glucose. The Krebs cycle (in mitochondria) COMPLETELY oxidises Acetyl-CoA to CO₂. The electron transport system GENERATES the VAST majority of ATP through oxidative phosphorylation. Fermentation is an ANCIENT, less efficient alternative for O₂-free conditions. Understanding respiration is FUNDAMENTAL to metabolism, bioenergetics, and the integrated functioning of organisms.
