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Short Notes Respiration in Plants - Short Notes for NEET
Definition and Overview
- Respiration: Catabolic process of breakdown of organic compounds to release energy
- Overall equation: C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy (ATP)
- Exergonic process: Releases energy
- Occurs in all living cells (24/7)
Types of Respiration
| Type | Aerobic Respiration | Anaerobic Respiration/Fermentation |
|---|---|---|
| O2 requirement | Required | Not required |
| Location | Cytoplasm + Mitochondria | Cytoplasm only |
| End products | CO2 + H2O | Ethanol/Lactate + CO2 |
| ATP yield | 36-38 ATP per glucose | 2 ATP per glucose |
| Complete oxidation | Yes | No (incomplete) |
Exchange of Gases
- In plants:
- Stomata (leaves), Lenticels (stems), General surface (roots)
- Gas exchange by simple diffusion
- Each cell respires independently
- No specialized respiratory organs
- Day vs Night:
- Day: Photosynthesis > Respiration; net O2 release, CO2 uptake
- Night: Only respiration; O2 uptake, CO2 release
Stages of Aerobic Respiration
| Stage | Location | Input | Output | ATP (Net) |
|---|---|---|---|---|
| 1. Glycolysis | Cytoplasm | Glucose (6C) | 2 Pyruvate (3C) 2 NADH 4 ATP | 2 ATP (4 produced - 2 consumed) |
| 2. Link Reaction (Oxidative decarboxylation) | Mitochondrial matrix | 2 Pyruvate | 2 Acetyl CoA (2C) 2 NADH 2 CO2 | 0 ATP |
| 3. Krebs Cycle (TCA/Citric acid cycle) | Mitochondrial matrix | 2 Acetyl CoA | 4 CO2 6 NADH 2 FADH2 2 ATP/GTP | 2 ATP |
| 4. Electron Transport Chain & Oxidative Phosphorylation | Inner mitochondrial membrane | 10 NADH 2 FADH2 | H2O | 34 ATP (28 from NADH + 6 from FADH2) |
Total ATP: 38 ATP (theoretical maximum)
- In reality: 30-32 ATP due to energy loss in transport
GLYCOLYSIS (EMP Pathway)
Overview
- Discovered by: Embden, Meyerhof, and Parnas (EMP pathway)
- Location: Cytoplasm
- Oxygen: Not required (occurs in both aerobic and anaerobic conditions)
- Definition: Breakdown of glucose to pyruvate
Steps of Glycolysis
A. Preparatory Phase (Energy Investment)
| Step | Reaction | Enzyme |
|---|---|---|
| 1 | Glucose (6C) → Glucose-6-phosphate Uses 1 ATP | Hexokinase |
| 2 | Glucose-6-phosphate → Fructose-6-phosphate | Phosphoglucoisomerase |
| 3 | Fructose-6-phosphate → Fructose-1,6-bisphosphate Uses 1 ATP | Phosphofructokinase (Rate-limiting enzyme) |
| 4 | Fructose-1,6-bisphosphate → DHAP + G3P (3C each) | Aldolase |
| 5 | DHAP ⇌ G3P (Both are triose phosphates) | Triose phosphate isomerase |
Result: 2 molecules of G3P from 1 glucose; 2 ATP consumed
B. Pay-off Phase (Energy Generation)
| Step | Reaction | Enzyme |
|---|---|---|
| 6 | 2 G3P → 2 1,3-bisphosphoglycerate Produces 2 NADH | G3P dehydrogenase (NAD+ reduced) |
| 7 | 2 1,3-bisphosphoglycerate → 2 3-PGA Produces 2 ATP (substrate-level) | Phosphoglycerate kinase |
| 8 | 2 3-PGA → 2 2-PGA | Phosphoglycerate mutase |
| 9 | 2 2-PGA → 2 PEP + 2 H2O | Enolase |
| 10 | 2 PEP → 2 Pyruvate Produces 2 ATP (substrate-level) | Pyruvate kinase |
Net Products of Glycolysis (per glucose)
- 2 Pyruvate (3-carbon)
- 2 NADH + 2H+
- Net 2 ATP (4 produced - 2 consumed)
Fate of Pyruvate
| Condition | Process | Location | Products |
|---|---|---|---|
| Aerobic (O2 present) | Enters mitochondria for Krebs cycle | Mitochondria | CO2 + H2O + ATP |
| Anaerobic (No O2) | Fermentation | Cytoplasm | See fermentation section |
FERMENTATION (Anaerobic Respiration)
Overview
- Definition: Incomplete oxidation of glucose without O2
- Location: Cytoplasm
- ATP yield: Only 2 ATP per glucose (from glycolysis only)
- Purpose: Regenerate NAD+ to allow glycolysis to continue
Types of Fermentation
| Type | Organism/Location | Reaction | Products |
|---|---|---|---|
| Alcoholic Fermentation | Yeast, plant roots in waterlogged soil | Pyruvate → Acetaldehyde + CO2 Acetaldehyde + NADH → Ethanol + NAD+ | Ethanol + CO2 2 ATP |
| Lactic Acid Fermentation | Muscle cells (during vigorous exercise), some bacteria (Lactobacillus) | Pyruvate + NADH → Lactate + NAD+ | Lactic acid (Lactate) 2 ATP |
Key Points
- NADH from glycolysis is oxidized back to NAD+
- NAD+ regeneration allows glycolysis to continue
- No additional ATP produced in fermentation steps
- Incomplete breakdown - much energy remains in end products
AEROBIC RESPIRATION
Link Reaction (Oxidative Decarboxylation of Pyruvate)
- Location: Mitochondrial matrix
- Reaction: Pyruvate (3C) + CoA + NAD+ → Acetyl CoA (2C) + CO2 + NADH + H+
- Enzyme complex: Pyruvate dehydrogenase complex
- Per glucose: Occurs twice (2 pyruvate molecules)
- Products: 2 Acetyl CoA, 2 NADH, 2 CO2
- Irreversible step
KREBS CYCLE (TCA Cycle / Citric Acid Cycle)
Overview
- Discovered by: Hans Krebs
- Location: Mitochondrial matrix
- Cyclic pathway: OAA regenerated at the end
- Occurs twice per glucose (once for each Acetyl CoA)
Steps of Krebs Cycle
| Step | Reaction | Enzyme | Carbons |
|---|---|---|---|
| 1 | Acetyl CoA (2C) + OAA (4C) → Citrate (6C) + CoA | Citrate synthase | 6C |
| 2 | Citrate → Isocitrate (isomerization) | Aconitase | 6C |
| 3 | Isocitrate → α-Ketoglutarate + CO2 + NADH (First decarboxylation) | Isocitrate dehydrogenase | 5C |
| 4 | α-Ketoglutarate + CoA → Succinyl CoA + CO2 + NADH (Second decarboxylation) | α-Ketoglutarate dehydrogenase | 4C |
| 5 | Succinyl CoA → Succinate + GTP/ATP (Substrate-level phosphorylation) | Succinyl CoA synthetase | 4C |
| 6 | Succinate → Fumarate + FADH2 | Succinate dehydrogenase (embedded in inner membrane) | 4C |
| 7 | Fumarate + H2O → Malate | Fumarase | 4C |
| 8 | Malate → OAA + NADH (OAA regenerated - cycle continues) | Malate dehydrogenase | 4C |
Net Products of Krebs Cycle (per Acetyl CoA / per turn)
- 2 CO2
- 3 NADH
- 1 FADH2
- 1 GTP/ATP (substrate-level phosphorylation)
Per Glucose (2 turns of cycle)
- 4 CO2
- 6 NADH
- 2 FADH2
- 2 ATP
Key Points
- Main function: Generate NADH and FADH2 for ETC
- All 6 carbons of glucose released as CO2 (2 in link reaction + 4 in Krebs)
- OAA is both first reactant and final product (cyclic)
ELECTRON TRANSPORT CHAIN (ETC) AND OXIDATIVE PHOSPHORYLATION
Overview
- Location: Inner mitochondrial membrane (cristae)
- Function: Oxidize NADH and FADH2; generate ATP via chemiosmosis
- Final electron acceptor: O2 (reduced to H2O)
- Produces maximum ATP
Components of ETC (Electron Carriers)
- Complex I: NADH dehydrogenase (accepts electrons from NADH)
- Coenzyme Q (Ubiquinone): Mobile carrier
- Complex III: Cytochrome bc1 complex
- Cytochrome c: Mobile carrier
- Complex IV: Cytochrome oxidase (transfers electrons to O2)
Mechanism
- NADH donates electrons to Complex I → NAD+ regenerated
- FADH2 donates electrons to Complex II (Coenzyme Q) → FAD regenerated
- Electrons pass through chain: Complex I/II → Q → Complex III → Cytochrome c → Complex IV
- Energy released pumps H+ from matrix to intermembrane space (at Complexes I, III, IV)
- Proton gradient created across inner membrane
- O2 accepts electrons at Complex IV: O2 + 4H+ + 4e- → 2H2O
Oxidative Phosphorylation (Chemiosmotic Hypothesis)
- Proposed by: Peter Mitchell
- Mechanism:
- H+ accumulates in intermembrane space (low pH)
- Matrix has low H+ concentration (high pH)
- Proton motive force drives H+ back through ATP synthase (F0-F1 complex)
- Energy released used to synthesize ATP from ADP + Pi
ATP Yield from ETC
- 1 NADH → approximately 2.5-3 ATP (conventional value: 3 ATP)
- 1 FADH2 → approximately 1.5-2 ATP (conventional value: 2 ATP)
ATP Accounting (Total per Glucose)
| Process | Direct ATP | NADH | FADH2 | ATP from NADH | ATP from FADH2 | Total ATP |
|---|---|---|---|---|---|---|
| Glycolysis | 2 | 2 | 0 | 2×3 = 6 (or 2×2 = 4)* | 0 | 8 (or 6)* |
| Link Reaction (×2) | 0 | 2 | 0 | 2×3 = 6 | 0 | 6 |
| Krebs Cycle (×2) | 2 | 6 | 2 | 6×3 = 18 | 2×2 = 4 | 24 |
| TOTAL | 4 | 10 | 2 | 30 (or 28)* | 4 | 38 (or 36)* |
*NADH from glycolysis in cytoplasm requires shuttle system to enter mitochondria; costs ATP
Practical ATP Yield
- Theoretical maximum: 38 ATP
- Actual yield: 30-32 ATP (due to proton leak, transport costs)
AMPHIBOLIC PATHWAYS
Definition
- Amphibolic: Pathways that function in both catabolism (breakdown) and anabolism (synthesis)
- Respiration is amphibolic - not just catabolic
Respiratory Substrates
| Substrate | Entry Point in Respiration |
|---|---|
| Carbohydrates | Glucose → Glycolysis |
| Fats/Lipids | Glycerol → DHAP (glycolysis) Fatty acids → Acetyl CoA (β-oxidation) |
| Proteins | Amino acids (after deamination) → Various points: - Pyruvate - Acetyl CoA - Krebs cycle intermediates (α-ketoglutarate, OAA) |
Anabolic Uses of Respiratory Intermediates
- Acetyl CoA: Fatty acid synthesis, cholesterol synthesis
- α-Ketoglutarate: Amino acid (glutamate) synthesis
- OAA: Amino acid (aspartate) synthesis
- Succinyl CoA: Porphyrin (heme) synthesis
Respiratory Quotient (RQ)
Definition & Formula
- Respiratory Quotient (RQ) = Volume of CO₂ evolved / Volume of O₂ consumed
- Indicates the type of respiratory substrate being oxidized
- Also called Respiratory Ratio
RQ Values for Different Substrates
| Substrate | RQ Value | Reason |
|---|---|---|
| Carbohydrates | 1.0 | Equal volumes of CO₂ released and O₂ consumed C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O |
| Fats/Lipids | 0.7 | More O₂ required for oxidation, less CO₂ produced 2(C₅₁H₉₈O₆) + 145O₂ → 102CO₂ + 98H₂O |
| Proteins | 0.8-0.9 | Intermediate value |
| Organic acids | >1.0 | More CO₂ released than O₂ consumed |
| Succulents (CAM plants) | 0 (during night) | Only CO₂ fixation, no O₂ consumption |
| Anaerobic respiration | ∞ (Infinity) | CO₂ released without O₂ consumption |
Significance
- Determines the type of respiratory substrate being utilized
- Helps in understanding metabolic state of organisms
- Used in Ganong's respirometer to measure RQ
- Mixed diet typically shows RQ = 0.85-0.95
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FAQs on Short Notes Respiration in Plants - Short Notes for NEET
| 1. What is respiration in plants? |  |
Ans. Respiration in plants is a biochemical process where glucose is broken down in the presence of oxygen to release energy, carbon dioxide, and water. This process occurs in the mitochondria of plant cells and is vital for cellular activities.
| 2. How does respiration differ from photosynthesis in plants? | |
Ans. Respiration and photosynthesis are complementary processes in plants. Photosynthesis occurs in the chloroplasts, where sunlight is used to convert carbon dioxide and water into glucose and oxygen. In contrast, respiration takes place in the mitochondria, where glucose is oxidised to release energy, utilising oxygen and producing carbon dioxide and water.
| 3. What are the main stages of respiration in plants? | |
Ans. The main stages of respiration in plants include glycolysis, the Krebs cycle, and the electron transport chain. Glycolysis occurs in the cytoplasm, breaking glucose into pyruvate. The Krebs cycle takes place in the mitochondria, further oxidising pyruvate to produce ATP, NADH, and FADH₂. Finally, the electron transport chain uses these electron carriers to generate more ATP through oxidative phosphorylation.
| 4. What is anaerobic respiration, and when does it occur in plants? | |
Ans. Anaerobic respiration is a type of respiration that occurs in the absence of oxygen. In plants, this process typically occurs during conditions like waterlogged soils where oxygen is limited. The end products of anaerobic respiration in plants are ethanol and carbon dioxide, along with a smaller yield of energy compared to aerobic respiration.
| 5. Why is respiration essential for plant growth and development? | |
Ans. Respiration is essential for plant growth and development as it provides the energy required for various metabolic activities, including nutrient absorption, cell division, and synthesis of biomolecules. Without sufficient energy from respiration, plants would be unable to grow, reproduce, or respond to environmental changes effectively.
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