🌬️ Plant Respiration
Study aerobic and anaerobic respiration in plants for CUET Agriculture. Glycolysis, Krebs cycle, ETC and respiratory quotient explained.
Definition
Respiration is a catabolic process in which organic substrates (mainly glucose) are oxidized (broken down) to release energy in the form of ATP for cellular activities. While photosynthesis builds molecules using light energy, respiration breaks them down to release stored chemical energy — these two processes are complementary.
Types of Respiration
Based on Substrate
| Type | Description |
|---|---|
| Starchy respiration | When carbohydrates and starch are used as the respiratory substrate — this is the most common type, also called floury respiration |
| Protoplasmic/Protein respiration | When protein is used as substrate — this occurs during extreme conditions like starvation, disease, or aging, when carbohydrate reserves are depleted |
NOTE
In leguminous plants, protein is the most abundant stored substrate and is the first to be oxidized. In these plants, protoplasmic respiration is also sometimes called floury respiration.
Based on O₂ Usage
| Feature | Aerobic Respiration | Anaerobic Respiration |
|---|---|---|
| O₂ used | Yes | No |
| Occurs in | All living organisms | Some fungi, germinating seeds in waterlogged conditions |
| Oxidation | Complete (glucose fully broken down) | Incomplete (glucose partially broken down) |
| Water formation | Yes | No |
| Location | Cytoplasm + Mitochondria | Only in Cytoplasm |
| ATP yield | 38 ATP (from one glucose) | 2 ATP |
| Equation | C₆H₁₂O₆ + 6O₂ + 6H₂O → 6CO₂ + 12H₂O + 686 Kcal + 38 ATP | C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 56 Kcal + 2 ATP |
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Definition
Respiration is a catabolic process in which organic substrates (mainly glucose) are oxidized (broken down) to release energy in the form of ATP for cellular activities. While photosynthesis builds molecules using light energy, respiration breaks them down to release stored chemical energy — these two processes are complementary.
Types of Respiration
Based on Substrate
| Type | Description |
|---|---|
| Starchy respiration | When carbohydrates and starch are used as the respiratory substrate — this is the most common type, also called floury respiration |
| Protoplasmic/Protein respiration | When protein is used as substrate — this occurs during extreme conditions like starvation, disease, or aging, when carbohydrate reserves are depleted |
NOTE
In leguminous plants, protein is the most abundant stored substrate and is the first to be oxidized. In these plants, protoplasmic respiration is also sometimes called floury respiration.
Based on O₂ Usage
| Feature | Aerobic Respiration | Anaerobic Respiration |
|---|---|---|
| O₂ used | Yes | No |
| Occurs in | All living organisms | Some fungi, germinating seeds in waterlogged conditions |
| Oxidation | Complete (glucose fully broken down) | Incomplete (glucose partially broken down) |
| Water formation | Yes | No |
| Location | Cytoplasm + Mitochondria | Only in Cytoplasm |
| ATP yield | 38 ATP (from one glucose) | 2 ATP |
| Equation | C₆H₁₂O₆ + 6O₂ + 6H₂O → 6CO₂ + 12H₂O + 686 Kcal + 38 ATP | C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 56 Kcal + 2 ATP |
IMPORTANT
Aerobic respiration produces 19 times more ATP than anaerobic respiration from the same amount of glucose (38 vs 2 ATP). This is why oxygen is so critical for most organisms.
Steps of Aerobic Respiration
Overview
Aerobic respiration occurs in four sequential steps, each in a specific cellular location:
| Step | Location | Process |
|---|---|---|
| 1 | Cytoplasm | Glycolysis (EMP pathway) |
| 2 | Mitochondrial matrix | Link Reaction (Pyruvate oxidation) |
| 3 | Mitochondrial matrix | Krebs Cycle (TCA cycle) |
| 4 | Inner mitochondrial membrane | ETS/ETC (Electron Transport Chain) |
1. Glycolysis (EMP Pathway)
- Discovered by Embden, Meyerhof, and Parnas — hence called the EMP pathway
- Occurs in the cytoplasm (cytosol) — does not require mitochondria
- Involves 10 enzymatic steps
- Does not require oxygen — occurs in both aerobic and anaerobic conditions (this is the shared first step)
Summary:
Glucose (6C) → 2 Pyruvic acid (3C) + 2 ATP (net) + 2 NADH₂
Detailed Steps:
| Step | Reaction | Enzyme | Energy Change |
|---|---|---|---|
| 1 | Glucose → Glucose-6-phosphate | Hexokinase (Mg²⁺) | −1 ATP |
| 2 | Glucose-6-P → Fructose-6-P | Phosphoglucoisomerase | — |
| 3 | Fructose-6-P → Fructose-1,6-bisphosphate | Phosphofructokinase (Mg²⁺) | −1 ATP |
| 4 | Fructose-1,6-bisphosphate → DHAP + G3P | Aldolase | — |
| 5 | DHAP ⇌ G3P | Triose phosphate isomerase | — |
| 6 | G3P → 1,3-bisphosphoglycerate | G3P dehydrogenase | +2 NADH |
| 7 | 1,3-BPG → 3-phosphoglycerate | Phosphoglycerate kinase | +2 ATP |
| 8 | 3-PG → 2-phosphoglycerate | Phosphoglycerate mutase | — |
| 9 | 2-PG → Phosphoenolpyruvate (PEP) | Enolase | — |
| 10 | PEP → Pyruvate | Pyruvate kinase | +2 ATP |
ATP Accounting in Glycolysis:
- Gross ATP produced = 10 ATP (including NADH conversion via ETS: 2 NADH × 3 = 6, plus 4 from substrate-level phosphorylation)
- ATP consumed = 2 ATP (steps 1 and 3)
- Net ATP = 8 ATP (when NADH₂ is converted to ATP via ETS)
- Net ATP without ETS = 2 ATP (from substrate-level phosphorylation alone)
- NADH₂ produced = 2 NADH₂
- ATP from substrate-level phosphorylation = 4 ATP (gross), 2 ATP (net after subtracting the 2 consumed)
Key Points:
- Glycolysis occurs wherever Mg²⁺ ions are present (cytoplasm)
- Both ATP production and consumption occur — it costs 2 ATP to "activate" glucose
- This pathway is common to both aerobic and anaerobic respiration — it is the evolutionary oldest metabolic pathway
2. Link Reaction (Pyruvate Oxidation)
This is the bridge between glycolysis and the Krebs cycle. It occurs in the mitochondrial matrix.
Reaction:
2 Pyruvic acid (3C) → 2 Acetyl CoA (2C) + 2CO₂ + 2NADH₂
- Catalyzed by the pyruvate dehydrogenase complex — a large multi-enzyme complex requiring 5 cofactors: Mg²⁺, NAD, CoA, Lipoic acid, Thiamine pyrophosphate (TPP)
- This reaction produces 2 NADH₂ = 6 ATP (via ETS)
- This is an irreversible reaction — pyruvate cannot be regenerated from acetyl CoA
NOTE
The link reaction is where the first CO₂ is released during aerobic respiration. Each pyruvate loses one carbon as CO₂, converting from a 3C to a 2C compound (acetyl CoA).
3. Krebs Cycle / TCA Cycle / Citric Acid Cycle
- Discovered by H.A. Krebs in 1937 — received the Nobel Prize in 1953
- Occurs in the mitochondrial matrix
- First stable product: Citric acid (6C)
- Also called Tricarboxylic Acid (TCA) cycle because citric acid has 3 carboxyl groups
The 8 Steps:
| Step | Substrate → Product | Enzyme | Products |
|---|---|---|---|
| 1 | Acetyl CoA (2C) + OAA (4C) → Citric acid (6C) | Citrate synthase | Citrate |
| 2 | Citric acid (6C) → Isocitric acid (6C) | Aconitase | Isocitrate |
| 3 | Isocitric acid → α-Ketoglutaric acid (5C) | Isocitrate dehydrogenase | CO₂ + NADH₂ |
| 4 | α-KG → Succinyl CoA (4C) | α-KG dehydrogenase | CO₂ + NADH₂ |
| 5 | Succinyl CoA → Succinic acid (4C) | Succinyl CoA synthetase | GTP (= 1 ATP) |
| 6 | Succinic acid → Fumaric acid (4C) | Succinate dehydrogenase | FADH₂ |
| 7 | Fumaric acid → Malic acid (4C) | Fumarase | — |
| 8 | Malic acid → OAA (4C) | Malate dehydrogenase | NADH₂ |
Per Turn of Krebs Cycle (one acetyl CoA):
- 3 NADH₂ = 9 ATP (via ETS)
- 1 FADH₂ = 2 ATP (via ETS)
- 1 GTP = 1 ATP (substrate-level phosphorylation)
- Total per turn = 12 ATP
- Per glucose (2 turns, since one glucose produces 2 acetyl CoA): 24 ATP
- CO₂ released per turn: 2 molecules
- Total CO₂ from Krebs: 4 molecules (from 2 turns)
4. Electron Transport Chain (ETS/ETC)
The ETS is where the majority of ATP is produced. It occurs on the inner mitochondrial membrane and consists of 5 complexes.
Complexes:
| Complex | Name | Electron Donor |
|---|---|---|
| I | NADH dehydrogenase | NADH₂ |
| II | Succinate dehydrogenase (FADH₂) | FADH₂ |
| III | Cytochrome b-c₁ complex | — |
| IV | Cytochrome c oxidase | — |
| V | ATP synthase | — (uses proton gradient) |
Electron Flow: NADH₂ → Complex I → UQ (Ubiquinone) → Complex III (Cyt b-c₁) → Cyt c → Complex IV → O₂ → H₂O
Key Components:
- UQ (Ubiquinone/Coenzyme Q): A mobile electron carrier that is lipid-soluble — it shuttles electrons between complexes I/II and complex III
- Cytochrome c: A small, mobile protein that carries electrons from complex III to complex IV
- Oxygen: The terminal electron acceptor — it receives electrons at the end of the chain and combines with H⁺ to form water
Chemiosmotic Hypothesis (Peter Mitchell)
- Proposed by Peter Mitchell — this elegant theory explains how ATP is synthesized using the energy from electron transport
- As electrons pass through complexes I, III, and IV, H⁺ ions (protons) are actively pumped from the matrix to the intermembrane space
- This creates a proton gradient — higher H⁺ concentration in the intermembrane space than in the matrix
- Protons flow back into the matrix through ATP synthase (Complex V) — this flow of protons drives ATP synthesis (like water turning a turbine in a hydroelectric dam)
- 3 H⁺ ions flowing through ATP synthase produce 1 ATP
- ATP synthase has two components: F₀ (the membrane channel through which protons flow) and F₁ (the catalytic unit in the matrix where ATP is actually made)
Total ATP Yield from One Glucose Molecule
| Pathway | NADH₂ | FADH₂ | ATP (substrate) | Total ATP |
|---|---|---|---|---|
| Glycolysis | 2 | — | 2 | 2 + 6 = 8 |
| Link Reaction | 2 | — | — | 6 |
| Krebs Cycle (×2) | 6 | 2 | 2 (GTP) | 18 + 4 + 2 = 24 |
| Total | 10 | 2 | 4 | 38 ATP |
Calculation:
- 10 NADH₂ × 3 ATP = 30 ATP
- 2 FADH₂ × 2 ATP = 4 ATP
- 4 ATP (substrate level) = 4 ATP
- Grand Total = 38 ATP
Alternative counting (some textbooks):
- If glycolysis NADH₂ enters mitochondria via malate-aspartate shuttle: each NADH₂ = 3 ATP → Total = 38 ATP
- If via glycerol phosphate shuttle: each NADH₂ = 2 ATP → Total = 36 ATP
Energy Efficiency:
- 1 pyruvic acid → 15 ATP (complete oxidation)
- 1 Acetyl CoA → 12 ATP (complete oxidation via Krebs)
- 1 glucose → 38 ATP total
- 1 ATP = 7.3 kcal of energy
- Total energy from 1 glucose = 686 kcal; energy captured in ATP = ~277 kcal → approximately 40% efficiency (the rest is lost as heat)
Anaerobic Respiration / Fermentation
Discovered by Buchner in 1897 using yeast extract — the enzyme responsible was named Zymase.
- Only glycolysis occurs (in cytoplasm) — pyruvate is then converted to other products without entering mitochondria
- Produces only 2 ATP per glucose
Types of Fermentation
| Type | Product | Organism | Equation |
|---|---|---|---|
| Alcoholic fermentation | Ethanol + CO₂ | Yeast (Saccharomyces) | Pyruvate → Acetaldehyde → Ethanol + CO₂ + 2 ATP |
| Lactic acid fermentation | Lactic acid | Lactobacillus, muscle cells | Pyruvate → Lactic acid + 2 ATP |
| Acetic acid fermentation | Acetic acid (vinegar) | Acetobacter | Ethanol → Acetic acid + 2 ATP |
| Butyric acid fermentation | Butyric acid (rancid smell) | Clostridium | Glucose → Butyric acid + 2 ATP |
WARNING
Methyl alcohol (methanol) is extremely toxic and can cause blindness or death. It is sometimes found as a contaminant in illegally produced liquor. Ethanol (ethyl alcohol) is the product of normal yeast fermentation and is the alcohol in beverages.
Respiratory Quotient (RQ)
The Respiratory Quotient is the ratio of CO₂ released to O₂ consumed during respiration. It reveals what type of substrate is being respired.
RQ = CO₂ released / O₂ consumed
RQ value depends on the substrate being respired. Measured using a Ganong's respirometer or Regnault and Reiset apparatus.
| Substrate | RQ Value |
|---|---|
| Carbohydrate | 1.0 (equal CO₂ released and O₂ consumed) |
| Fat/Lipid | 0.7 (fats need more O₂ to oxidize, being more reduced) |
| Protein | 0.9 |
| Organic acid | > 1.0 (organic acids are already partially oxidized, need less O₂) |
| Citric acid | 1.14 |
| Malic acid | 1.33 |
| Oxalic acid | 4.0 |
| CAM/Succulent plants | 0 (stomata closed during day — no external gas exchange) |
| Anaerobic respiration | Infinity (no O₂ consumed, but CO₂ is released) |
TIP
If RQ = 1 → carbohydrate substrate. If RQ < 1 → fat or protein. If RQ > 1 → organic acids. If RQ = ∞ → anaerobic respiration.
Pentose Phosphate Pathway (HMP Shunt / PPP / Warburg-Dickens Pathway)
- Discovered by Warburg and Dickens (detailed study by Racker in 1958)
- Also called Hexose Monophosphate (HMP) pathway or Pentose Phosphate Pathway (PPP)
- Occurs in the cytoplasm (not in mitochondria) — an alternative to glycolysis
Process: Glucose (6C) → Ribulose-5-phosphate (5C) + CO₂ + NADPH₂
Key Points:
- Produces NADPH₂ (reducing power for biosynthesis) — note: this is NADPH₂, not NADH₂! NADPH₂ is used for building molecules, while NADH₂ feeds into energy production
- Generates ribose-5-phosphate — essential for nucleotide synthesis (DNA and RNA building blocks)
- Each cycle produces 2 NADPH₂
- Complete oxidation of 1 glucose via HMP: 12 NADPH₂ = 36 ATP
- Important in cells with high biosynthetic activity (fat synthesis, steroid synthesis, actively dividing cells)
- This pathway runs in 6 steps with 6 CO₂ released per complete cycle
Shuttle Systems
The NADH₂ produced during glycolysis (in the cytoplasm) cannot directly enter the mitochondria. Special shuttle systems transport the electrons across the mitochondrial membrane:
| Shuttle | NADH₂ enters as | ATP per NADH₂ | Total ATP from 1 glucose |
|---|---|---|---|
| Malate-Aspartate Shuttle | NADH₂ (enters ETS at Complex I) | 3 ATP | 38 ATP |
| Glycerol Phosphate Shuttle | FADH₂ (enters ETS at Complex II) | 2 ATP | 36 ATP |
NOTE
The shuttle system used varies by organism and tissue type. The malate-aspartate shuttle is more efficient (yielding 38 ATP total) but is found mainly in liver and heart cells. The glycerol phosphate shuttle (36 ATP total) is found in brain and skeletal muscle cells.
Pasteur Effect
When oxygen is introduced to an anaerobic environment, the rate of glucose consumption decreases dramatically. This is because aerobic respiration is much more efficient — producing 38 ATP vs 2 ATP per glucose. The organism needs far less glucose to meet its energy needs. This phenomenon is called the Pasteur Effect, named after Louis Pasteur who first observed it in yeast.
Key Facts for Exam Revision
| Fact | Detail |
|---|---|
| Glycolysis discovered by | Embden, Meyerhof, Parnas |
| Krebs cycle discovered by | H.A. Krebs (Nobel Prize 1953) |
| Chemiosmotic theory by | Peter Mitchell |
| Fermentation discovered by | Buchner (1897) |
| Glycolysis location | Cytoplasm |
| Krebs cycle location | Mitochondrial matrix |
| ETS location | Inner mitochondrial membrane |
| Net ATP from glycolysis | 8 (with ETS) or 2 (without) |
| ATP from one Acetyl CoA | 12 ATP |
| Total ATP per glucose | 38 ATP (aerobic) |
| ATP per glucose (anaerobic) | 2 ATP |
| RQ of carbohydrate | 1.0 |
| RQ of fat | 0.7 |
| RQ of protein | 0.9 |
| Terminal electron acceptor | O₂ |
| 1 ATP energy | 7.3 kcal |
| Total energy from 1 glucose | 686 kcal |
| HMP pathway produces | NADPH₂ + Ribose-5-P |
| Pyruvate dehydrogenase cofactors | Mg²⁺, NAD, CoA, Lipoic acid, TPP |
| ATP synthase components | F₀ (channel) + F₁ (catalytic) |
| 3 H⁺ through ATP synthase | 1 ATP |
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Respiration definition | Catabolic process; organic substrates oxidized to release ATP |
| Starchy respiration | Uses carbohydrates/starch as substrate; most common type |
| Protoplasmic respiration | Uses protein as substrate; occurs during starvation/disease |
| Aerobic respiration | Requires O₂; complete oxidation; cytoplasm + mitochondria; yields 38 ATP |
| Anaerobic respiration | No O₂; incomplete oxidation; cytoplasm only; yields 2 ATP |
| Aerobic equation | C₆H₁₂O₆ + 6O₂ + 6H₂O → 6CO₂ + 12H₂O + 686 Kcal + 38 ATP |
| Glycolysis (EMP pathway) | By Embden, Meyerhof, Parnas; in cytoplasm; 10 steps; no O₂ needed |
| Glycolysis summary | Glucose (6C) → 2 Pyruvic acid (3C) + 2 ATP (net) + 2 NADH₂ |
| Glycolysis net ATP | 8 ATP (with ETS) or 2 ATP (without ETS) |
| Key glycolysis enzymes | Hexokinase (step 1), Phosphofructokinase (step 3), Pyruvate kinase (step 10) |
| Link reaction | In mitochondrial matrix; 2 Pyruvate → 2 Acetyl CoA + 2CO₂ + 2NADH₂ |
| Pyruvate dehydrogenase cofactors | Mg²⁺, NAD, CoA, Lipoic acid, TPP (5 cofactors) |
| Krebs cycle | By H.A. Krebs (1937); Nobel Prize 1953; in mitochondrial matrix; 8 steps |
| Krebs first product | Citric acid (6C); also called TCA cycle (3 carboxyl groups) |
| Only 5C compound in Krebs | α-Ketoglutaric acid |
| Krebs per turn | 3 NADH₂ + 1 FADH₂ + 1 GTP = 12 ATP |
| Krebs per glucose (2 turns) | 24 ATP total; 4 CO₂ released |
| Krebs is amphibolic | Serves both catabolic and anabolic purposes |
| ETS/ETC | On inner mitochondrial membrane; 5 complexes |
| ETS complexes | I (NADH dehydrogenase), II (Succinate dehydrogenase), III (Cyt b-c₁), IV (Cyt c oxidase), V (ATP synthase) |
| Terminal electron acceptor | O₂ → combines with H⁺ to form H₂O |
| Chemiosmotic hypothesis | By Peter Mitchell; H⁺ pumped to intermembrane space → flows back through ATP synthase (F₀ + F₁) |
| H⁺ per ATP | 3 H⁺ through ATP synthase = 1 ATP |
| Total ATP per glucose | 38 ATP (malate-aspartate shuttle) or 36 ATP (glycerol phosphate shuttle) |
| 1 ATP energy | 7.3 kcal; total from 1 glucose = 686 kcal; efficiency ~40% |
| 1 Acetyl CoA | 12 ATP; 1 pyruvate = 15 ATP |
| Fermentation | Discovered by Buchner (1897); enzyme = Zymase |
| Alcoholic fermentation | Pyruvate → Ethanol + CO₂; by yeast (Saccharomyces); 2 ATP |
| Lactic acid fermentation | Pyruvate → Lactic acid; by Lactobacillus / muscle cells; 2 ATP |
| RQ formula | RQ = CO₂ released / O₂ consumed |
| RQ values | Carbohydrate = 1.0, Fat = 0.7, Protein = 0.9, Organic acid = >1, Anaerobic = ∞, CAM = 0 |
| RQ measured by | Ganong's respirometer |
| Pentose Phosphate Pathway (HMP) | By Warburg & Dickens; in cytoplasm; produces NADPH₂ + Ribose-5-P |
| Pasteur Effect | O₂ introduction → glucose consumption decreases (aerobic more efficient: 38 vs 2 ATP) |
| Shuttle systems | Malate-aspartate (3 ATP/NADH₂ → 38 total) vs Glycerol phosphate (2 ATP/NADH₂ → 36 total) |
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