Respiration in Plants
KVPY – Biology – Historic
Respiration is the cellular process that converts stored energy in sugars into usable ATP, releasing CO₂ and H₂O as waste products. In plants, this occurs in every cell and is essential for growth, development, and survival.
1. Simple Explanation
- What happens?
Glucose (or other organic molecules) is oxidized with O₂ to produce CO₂, H₂O, and ATP. - Where does it happen?
In the mitochondria of every plant cell (also in chloroplasts during photorespiration). - Why is it important?
Provides energy (ATP) for biosynthetic pathways, maintains cellular ion balances, and drives active transport.
It also balances the photosynthetic CO₂ uptake by releasing CO₂ during the night or when light is absent.
2. Key Points
| Concept | Details |
|---|---|
| Definition | Catabolic process where organic molecules are oxidized to generate ATP. |
| Primary Site | Mitochondria (matrix, inner membrane). |
| Stages | 1. Glycolysis (cytoplasm) 2. Link reaction (pyruvate → acetyl‑CoA) 3. Krebs (TCA) cycle (matrix) 4. Electron Transport Chain (ETC) (inner membrane) + oxidative phosphorylation |
| Net Equation | C₆H₁₂O₆ (glucose) + 6 O₂ → 6 CO₂ + 6 H₂O + ~30–38 ATP |
| ATP Yield | ~30–36 ATP per glucose in plant mitochondria (variable). |
| Regulation | Allosteric enzymes (hexokinase, phosphofructokinase), hormonal (auxin, gibberellin), energy status (ATP/ADP). |
| Factors Affecting Rate | Temperature, O₂ availability, light (indirectly via photosynthesis), nutrient status, plant age. |
| Special Cases | Photorespiration (occurs in chloroplasts when O₂ high, CO₂ low) – a wasteful form of respiration that reduces net photosynthetic yield. Anaerobic respiration (fermentation) occurs in roots or fruit under low O₂; produces ethanol or lactate, generating only 2 ATP. |
| Physiological Roles | 1. Energy supply for growth, flowering, fruiting. 2. Maintenance of stomatal opening/closing. 3. Seed germination (mobilization of stored reserves). 4. Stress response (e.g., cold, drought). |
3. Worked Example
Example 1 – ATP Yield from 1 g of Glucose
| Step | Reaction | ATP Produced |
|---|---|---|
| 1 | Glycolysis | 2 ATP (net) + 2 NADH |
| 2 | Pyruvate → Acetyl‑CoA | 2 NADH |
| 3 | TCA Cycle | 2 ATP (GTP) + 6 NADH + 2 FADH₂ |
| 4 | ETC + Oxidative Phosphorylation | 4 NADH × 2.5 ATP = 10 ATP 2 FADH₂ × 1.5 ATP = 3 ATP |
| Total | 30 ATP |
Calculation
- One glucose (180 g/mol) → 1 mol.
- 30 ATP per mol → 30 mol ATP per 180 g glucose.
- 1 g glucose → (30 mol / 180 g) ≈ 0.167 mol ATP ≈ 100 kJ of biochemical energy.
Example 2 – CO₂ Production in a Leaf
- A 5 g leaf consumes 0.75 µmol O₂ min⁻¹ at 25 °C.
- Stoichiometry: 1 mol O₂ → 1 mol CO₂.
- CO₂ produced = 0.75 µmol min⁻¹.
- Over 60 min → 45 µ