Biology - Botany - How to Solve: Respiration in Plants (Glycolysis, Krebs Cycle, Electron Transport, Fermentation) – NEET UG Guide

How to Solve: Respiration in Plants (Glycolysis, Krebs Cycle, Electron Transport, Fermentation) – NEET UG Guide

Introduction

"Mastering plant respiration unlocks 3-5 direct NEET questions—worth 12-20 marks—on glycolysis, Krebs cycle, ETC, and fermentation. Miss this, and you lose easy marks on energy yield, ATP count, and anaerobic vs. aerobic pathways."

WHAT YOU NEED TO KNOW FIRST

1. Basic cell structure – Mitochondria (site of Krebs & ETC), cytoplasm (site of glycolysis).
2. ATP & redox reactions – ATP = energy currency; NAD⁺/FAD = electron carriers.
3. Enzyme basics – Substrate-level vs. oxidative phosphorylation.

KEY TERMS & FORMULAS

Key Terms

Formulas

1. Glycolysis Net Reaction Glucose + 2 ADP + 2 Pi + 2 NAD⁺ → 2 Pyruvate + 2 ATP + 2 NADH + 2 H⁺ + 2 H₂O

2. MEMORISE THIS: 2 ATP net gain (4 made, 2 used).

3. Krebs Cycle (Per Pyruvate) Pyruvate + 4 NAD⁺ + FAD + ADP + Pi → 3 CO₂ + 4 NADH + FADH₂ + ATP

4. MEMORISE THIS: 1 ATP (GTP), 4 NADH, 1 FADH₂ per pyruvate.

5. ETC ATP Yield (Per NADH/FADH₂)

6. 1 NADH → 3 ATP (via Complex I, III, IV)
7. 1 FADH₂ → 2 ATP (via Complex II, III, IV)

8. MEMORISE THIS: Total ATP from glucose = 38 (prokaryotes) / 36 (eukaryotes) (due to NADH transport cost).

9. Fermentation (Ethanol Pathway) Pyruvate → Acetaldehyde + CO₂ → Ethanol (via alcohol dehydrogenase)

10. MEMORISE THIS: No ATP gain (only regenerates NAD⁺).

STEP-BY-STEP METHOD

Step 1: Identify the Pathway

• Aerobic? → Glycolysis → Krebs → ETC.
• Anaerobic? → Glycolysis → Fermentation.

Step 2: Glycolysis Breakdown

1. Glucose (6C) → 2 Pyruvate (3C) (cytoplasm).
2. Net ATP: 2 (4 made, 2 used).
3. NADH: 2 (used in ETC or fermentation).

Step 3: Pyruvate Fate

• Aerobic: Pyruvate → Acetyl-CoA (mitochondria) → Krebs.
• Anaerobic: Pyruvate → Ethanol (plants) or lactate (animals).

Step 4: Krebs Cycle (Per Pyruvate)

1. Acetyl-CoA (2C) + Oxaloacetate (4C) → Citrate (6C).
2. Citrate → 2 CO₂ + 3 NADH + 1 FADH₂ + 1 ATP (GTP).
3. Regenerate oxaloacetate (cycle repeats).

Step 5: Electron Transport Chain (ETC)

1. NADH/FADH₂ donate electrons to ETC (inner mitochondrial membrane).
2. Electrons flow: Complex I → III → IV (NADH) or II → III → IV (FADH₂).
3. H⁺ pumped into intermembrane space → proton gradient.
4. ATP Synthase: H⁺ flow back → ATP made (oxidative phosphorylation).

Step 6: Fermentation (If Anaerobic)

1. Pyruvate → Ethanol (plants) or lactate (animals).
2. Regenerates NAD⁺ (keeps glycolysis running).
3. No ATP gain (only 2 ATP from glycolysis).

Step 7: Total ATP Calculation

• Why 2 ATP per NADH in glycolysis?
• NADH from glycolysis must be shuttled into mitochondria (costs 1 ATP per NADH).

WORKED EXAMPLES

Example 1 – Basic: ATP Yield from 1 Glucose

Question: How many ATP are produced from 1 glucose in aerobic respiration in eukaryotes?

Solution:
1. Glycolysis: 2 ATP (net) + 2 NADH → 4 ATP (ETC).
2. Pyruvate → Acetyl-CoA: 2 NADH → 6 ATP.
3. Krebs Cycle: 2 ATP + 6 NADH → 18 ATP + 2 FADH₂ → 4 ATP.
4. Total: 2 + 4 + 6 + 2 + 18 + 4 = 36 ATP.

What we did and why: - Broke down each stage’s ATP contribution. - Accounted for NADH/FADH₂ transport costs in eukaryotes.

Example 2 – Medium: Fermentation vs. Aerobic Respiration

Question: Compare ATP yield and end products of aerobic respiration vs. ethanol fermentation in plants.

Solution: | Feature | Aerobic Respiration | Ethanol Fermentation | |---------|---------------------|----------------------| | ATP Yield | 36 ATP | 2 ATP | | End Products | CO₂ + H₂O | Ethanol + CO₂ | | NAD⁺ Regeneration | Via ETC | Via fermentation | | Oxygen Required? | Yes | No |

What we did and why: - Compared energy efficiency and metabolic outputs. - Highlighted NAD⁺ recycling as key difference.

Example 3 – Exam-Style: Disguised Question

Question: A plant cell is placed in an anaerobic environment. Which of the following will not occur? a) Glycolysis b) Krebs cycle c) Ethanol formation d) NAD⁺ regeneration

Solution:
1. Glycolysis occurs (anaerobic).
2. Krebs cycle stops (needs O₂ for ETC).
3. Ethanol forms (fermentation).
4. NAD⁺ regenerates (via fermentation). Answer: b) Krebs cycle.

What we did and why: - Identified the anaerobic block (Krebs/ETC require O₂). - Eliminated options systematically.

COMMON MISTAKES

EXAM TRAPS

1-MINUTE RECAP

"Listen up—this is your 60-second NEET rescue for plant respiration. Glycolysis: glucose → 2 pyruvate, 2 ATP, 2 NADH in the cytoplasm. Aerobic? Pyruvate → mitochondria → Krebs cycle (2 ATP, 8 NADH, 2 FADH₂ per glucose). ETC: NADH/FADH₂ → ATP (3 per NADH, 2 per FADH₂). Anaerobic? Fermentation: pyruvate → ethanol, no ATP, just NAD⁺ recycled. Total ATP: 36 in eukaryotes (2 lost in transport). Common traps: 38 ATP (wrong for plants), fermentation making ATP (it doesn’t), Krebs in cytoplasm (no—mitochondria!). Memorise the ATP counts, locations, and end products. You’ve got this—go ace that exam!