Respiration in Plants

NCERT Line-by-Line Breakdown for NEET 2026

Unit 4: Plant Physiology

All living organisms need energy to carry out daily life activities. This energy comes from the oxidation of macromolecules (food). In this chapter, we explore how plants break down glucose through Glycolysis, the Krebs Cycle, and the Electron Transport System (ETS) to generate ATP.

1. Glycolysis (EMP Pathway)

Site: Cytoplasm of the cell.
Occurrence: Present in ALL living organisms (Aerobes and Anaerobes).
Process: Partial oxidation of Glucose (6C) into two molecules of Pyruvic Acid (3C).

[Image of glycolysis pathway steps]

💡 NEET Key Points:

ATP Consumed: 2 steps (Glucose → Glucose-6-P; Fructose-6-P → Fructose-1,6-BP).
ATP Produced: 4 molecules (Direct synthesis/Substrate Level Phosphorylation).
Net Gain: 2 ATP + 2 NADH + 2 Pyruvate.
Key Enzyme: Phosphofructokinase (PFK) – The Pacemaker enzyme.

2. Fermentation (Anaerobic Respiration)

Occurs when oxygen is absent. Incomplete oxidation of glucose.

🍻 Alcoholic Fermentation

Organism: Yeast.

Products: Ethanol + CO₂.

Enzymes: Pyruvate decarboxylase, Alcohol dehydrogenase.

💪 Lactic Acid Fermentation

Organism: Some bacteria, Animal muscles (during exercise).

Products: Lactic Acid (No CO₂).

Enzyme: Lactate dehydrogenase.

Note: Less than 7% of energy in glucose is released. Yeast poisons themselves when alcohol reaches 13%.

3. Aerobic Respiration

Complete oxidation of organic substances in the presence of oxygen. Takes place in Mitochondria.

A. Link Reaction (Gateway Step)

Pyruvate enters the mitochondrial matrix and undergoes Oxidative Decarboxylation.

Pyruvate + CoA + NAD⁺ → Acetyl CoA + CO₂ + NADH + H⁺
(Enzyme: Pyruvate Dehydrogenase Complex)

B. Tricarboxylic Acid Cycle (TCA / Krebs Cycle)

Site: Mitochondrial Matrix.
First Product: Citric Acid (6C).

[Image of krebs cycle steps]

Acetyl CoA (2C) condenses with OAA (4C) to form Citrate.
Decarboxylation: Occurs at two steps (Alpha-ketoglutarate formation & Succinyl-CoA formation).
Substrate Level Phosphorylation: Succinyl-CoA → Succinic Acid (GTP/ATP formed).
Output per Glucose (2 turns): 6 NADH + 2 FADH₂ + 2 ATP + 4 CO₂.

C. Electron Transport System (ETS)

Site: Inner Mitochondrial Membrane.
Goal: Release and utilize energy stored in NADH and FADH₂ to form ATP (Oxidative Phosphorylation).

[Image of electron transport chain mitochondria]

Complex	Name/Function
Complex I	NADH Dehydrogenase (Oxidizes NADH).
Complex II	Succinate Dehydrogenase (Oxidizes FADH₂).
Complex III	Cytochrome bc1 complex.
Complex IV	Cytochrome c oxidase (Contains Cu centers). Reduces O₂ to H₂O.
Complex V	ATP Synthase (F0-F1 particles).

Final Electron Acceptor: Oxygen.

4. The Energy Balance Sheet

How much ATP is produced from one Glucose molecule?

Glycolysis: 2 ATP (direct) + 2 NADH (6 ATP) = 8 ATP.
Link Reaction: 2 NADH (6 ATP).
Krebs Cycle: 2 ATP (direct) + 6 NADH (18 ATP) + 2 FADH₂ (4 ATP) = 24 ATP.
Total: 38 ATP (or 36 in some cells due to shuttle systems).

5. Amphibolic Pathway & RQ

Respiration is not just Catabolic (breaking down). Intermediates enter synthesis pathways (Anabolic). Hence, it is Amphibolic.

Acetyl CoA: Synthesis of Fatty acids and Gibberellins.
Succinyl CoA: Synthesis of Chlorophyll.
Alpha-ketoglutarate: Synthesis of Amino acids.

Respiratory Quotient (RQ)

Ratio of volume of CO₂ evolved to volume of O₂ consumed.

Carbohydrates: RQ = 1.0 Fats (Tripalmitin): RQ = 0.7 Proteins: RQ = 0.9 Organic Acids: RQ > 1

📝 Rapid Fire MCQs

Q1. Which of the following is the connecting link between glycolysis and Krebs cycle?

A) Citric acid
B) Acetyl CoA
C) Succinic acid
D) Pyruvic acid

Click to check Answer

Answer: B) Acetyl CoA.

Q2. Cytochromes are found in:

A) Cristae of mitochondria
B) Lysosomes
C) Matrix of mitochondria
D) Outer wall of mitochondria