Board exam focus — Respiration in Plants (CBSE & HBSE)

This chapter explains how plants break down food (glucose) to release energy in the form of ATP. It covers cellular respiration, glycolysis, fermentation (anaerobic respiration), aerobic respiration (the Krebs cycle, electron transport system and oxidative phosphorylation), the respiratory balance sheet of ATP, the amphibolic nature of respiratory pathways, and the respiratory quotient (RQ). It is a core chapter for NEET and board exams, with numerical questions on ATP yield and RQ.

Glycolysis and Fermentation

Cellular Respiration

The breakdown of C-C bonds of complex compounds (glucose) through oxidation within the cell, leading to release of energy (ATP) is called cellular respiration. The compounds oxidised are called respiratory substrates (usually carbohydrates, sometimes fats and proteins).

Glycolysis (EMP Pathway)

• Occurs in the cytoplasm; common to both aerobic and anaerobic respiration.
• Scheme given by Embden, Meyerhof and Parnas (EMP pathway).
• One molecule of glucose (6-C) → 2 molecules of pyruvate (3-C).

Net products of glycolysis (per glucose):

Key step: Glucose is first phosphorylated to glucose-6-phosphate (uses ATP). Two ATP are invested in the early steps; four are produced later → net gain of 2 ATP.

Fate of Pyruvate

Depends on oxygen availability:

Fermentation (Anaerobic Respiration)

Incomplete oxidation of glucose without oxygen.

1. Alcoholic fermentation (yeast):

Pyruvate --(pyruvate decarboxylase, alcohol dehydrogenase)--> Ethanol + CO2

2. Lactic acid fermentation (muscle cells, some bacteria):

Pyruvate --(lactate dehydrogenase)--> Lactic acid

NEET Trap: Fermentation yields less than 7% of the energy in glucose and only 2 net ATP (from glycolysis). Both alcohol and lactic acid are harmful; alcohol above ~13% kills yeast.

Aerobic Respiration — Krebs Cycle and ETS

Aerobic Respiration

Occurs in the mitochondria. Pyruvate enters the mitochondrial matrix and is completely oxidised. It has two major steps: the Krebs cycle (TCA cycle) and the electron transport system (ETS).

Link Reaction (Pyruvate → Acetyl CoA)

In the mitochondrial matrix, pyruvate is oxidatively decarboxylated by pyruvate dehydrogenase:

Pyruvate + CoA + NAD+ --> Acetyl CoA + CO2 + NADH + H+

(per glucose: 2 pyruvate → 2 acetyl CoA, 2 CO2, 2 NADH)

Krebs Cycle (TCA / Citric Acid Cycle)

Discovered by Hans Krebs; occurs in the mitochondrial matrix. Acetyl CoA (2-C) combines with OAA (4-C) to form citric acid (6-C).

Per turn of the cycle (one acetyl CoA):

(per glucose = 2 turns, so double the above)

Electron Transport System (ETS) and Oxidative Phosphorylation

Located in the inner mitochondrial membrane. NADH and FADH2 are oxidised; electrons pass through complexes (I–IV), and O2 is the final electron acceptor, forming water.

• Complex I — NADH dehydrogenase
• Complex II — Succinate dehydrogenase (FADH2 enters here)
• Complex III — Cytochrome bc1
• Complex IV — Cytochrome c oxidase (O2 → H2O)
• Complex V — ATP synthase

Energy yield by chemiosmosis:

• 1 NADH → 3 ATP
• 1 FADH2 → 2 ATP

NEET Trap: Oxidative phosphorylation = ATP synthesis driven by the proton gradient as electrons flow to O2. The role of O2 is only at the very end (terminal electron acceptor); it is indispensable because it removes hydrogen, keeping the chain running.

ATP Balance Sheet, Amphibolic Pathway and RQ

Respiratory Balance Sheet

The net ATP yield from the complete aerobic oxidation of one glucose molecule is calculated below (assumptions: sequential, orderly pathway; NADH/FADH2 immediately oxidised; substrate used is only glucose).

Net total = 38 ATP per glucose (theoretical maximum).

Note: In many textbooks the net yield is shown as 36 ATP (eukaryotes) because the 2 NADH from glycolysis spend ATP in being shuttled into the mitochondria. NCERT gives net 38 ATP under ideal assumptions.

Amphibolic Pathway

The respiratory pathway is both catabolic and anabolic, hence called amphibolic.

• Catabolism: breakdown of glucose, fats, proteins for energy.
• Anabolism: intermediates of respiration (e.g. acetyl CoA, OAA, α-ketoglutarate) are withdrawn to synthesise fatty acids, amino acids, etc.

Example: Acetyl CoA is a precursor for fatty acid synthesis; α-ketoglutarate and OAA are used for amino acid synthesis. So respiratory intermediates serve as building blocks.

Respiratory Quotient (RQ)

The ratio of the volume of CO2 evolved to the volume of O2 consumed in respiration:

RQ = Volume of CO2 evolved / Volume of O2 consumed

NEET Trap: For glucose, RQ = 6 CO2 / 6 O2 = 1.0. Fats give RQ < 1 because they require more oxygen for complete oxidation (more H atoms). In pure fermentation (anaerobic), no O2 is used, so RQ is infinity (∞).

Frequently asked questions

Are these Respiration in Plants notes free?

Yes — the Respiration in Plants notes for Biology (Class 11) on Siksha Sarovar are completely free to read, with no account required.

Do these notes follow CBSE and HBSE?

Yes. The Respiration in Plants notes are NCERT-aligned and include guidance for both CBSE and Haryana Board (HBSE), with important questions and MCQs for revision.

What does the Respiration in Plants chapter cover?

Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Respiration in Plants.