Respiration in Plants

1. Chapter at a glance

• All living cells oxidise respiratory substrates (mainly glucose) by breaking C–C bonds to release energy, which is trapped as ATP; this process is called respiration.
• Glycolysis (EMP pathway) occurs in the cytoplasm and converts one glucose into two molecules of pyruvic acid with a net gain of 2 ATP and 2 NADH + H⁺.
• Pyruvate has three possible fates: lactic acid fermentation, alcoholic fermentation (both anaerobic) or entry into mitochondria for complete oxidation.
• In aerobic respiration, pyruvate undergoes oxidative decarboxylation to acetyl CoA, which enters the TCA (Krebs’) cycle in the mitochondrial matrix, producing CO₂, NADH + H⁺ and FADH₂.
• Electrons from NADH + H⁺ and FADH₂ are passed through the electron transport system (ETS) on the inner mitochondrial membrane; O₂ is the final acceptor, and the energy released drives oxidative phosphorylation.
• The respiratory pathway is amphibolic: it serves both catabolic breakdown and anabolic synthesis of fats, proteins and carbohydrates.
• Respiratory quotient (RQ) = volume of CO₂ evolved / volume of O₂ consumed; its value indicates the nature of the respiratory substrate.

2. Definitions and laws (exact NCERT wording)

• Respiration: “The breaking of the C-C bonds of complex compounds through oxidation within the cells, leading to release of considerable amount of energy is called respiration.”
• Glycolysis: “This breakdown of glucose to pyruvic acid is called glycolysis.”
• Fermentation: “In fermentation, say by yeast, the incomplete oxidation of glucose is achieved under anaerobic conditions by sets of reactions where pyruvic acid is converted to CO₂ and ethanol.”
• Aerobic respiration: “Aerobic respiration is the process that leads to a complete oxidation of organic substances in the presence of oxygen, and releases CO₂, water and a large amount of energy present in the substrate.”
• Respiratory Quotient (RQ): “The ratio of the volume of CO₂ evolved to the volume of O₂ consumed in respiration is called the respiratory quotient (RQ) or respiratory ratio.”
  RQ = volume of CO₂ evolved / volume of O₂ consumed
• Amphibolic pathway: “Because the respiratory pathway is involved in both anabolism and catabolism, it would hence be better to consider the respiratory pathway as an amphibolic pathway rather than as a catabolic one.”

3. Important diagrams and activities

• Figure 12.1 – Steps of glycolysis: shows the ten enzyme-catalysed reactions from glucose to pyruvate with ATP and NADH + H⁺ balance.
• Figure 12.2 – Major pathways of anaerobic respiration: illustrates lactic acid fermentation and alcoholic fermentation from pyruvate.
• Figure 12.3 – The Citric acid cycle (TCA cycle): depicts the cyclic pathway from acetyl CoA to oxaloacetic acid with CO₂, NADH + H⁺, FADH₂ and GTP production.
• Figure 12.4 – Electron Transport System (ETS): shows complexes I–IV, ubiquinone, cytochrome c and ATP synthase on the inner mitochondrial membrane.
• Figure 12.5 – Diagrammatic presentation of ATP synthesis in mitochondria: illustrates F₀–F₁ complex and proton flow for oxidative phosphorylation.
• Figure 12.6 – Interrelationship among metabolic pathways: shows entry points of carbohydrates, fats and proteins into the respiratory pathway.

4. Common misconceptions and exam pitfalls

• Assuming a net gain of 38 ATP always occurs in living cells (text states this is only theoretical; actual yield is lower because intermediates are withdrawn and NADH from glycolysis needs shuttle).
• Believing plants do not respire or that only non-green parts respire (all living plant cells respire; green cells also respire continuously).
• Confusing fermentation with aerobic respiration: fermentation yields only 2 ATP and produces ethanol/lactic acid; aerobic respiration yields far more ATP and produces CO₂ + H₂O.
• Stating RQ = 1 for all substrates (RQ = 1 only for carbohydrates; <1 for fats, ≈0.9 for proteins).
• Writing that O₂ is used in Krebs’ cycle (O₂ accepts electrons only at the end of ETS).

5. Formula sheet