Revision: Class 11 >> Respiration in Plants NEET (UG) Respiration in Plants

The breakdown of glucose in the presence of oxygen to produce carbon dioxide, water, and energy is called aerobic respiration.

The breakdown of glucose in the absence of oxygen to produce alcohol or lactic acid and a small amount of energy is called anaerobic respiration.

It is a process of release of energy from food substances such as glucose and fats under the control of enzymes, to carry out life processes, by the living organisms.

The process occurring in the cytoplasm where one glucose molecule is stepwise oxidized to form two molecules each of pyruvic acid, ATP, NADH₂, and water is called glycolysis.

The cyclic series of reactions occurring in the mitochondria, where acetyl-CoA is completely oxidized to produce CO₂, H₂O, NADH₂, and FADH₂, is called the tricarboxylic acid cycle or Krebs cycle.

The ratio of the volume of CO₂ evolved to the volume of O₂ consumed in respiration is called Respiratory Quotient (RQ) or respiratory ratio.

Respiratory quotient (RQ) is the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed in respiration over a period of time.

\[\mathrm{RQ=\frac{Volume~ofCO_{2}~evolved}{Volume~ofO_{2}~consumed}}\]

• Cellular respiration is the process where food (glucose) is oxidised inside the cell to release energy, stored as ATP via phosphorylation.
• Oxidation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 686 Kcal | Phosphorylation: ADP + iP + 7.3 Kcal → ATP
• First step occurs in cytoplasm — glucose breaks down into pyruvate (3-carbon molecule), releasing some energy.
• Without O₂ (Anaerobic) — Pyruvate → Ethanol + CO₂ (yeast) or Lactic acid (muscles). Less energy released.
• With O₂ (Aerobic) — Pyruvate breaks down in mitochondria into CO₂ + H₂O, releasing a large amount of energy as ATP.

• Glycolysis breaks one glucose into two pyruvic acid molecules in the cytoplasm.
• It has 10 steps: 5 preparatory (use ATP) and 5 pay‑off (produce ATP).
• There is a net gain of 8 ATP per glucose in the given scheme.
• With oxygen, pyruvic acid is fully oxidised to CO₂ and water.
• Without oxygen, pyruvic acid forms lactic acid or alcohol.
• Insulin promotes glycolysis by increasing glucose uptake.
• Glucagon and epinephrine help release glucose from glycogen, indirectly supporting glycolysis.

• It requires oxygen and completely breaks down substrates like glucose to CO₂ and H₂O.
• Pyruvic acid produced in glycolysis is transported into the mitochondrial matrix.
• There, pyruvic acid is converted to acetyl Co‑A by the pyruvate dehydrogenase complex in a reaction called the link reaction.
• In this reaction, pyruvic acid combines with coenzyme A and NAD⁺ to form acetyl Co‑A, CO₂ and NADH + H⁺.
• Acetyl Co‑A serves as the connecting link between glycolysis and the citric acid (Krebs) cycle, entering the cycle for further oxidation.

• It is a common oxidative pathway where acetyl Co‑A (from pyruvic acid via link reaction) is completely oxidised to CO₂.
• The cycle also supplies intermediates (e.g., α‑ketoglutarate, oxaloacetate) for synthesis of amino acids such as glutamate and aspartate.
• Per pyruvic acid, the cycle produces 3 CO₂, 4 NADH + 4H⁺, 1 FADH₂ and 1 ATP (or GTP) in the mitochondrial matrix.
• For each glucose (2 pyruvates), Krebs cycle output is 6 CO₂, 8 NADH + 8H⁺, 2 FADH₂ and 2 ATP molecules.
• Considering the whole respiratory pathway, glucose breakdown yields CO₂, 8 NADH + H⁺, 2 FADH₂ and 2 ATP at the Krebs‑cycle level.
• Because its intermediates are used both for breakdown (catabolism) and for biosynthesis (anabolism), the respiratory pathway is termed an amphibolic pathway.

• Respiratory quotient (RQ) is the ratio of volume of CO₂ evolved to volume of O₂ consumed during respiration.
• For carbohydrate respiration, RQ = 1.
• For fats, RQ is less than 1.
• For proteins, RQ is approximately 0.9.
• Respiration helps maintain the balance between CO₂ and O₂ in the atmosphere.
• Respiration provides energy required for synthesis of biomolecules and other cellular activities.