Respiration in Plants: Significance of Krebs ′ Cycle and Acetyl CoA and Amphibolic Pathway (For CBSE, ICSE, IAS, NET, NRA 2022)

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  • It is the major pathway to release reduced coenzymes and energy, in a controlled manner.
  • Common pathway for oxidative breakdown of glucose, carbohydrates, fatty acids and amino acids. The fatty acids undergo oxidation to produce acetyl CoA and amino acids from proteins enter Krebs՚ Cycle after deamination of amino acids.
  • Krebs՚ Cycle provides series of intermediate compounds needed for the synthesis of other biomolecules like amino acids, nucleotides, chlorophylls and fats.

Amphibolic Pathway

An Amphibolic pathway is the metabolic pathway used in both catabolism and metabolism. These pathways are central metabolic pathways which furnish, from catabolic sequences, the intermediates, which form the substrates of metabolic processes. An important example of an amphibolic pathway is the Krebs՚ Cycle, which involves both the catabolism of carbohydrates and fatty acids and the synthesis of anabolic precursors for amino acid synthesis.

The intensity of light at which uptake of CO2 for photosynthesis is equal to the release of CO2 by respiration is called compensation point. At compensation point the rate of photosynthesis and respiration balance each other and there is no exchange of oxygen or carbon dioxide. At this point there is neither weight gain nor weight loss. As the light intensity increases beyond compensation point, the rate of photosynthesis increases, and photosynthesis products start accumulating in the cell cytoplasm leading to plant growth.

Rate of Respiration and Factors Affecting It

  • The rate of respiration can be measured by the amount of CO2 released per unit time. The rate of respiration can be measured in terms of gas exchange that is consumption of the respiratory substrate oxygen or evolution of carbon dioxide. During aerobic respiration, oxygen is consumed, and carbon dioxide is released. The respiratory quotient (RQ) is the ratio of CO2 produced to that of the O2 consumed while food is being metabolized.

Affecting Factors of Respiration May Either be External or Internal

External factors

  • Temperature: Below and above the rate of respiration reduces. The optimum temperature for respiration is .
  • Oxygen: It the amount of oxygen is reduced the rate of respiration will reduce.
  • Water: Optimum supply of water maintains the rate of respiration normal. Rate of respiration reduces for scarcity of water.
  • Light: The stomata open in light, so rate of respiration comparatively high in daytime.
  • Carbon dioxide: with the increases of CO2 in air, rate of respiration decreases slightly.

Internal factors

  • Food materials: During respiration food materials break to release water and CO2. So, the quality and quantity of food materials in the cell regulates the rate of respiration.
  • Age of cell: rate of respiration is maximum in young cells especially in meristematic cells.
  • Enzyme: Different reactions of respiration need different kinds of enzymes. The process completely depends on their presence.


  • The respiration that occurs in green cells in the presence of light and result in excess evolution of carbon dioxide is termed as photorespiration. Photorespiration process accomplished in three different cell organelles: chloroplasts, peroxisomes and mitochondria.
  • During dark reaction of photosynthesis, the enzyme RUBISCO catalyzes the carboxylation of RUBP.

  • Oxygen competes with CO2 for the active site of RuBP carboxylase (Rubisco) and the result is only one molecule of Phosphoglyceric acid (PGA) .

  • This occurs because RUBISCO enzyme has the same active site for both CO2 and O2.
  • Oxygenation of RUBP in presence of oxygen leads to a loss of about carbon fixed by plants during the dark reaction.
  • Use: Protects the plants from photo oxidative damage by utilizing part of the solar energy which would otherwise damage the plant pigments.

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