Summary

  • Plants unlike animals have no special systems for breathing or gaseous exchange.
  • Stomata and lenticels allow gaseous exchange by diffusion.
  • Almost all living cells in a plant have their surfaces exposed to air.
  • The breaking of C-C bonds of complex organic molecules by oxidation cells leading to the release of a lot of energy is called cellular respiration.
  • Glucose is the favoured substrate for respiration. Fats and proteins can also be broken down to yield energy.
  • The initial stage of cellular respiration takes place in the cytoplasm.
  • Each glucose molecule is broken through a series of enzyme catalysed reactions into two molecules of pyruvic acid. This process is called glycolysis. The fate of the pyruvate depends on the availability of oxygen and the organism.
  • Under anaerobic conditions either lactic acid fermentation or alcohol fermentation occurs.
  • Fermentation takes place under anerobic conditions in many prokaryotes, unicellular eukaryotes and in germinating seeds. In eukaryotic organisms aerobic respiration occurs in the presence of oxygen.
  • Pyruvic acid is transported into the mitochondria where it is converted into acetyl CoA with the release of CO2.
  • Acetyl CoA then enters the tricarboxylic acid pathway or Krebs’ cycle operating in the matrix of the mitochondria.
  • NADH + H+ and FADH2 are generated in the Krebs’cycle. The energy in these molecules as well as that in the NADH+ H+ synthesised during glycolysis are used to synthesise ATP.
  • This is accomplished through a system of electron carriers called electron transport system (ETS) located on the inner membrane of the mitochondria.
  • The electrons, as they move through the system, release enough energy that are trapped to synthesise ATP.
  • This is called oxidative phosphorylation. In this process O2 is the ultimate acceptor of electrons and it gets reduced to water.
  • The respiratory pathway is an amphibolic pathway as it involves both anabolism and catabolism.
  • The respiratory quotient depends upon the type of respiratory substance used during respiration.

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