Plants do not have any special systems for breathing or gaseous exchange. Stomata and lenticels allow gaseous exchange by diffusion.
The breaking of C-C bonds of complex organic molecules by oxidation cells leading to the release 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 catalyzed reactions into two molecules of pyruvic acid. This process is called glycolysis.
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 Kreb’s cycle operating in the matrix of the mitochondria.
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.