→ 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 favored 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.

→ 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 anaerobic conditions in prokaryotes, unicellular eukaryotes, and germinating seeds.

→ In eukaryotic organisms in the presence of oxygen aerobic respiration occurs.

→ 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 FADH, are generated in the Krebs’ cycle.

→ The energy in these molecules as well as that in the NADH⁺+H⁺ synthesized during glycolysis are used to synthesis ATP. This is accomplished through a system of electron carriers called the electron transport system (ETS) located on the inner membrane of the mitochondria.

→ The electrons as they move through the system release enough energy that is trapped to synthesize ATP. This is called oxidative phosphorylation. In this process 02 is the ultimate acceptor of electrons and it gets reduced 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.

→ Respiration: The breaking of the C-C bonds of complex compounds through oxidation within the cells, leading to the release of a considerable amount of energy is called respiration.

→ Respiratory substrates: The compounds that are oxidized during this process are known as respiratory substrates.

→ Glycolysis: In any case, all living organisms retain the enzymatic machinery to partially oxidize glucose without the help of oxygen. This breakdown of glucose to pyruvic acid is called glycolysis

→ Aerobic respiration: Aerobic respiration is the process that leads to a complete oxid&tion of organic substances in the presence of oxygen, and releases CO₂, water, and a large amount of energy present in the substrate.

→ Electron transport system (ETS): The metabolic pathway through which the electron passes from one carrier to another, is called the electron transport system.

→ Oxidative phosphorylation: Unlike photophosphorylation where it is the light energy that is utilized for the production of proton gradient required for phosphorylation, in respiration it is the energy of oxidation-reduction utilized for the same process. It is for this reason that the process is called oxidative phosphorylation.

→ Respiratory quotient: During aerobic respiration. O₂ is consumed and CO₂ is released. 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.