The oxidation of glucose in the cell to release energy is known as cellular respiration and it occurs in the mitochondria of all living cells. There are two types of cellular respiration i.e. aerobic and anaerobic respiration



When cellular respiration takes place in the presence of oxygen is known as aerobic respiration. The largest amount of ATP possible is generated through it from one molecule of glucose with the release of carbon (iv) oxide and water as by product.

C6 H12 O6           +       6O2                                                        6CO2             +       6H2O     +      38ATP



In some organisms such as bacteria, fungi and endoparasites, the cells gets energy from breaking down glucose in the absence of oxygen this is known as anaerobic respiration. In this type of respiration, lesser amount of ATP is produced. The pyruvic acid produced is converted to alcohol in plants (alcoholic fermentation) while in animals, lactic acid is produced which leads to muscle fatigue in athletes


In plants

C6 H12 O6                                                           CH2COCOOH                                     C2H5OH   +   CO2   + 2ATP


In animals

C6 H12 O6                                 CH2COCOOH                           CH2 CHOHCOOH    +   CO2   + 2ATP



Cellular respiration involve two main stages

  1. Glycolysis: this occurs in the cytoplasm of the cells. First the glucose molecule is phosphorylated by the addition of a phosphate group to the glucose to become glucose-6-phosphate which is carried out by an enzyme known as hexose kinase. There are ten steps in this pathway that leads to the breakdown of One molecule of 6 – carbon into two molecules of the 3 carbon pyruvic acid by the enzymes in the cytoplasm. This process does not require oxygen. Glycolysis takes place in the cytoplasm. At the end of the pathway, 2 ATP is used to produce 4 ATP (2 ATPs from each pyruvate) so that a net energy of 2 ATP is generated in glycolysis, this is called “substrate level phosphorylation”.


  1. Kreb’s cycle (also known as citric acid cycle CAC or Tricarboxylic acid cycle TCA): Here, each pyruvic acid is further oxidized completely to carbon dioxide and water in the mitochondria. The pyruvic acid from the glycolysis is coverted to acetic acid through the removal of one molecule of CO2. The acetic is carried into the kreb’s cycle by co enzyme A. The combination of acetic acid and co enzyme A forms acetyl-coA. Acetyl-coA combines with oxaloacetatic acid to form citric acid which is an important started of the kreb’s cycle. Kreb’s cycle takes place in the matrix of the mitochondrion. Most of the ATP is generated in the cycle. The oxidation process in kreb’s cycle leads to the production of 36 ATPs (18 ATP from each pyruvate). The process used in the production of ATP in kreb’s cycle is called Oxidative phosphorylation. A total of 38 ATP is generated from the aerobic breakdown of glucose. The kreb’s cycle is particularly important because it is key pathway that connects protein, fats and carbohydrates.

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