What do you understand by respiration? Explain the entire process of glycolysis

What is Respiration

Respiration can be defined as the process by which living cells break down complex high-energy food molecules into simple low energy molecules, releasing the energy trapped within the chemical bonds.

Synthesis of energy-rich compounds like carbohydrates, fats, proteins etc., also need certain amount energy and thus, are endergonic (energy absorbing) whereas respiration is an exergonic (energy releasing) reaction.

In respiration, many types of high-energy compounds are oxidised.  These are called respiratory substrates and may include carbohydrates, fats, and proteins.  Of these, carbohydrates such as glucose, fructose (hexoses), sucrose etc., are the main substrates.  Hexoses are the first energy-rich compounds to be oxidised during respiration.

Types of Respiration:

Depending on the use of molecular oxygen in oxidation, respiration can be classified into two types:  aerobic and anaerobic respiration.

  1. Aerobic respiration: Most of the organism use molecular oxygen for oxidation of their foods.  It is known as aerobic respiration or oxyrespiration.
  2. Anaerobic respiration: When oxygen is not available for the oxidation of carbohydrates, then carbon dioxide and ethyl alcohol or lactic acid are formed.  This is known as anaerobic respiration or an oxyrespiration.  It results in incomplete oxidation of the substrate, hence, the energy released is much less than in aerobic respiration.



The mechanism of respiration involves following two processes.

  1. Glycolysis: The sequence of reactions which converts glucose into pyruvic acid with the production of ATP is termed as glycolysis. It is a series of reactions in which 6 carbon glucose molecules is converted into two 3 carbon molecules of pyruvic acid. In this process, 4 ATP molecules are produced.  Of these, two molecules of ATP are used up during degradation of glucose into pyruvic acid, hence, the net gain is 2 ATP molecules. All reactions of glycolysis occur in the cytoplasm.  Two molecules of NADH2 are also produced during glycolysis.
  2. Further break down of pyruvic acid molecules by aerobic or anaerobic methods. In the presence of oxygen, the pyruvic acid is completely oxidized .Carbon dioxide and water are formed as the end products.  This oxidation is carried out by a cyclic series of reactions known as tricarboxylic acid cycle or citric cyclic or Krebs cycle.  All reactions of Krebs cycle occur within mitochondria.

In the absence of oxygen, pyruvic acid is first converted into acetaldehyde and finally into ethyl alcohol.  Under certain conditions, pyruvic acid may be degraded into lactic acid.  These reactions take place in the cytoplasm.  Under anaerobic conditions, one molecule of glucose produces 2 ATP (from glycolysis).


Glycolysis involves a series of reactions given by three scientists, Embden–Meyerhoff–Parnas.  Hence, it is known as EMP pathway.  The process may be subdivided into the following steps.

Phosphorylation of sugar:

 In the first step of glycolysis, glucose, a stable 6 carbon sugar is phosphorylated first into fructose 6 phosphate and then to fructose 1, 6 biphosphate .  In the process, two ATP molecules are utilized.  The steps involved in phosphorylation are as follows:

  1. First, phosphorylation: A phosphate group from ATP is donated to the 6th carbon atom of the glucose molecule in the presence of enzyme, hexokinase to form glucose 6 phosphate.
  2. Isomerization: It involves isomerization of glucose 6 phosphate molecule to form a 6 carbon compound, fructose 6 phosphate in the presence of enzyme, phosphoglucoisomerase.
  3. Second phosphorylation: Fructose 6 phosphate, thus, formed is again phosphorylated by another ATP molecule to form fructose 1, 6 biphosphate .  Thus, the phosphorylation of glucose into fructose 1, 6 biphosphate requires two molecules of ATP.

Splitting of sugar:

 In this step, fructose 1, 6 biphosphate , a 6 carbon molecule splits into two 3 carbon molecules, each with phosphate group attached to one end.

  1. Production of phosphoglyceraldehyde: Fructose 1, 6 biphosphate splits into two 3 carbon molecules – phosphoglyceraldehyde (PGAL) and dihydroxyacetone phosphate (DHAP).  Of these two, only three phosphoglyceraldehyde is oxidised.  Dihydroxyacetone phosphate is enzymatically converted into phosphoglyceraldehyde.

Formation of pyruvic acid:

In this step, three phosphoglyceraldehyde molecules are further oxidised into pyruvic acid.  The following steps are as follows:

  1. Production of 1, 3 diphosphoglyceric acid: In the presence of enzyme, 3 phosphoglyceraldehyde dehydrogenase, 3 phosphoglyceraldehyde loses hydrogen to NAD (nicotinamide adenine dinucleotide) to form NADH2  and two molecules of 1, 3 diphosphoglyceric acid.
  2. Production of 3 phosphoglyceric acid: In this reaction, one high-energy phosphate group from each of the two diphosphoglyceric acid molecules are transferred to two ADP molecules, thus, forming two ATP molecules in the presence of enzyme, phosphoglycerokinase.  Thus, 1, 3 diphosphoglyceric acid is converted to 3 phosphoglyceric acid.
  3. Production of 2 phosphoglyceric acid: Two molecules of 3 phosphoglyceric acid gives rise to two molecules of 2 phosphoglyceric acid in the presence of enzyme, phosphoglyceromutase.
  4. Production of phosphoenolpyruvic acid: A molecule of water is removed from each molecule of 2 phosphoenolpyruvic acid leading to the formation of two molecules of phosphoenolpyruvic acid in the presence of enzyme, enolase.
  5. Production of pyruvic acid: In this last step of glycolysis, the remaining phosphate group of phosphoenolpyruvic acid is used to phosphorylate ADP to ATP.  As a result, phosphoenolpyruvic acid is converted into two molecules of a 3 carbon compound, pyruvic acid.




  1. Each molecule of glucose produces two molecules of pyruvic acid at the end of glycolysis.
  2. There is a net gain of 2 molecules of ATP in glycolysis..
  3. There is a net gain of two molecule of NADH2 .
  4. During aerobic respiration, each NADH2 forms 3 ATP and H2O through electron transport system of mitochondria. During aerobic respiration there is an additional gain of 6 ATP molecules in glycolysis, resulting in a total net gain of 8 ATP.
  5. Reactions of glycolysis do not require oxygen.
  6. No carbon dioxide is evolved in glycolysis.