Glycolysis

Most ancient metabolic pathway with the sequence of reactions which converts glucose to pyruvic acid with the production of ATP is termed as glycolysis.

It is an anaerobic process, i.e., it does not require oxygen and is common to both aerobic and anaerobic respiration.

Therefore, it is regarded as the fundamental step in respiratory breakdown of glucose.

It is the only process in respiration among anaerobes.

Glycolysis involves a series of ten reactions, which were elucidated by three scientists - Embden, Meyerhof and Parnas, thus also known as EMP pathway.

These reactions occur in the cytoplasm of cells and do not required oxygen.

The first half of this pathway activates glucose (glucose activation phase).

The second half extracts the energy (energy extraction phase).

Glycolysis yields only 5% of total ATP production and less than 7 % of total energy content of glucose.

Special features of glycolysis can be summarised as follows:

(1) Each molecule of glucose produces 2 molecules of pyruvic acid after partial oxidation, at the end of the glycolysis.

(2) The net gain of ATP in this process is 2 ATP molecules four ATPs are formed in glycolysis but two are already used up in the reaction).

(3) During the conversion of 1,3-diphosphoglyceraldehyde into 1,3-diphosphoglyceric acid, one molecule of NADH2 is formed. As each molecule of glucose yields two molecules of 1, 3-diphosphoglyceric acid, hence, each molecule of glucose forms 2 molecules of NADH2.

(4) During aerobic respiration (when oxygen is available) eachNADH2 forms 3ATP and H2O through electron transport system of mitochondria. In this process 1/2 O2 molecule is utilized for the synthesis of each water molecule.

All the reactions of EMP are reversible except for those catalysed by Hexokinase, Phosphofructokinase and Pyruvic kinase.

Nearly all enzymes require Mg+2 as cofactor.

Brain, retina and skin derive most of their energy from glycolysis.

It is the only source of energy in RBC.

Figure: Steps involved in glycolysis

Summary of glycolysis