- Glycolysis is series of enzyme catalyzed reaction occurring in living cells which converts Glucose into Pyruvate with the release of energy.
- It is the metabolic process that is a foundation for aerobic as well as anaerobic cellular respiration.
- It occurs in the cytoplasm of all
- First and Foremost pathway for breakdown of glucose.
End product, Pyruvate can be used through:
- Aerobic respiration via TCA cycle which yields much more usable energy (ATP)
- Anaerobic respiration in absence of oxygen i.e.
Phases of Glycolysis:
Glycolysis consists of two phases which are:
- Preparatory Phase: In this phase glucose molecule is converted to two molecules of three carbon sugars namely Glyceraldehyde-3-phosphate (G3P) and Di-hydroxy-acetone phosphate (DHAP). This phase is also called investment phase as 2 molecules of ATP are invested or spent during production of 3-carbon compounds.
- Pay-off Phase: In this phase 3- carbon compounds are converted to final end product, It is characterized by a net gain of the energy-rich molecules ATP and NADH. For a single molecule of glucose, each reaction of pay-off phase occurs twice as glucose leads to two triose sugars in the preparatory phase. A total of 4 ATP molecules and 1 NADH molecule are produced in pay off phase.
Functions of Glycolysis:
- Major function is the breakdown of glucose,
- Formation of NADH and ATP as source of energy.
- End product Pyruvate is the precursor for Citric Acid Cycle (TCA).
- Intermediate compounds during the process can be used in various cellular purposes.
Importance of Glycolysis:
- Major source of energy
- It is the only source of energy for cells lacking mitochondria such as Red Blood Cells (RBC).
- For certain anaerobic organisms [some bacteria and fermentation yeasts], glycolysis is the sole source of energy.
- It is essential for aerobic respiration, anaerobic respiration as well as a number of fermentations such as alcoholic fermentation, lactic acid fermentation, etc.
Stages of Glycolysis: The glycolytic pathway can be divided into three stages:
- Trapping of Glucose and destabilization
- Generation of two inter-convertible three-carbon molecules [ G3P and DHAP]
- Generation of ATP.
ATP production from Glycolysis
- Net ATP production: Two molecules of ATP are used during preparatory phase while 4 molecules of ATP are generated in pay-off phase. Thus,
Net ATP production = ATP generated- ATP used
=4 ATP- 2 ATP
= 2 ATP molecules
- Total ATP production: NADH molecule generated during glycolysis process on entering the Electron Transport Chain (ETC) produces 3 molecules of ATP. Thus,
Number of NADH generated = 2 NADH molecules
Number of ATP from ETC cycle = 2×3 ATP
= 6 ATP
Total ATP = Net ATP + ATP from ETC cycle
= 2 ATP + 6 ATP
= 8 ATP molecules
Interesting Fact: Glycolysis is somewhat similar to a reversal of photosynthesis.