Tue. Sep 17th, 2019

Glycolysis & Net ATP

2 min read
Glycolysis is breakdown of glucose into two pyruvate molecules with the capture of energy. Glycolysis has two phases: Preparatory and Pay-off. It is sole source of energy for cells lacking mitochondria.

Glycolysis Cycle

Glycolysis [Gr. Glyco= Sugar , Lysis=Breakdown]

  • 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:

  1. Aerobic respiration via TCA cycle which yields much more usable energy (ATP)
  2. Anaerobic respiration in absence of oxygen i.e.

Glycolysis is oxidative breakdown of glucose into two pyruvate molecules with the capture of energy in the form of ATP and NADH. At least six enzymes operate in the metabolic pathway.


Phases of Glycolysis:

Glycolysis consists of two phases which are:

  1. 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.


  1. 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:

  1. Major function is the breakdown of glucose,
  2. Formation of NADH and ATP as source of energy.
  3. End product Pyruvate is the precursor for Citric Acid Cycle (TCA).
  4. Intermediate compounds during the process can be used in various cellular purposes.


Importance of Glycolysis:

  1. Major source of energy
  2. It is the only source of energy for cells lacking mitochondria such as Red Blood Cells (RBC).
  3. For certain anaerobic organisms [some bacteria and fermentation yeasts], glycolysis is the sole source of energy.
  4. 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:

  1. Trapping of Glucose and destabilization
  2. Generation of two inter-convertible three-carbon molecules [ G3P and DHAP]
  3. Generation of ATP.


ATP production from Glycolysis

  1. 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


  1. 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.

Copyright © All rights reserved @microbionotes.com. | Newsphere by AF themes.