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The glycolysis or glycolytic (from the Greek Glycos, sugar and lysis, rupture), is the metabolic pathway responsible for oxidizing the glucose in order to obtain energy for the cell. It consists of ten consecutive enzymatic reactions that convert glucose (six carbon atoms) into two pyruvate molecules (three carbon atoms each), capable of following other metabolic pathways and continuing to produce energy for the body.

Glycolysis occurs in the cytosol, does not need oxygen, and is made up of several chemical reactions.

The final energy yield of glycolysis is two ATP molecules, since two are consumed, but four are obtained. In addition, two NADH molecules are formed, with reducing power.

Phases of glycolysis

Glycolysis is divided into two main phases and ten enzymatic reactions:

1. Energy expenditure phase or “hexose phase” or “preparative phase”.

This first phase of glycolysis consists of five reactions and consists of transforming a glucose molecule into two glyceraldehyde 3-phosphate molecules. 

It is a degradative stage, no oxidation occurs. ATP is not obtained , but 2 ATP molecules are used for each glucose molecule.

2. Energy obtaining phase or “triose phase” or “oxidative” phase.

It consists of five reactions in which NAD is reduced, which is transformed into NADH + H+, forming 4 ATP molecules by transfer of phosphate groups to ADP. One molecule of pyruvate (ionized form of pyruvic acid ) is obtained for each one of glyceraldehyde, two for each glucose.

Glycolysis energy balance

Almost all organisms carry out glycolysis, from prokaryotes to multicellular beings. You need to use two molecules of ATP to start, but then you will get two molecules of NADH and four molecules of ATP. Therefore, the final energy balance is two molecules of NADH (which will generate more ATP in the event that a respiration process is followed later) and two molecules of ATP for each molecule of glucose.

Remember: The global equation for glycolysis is:

Glucose + 2NAD+  + 2ADP + 2Pi  → 2 pyruvic acid + 2 NADH + 2ATP + 2H+  + 2H2O

For each glucose molecule, the following has been obtained:

  • 2 molecules of pyruvic acid.
  • ATP molecules (4-2).
  • 2 molecules of NADH.

If glucose comes from glycogen, it comes in the form of glucose-6-phosphate, and one more ATP is produced, since glucose does not have to be phosphorylated.



Glucose + 2 ADP + 2 Pi + 2 NAD + → 2 pyruvate + 2 ATP + 2 NADH

Key stages of glycolysis

One of the most important stages of glycolysis is stage 6 (in some books it appears within stage 5), in which NADH is obtained. If the NADH is not re-oxidized, the route will stop. Depending on the availability of oxygen, it can be oxidized in two ways:

  • In aerobic conditions, molecules of NADH lose their electrons to the electron transport chain, that take up the oxygen (O2) to yield water and regenerating NAD+, which is reused in glycolysis. Afterwards, pyruvic acid enters the mitochondria and is transformed into acetyl groups, which will form acetyl coenzyme A (acetyl-CoA), which will intervene in cellular respiration.
  • In anaerobic conditions, in cells under anoxia (no O2, as in the muscle under anaerobic conditions), the NADH is oxidized to NAD+ by reducing the pyruvic acid. Obtaining energy under anaerobic conditions is called fermentation (the ultimate electron acceptor being an organic compound) and it takes place in the cytosol.

What to remember about glycolysis:

  • It is produced in the cytoplasm.
  • ATP is obtained by phosphorylation at the substrate level.
  • Its energy efficiency is low (2 ATP for each glucose molecule).
  • Generates reducing power (NADH ).
  • It supplies the cell with metabolic precursors.
  • It does not require the presence of oxygen.

Functions of glycolysis

The functions of glycolysis are:

  • The generation of high-energy molecules (ATP and NADH) as a source of cellular energy in processes of aerobic respiration (presence of oxygen) and fermentation (absence of oxygen).   
  • The generation of pyruvate that will pass to the Krebs cycle, as part of aerobic respiration.  
  • The production of 6 and 3 carbon intermediates that can be used in other cellular processes.

Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

Galicia, September 2018, option A, question 2.

Briefly develop the glycolysis process and indicate:

a) What compound does it start with and what does it end with?

b) where does it take place?

c) what is generated?

d) what is it for?

Murcia, June 2019, option A, question 3.

Answer the following points:

a) What metabolic pathway is the following? (0.25 points):

Glucose + 2 NAD+ + 2 ADP + 2 P 2 Pyruvate + 2 NADH + 2 H+ + 2 ATP + 2 H2O

b) In which cell compartment is it produced? (0.25 points).

c) Explain the possible metabolic fates that the pyruvate produced may have (1 point).

Castilla La Mancha, July 2020, block 2.4

Glycolysis is one of the metabolic processes that previously appeared on the planet.

a. Justify if it is an anabolic or catabolic process and aerobic or anaerobic.

b. Where is it located in the eukaryotic cell? What are your THREE end products?

c. Under anaerobic conditions, explain what metabolic pathway pyruvate can follow.

Basque Country, July 2020, extraordinary test, question 2B

Metabolic pathways: glycolysis and gluconeogenesis.

a) (1 point) Describe with the help of a drawing or diagram what glycolysis consists of. It indicates the molecules involved at the beginning and end of the pathway, where in the cell it is produced, and whether it is anabolic or catabolic.

b) (1 point) Describe with the help of a drawing or diagram what gluconeogenesis consists of. It indicates the molecules involved at the beginning and end of the pathway, where in the cell it is produced, and whether it is anabolic or catabolic.

c) (0.5 points) Can both routes occur simultaneously or are they incompatible? Give reasons for your answer.

Castilla y León, June 2021, question 3

3. In relation to glycolysis:

a) Is it an anabolic or catabolic process? Reason for the answer. (0,4)
b) What is the final product that is obtained and what is the balance of ATP and reducing power? (0.6)
c) In which compartment does it take place? (0.2)
d) Indicate the two destinations that the final product obtained in glycolysis can follow and the differences between them in terms of the need for oxygen and ATP production). (0.8)

Murcia, July 2021, question 2.4

Regarding glycolysis:

a) Explain reasonably if it is an anabolic or catabolic process (0.35 points).
b) Indicate which are the products of this metabolic process and their location at the cellular level (0.75 points).
c) Explain how ATP synthesis occurs in glycolysis (0.4 points).

Fundamental Ideas About Glycolysis


  • The glycolysis metabolic pathway responsible for oxidizing the glucose in order to obtain energy for the cell.  
  • From glucose (six carbon atoms), two pyruvate molecules (three carbon atoms each), two ATP molecules, and two NADH molecules are obtained, with reducing power.
  • Glucose + 2  ADP  + 2 Pi + 2 NAD+  → 2 pyruvate + 2 ATP + 2  NADH
  • Glycolysis occurs in the cytosol.
  • It does not need oxygen.
  • The pyruvate obtained can have two destinations:


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