August 27, 2016 | 0 Comments

Our body is constantly in need of energy to function. Even during resting state, a substantial amount of energy is needed for the fundamental functioning of the cells. Three fourth of this energy is derived from the carbohydrates which we consume in our food.

Glucose is the simplest form of sugar, to which the dietary carbohydrates gets broken down and the steps to production of energy begins with this molecule. Thus, Glucose is the major fuel to our body.

What is Glycolysis : Definition

Glycolysis is the first step in the process of energy production from glucose molecule. By definition, Glycolysis is the enzymatic degradation of glucose for the production of the energy providing molecule, the ATP (Adenosine triphosphate).

The process and where does glycolysis take place ?

As each and every cell in the body needs energy, glycolysis happens in all the cells and the location of glycolysis is the cytoplasm. Owing to its inevitability, glycolysis can occur both in the presence and in the absence of oxygen. Accordingly, the two types are known as aerobic glycolysis and anaerobic glycolysis respectively.

The anaerobic glycolysis is of importance in cells like Red Blood Cells which lack mitochondria that is essential for anaerobic reactions and in skeletal muscles where frequent spells of lack of oxygen is not uncommon

equation of aerobic glycolysis
Picture 1 : The equation of aerobic glycolysis
Image source :

The starting substance or the reactant in glycolysis is Glucose. It undergoes a series of steps to form the end product, the Pyruvate.

glycolysis steps
Picture 2: The process of Glycolysis
Image source :

The process of glycolysis involves a series of steps starting with glucose as the reactant. The first step is important and one of the three irreversible steps in the process. In the presence of the enzyme hexokinase, glucose takes up a phosphate group from the ATP and forms Glucose-6-Phosphate.

Apart from glycolysis, this compound is the gateway to many other important processes like glycogen and lipid synthesis.

In the next step, there is no new compound synthesis but just rearrangement of the atoms (isomerisation) to form fructose-6-phosphate from glucose-6-phosphate and this step is catalysed by enzyme phosphohexose isomerise. The next step is the second irreversible step in the process and the one which is more important in the regulation of glycolysis. Fructose 6 phosphate is phosphorylated by ATP to Fructose-1,6 bisphosphate in presence of phosphofructokinase 1.

The Fructose-1,6-bisphosphate  is a 6 carbon compound same as that of glucose. In this step, it is cleaved to two different compounds – Glyceradehyde-3-phosphate and dihydroxyacetone phosphate by the enzyme aldolase. Both are 3 carbon compounds which are interchangeable to each other.

From the next step, consider that 2 molecules of Glyceraldehyde-3-phosphate are undergoing the process simultaneously.

The Glyceraldehyde-3-phosphate (G3P) is phosphorylated to 1,3 bisphosphoglycerate (1,3 BPG) by inorganic phosphate by the enzyme glyceraldehyde-3-phosphate dehydrogenase. This reaction is NAD mediated. 1,3 BPG is converted to 3 phosphoglycerate by phosphoglycerate kinase enzyme and the phosphate removed from it is used to synthesis an ATP from ADP. Thus 2 ATP molecules are generated in this step considering that 2 molecules of G3P are undergoing glycolysis simultaneously

The 3-phosphoglycerate is converted to phosphoenolpyruvate (PEP) by enolase. The phosphate group is removed from PEP by which ADP is phosphorylated to ATP in presence of pyruvate kinase. The end product is Pyruvate. This is the last step in aerobic glycolysis which is irreversible and yields 2 ATP molecules

Glycolysis Steps (Enzymes and Pathway)

glycolysis steps

Glycolysis animation part 1

Glycolysis animation part 2:

Aerobic Vs Anaerobic Glycolysis : Differences

When there is sufficient amount of oxygen, the pyruvate gets converted to Acetyl Co A in the mitochondria and then enters the Citric Acid Cycle. In anaerobic conditions, the pyruvate gets converted to lactate in presence of lactate dehydrogenase.

The advantage of this step is that, NADPH gets oxidised to NAD which re-enters the glycolysis at the G3P dehydrogenase step and proceeds to generate ATP through the usual steps. Thus energy is produced even in the absence of oxygen.

Aerobic vs Anaerobic glycolysis

Picture 3: Aerobic vs Anaerobic glycolysis
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The anaerobic glycolysis takes place in skeletal muscles, brain, kidneys and liver when there is lack of oxygen while in RBCs, it takes place even in the presence of oxygen. Due to the absence of mitochondria which is inevitable for the steps following the production of pyruvate, glycolysis in RBCs is always anaerobic.

Regulation of Glycolysis

As mentioned above, the three irreversible steps which are catalysed by the enzymes Hexokinase, Phosphofructokinase and Pyruvate kinase are the regulatory steps of Glycolysis.

Regulating Steps of Glycolysis

Picture 4: Regulating Steps of Glycolysis
Image source

  • As for any chemical reaction, when the reactant (Glucose) is available in excess, there is an increase in the reaction rate. The enzymes of glycolysis get activated.
  • Also, increased amount of Glucose results in insulin secretion which in turn increases the glycolytic enzyme activities.
  • Glucagon and Epinerphrine sense the fall in blood glucose level and inhibit the glycolytic enzymes. They increase the cAMP level which inhibits the pyruvate kinase enzyme by phosphorylating and rendering it inactive. This mode of regulation is very fast.
  • 5’-AMP is an indirect indicator of the amount of ATP in the system. When the ATP gets used up, resulting in increased AMP formation, it is sensed as lack of energy in the system. Thus AMP activates Phosphofructokinase enzyme and causes instantaneous change in the rate of glycolysis.
  • ATP and citrate inhibits Phosphofructokinase enzyme while the inhibition by ATP is blocked by Fructose 2,6 bisphosphate which is formed from Phosphofructokinase II

ATP produced by Glycolysis

Catalysing Enzymes Mode of ATP production Number of ATP
Glyceraldehyde-3-phosphate dehydrogenase Respiratory chain oxidation of 2 NADH 6*
Phosphoglycerate kinase Substrate level phosphorylation 2
Pyruvate kinase Substrate level phosphorylation 2
Total 10

After allowing the 2 ATP molecules consumed in reactions catalyzed by hexokinase and phosphofructokinase enzyme, the net ATP production is 8

Substrate Level Phosphorylation

When the phosphate group is transferred from the substrate to the ADP to form ATP, it is called as substrate level phosphorylation. In glycolysis, the substrate level phosphorylation happens in 2 different steps


Picture  5: Substrate level phosphorylation
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glycolysis atp flow chart

Picture 6: In Glycolysis – ATPs produced in step 7 and 10 in the figure is  by substrate level phosphorylation
Image source :

Difference between Glycolysis and Kreb’s Cycle

Glycolysis is the first step in the process of energy production from a glucose molecule which ends with the production of 2 molecules of pyruvate which then gets converted to citric acid and enters the citric acid cycle also known as Kreb’s cycle for further production of energy.

The reactant is one molecule of glucose The reactant is pyruvate which gets converted to citric acid which then enters the cycle
The product is 2 molecule of pyruvic acid Pyruvate is oxidised to carbondioxide and water
It occurs in cytoplasm of a cell It occurs in the mitochondria of a cell
It can take place both aerobically and anaerobically It can take place only in the presence of oxygen
8 ATP molecules produced 24 ATP molecules produced
2 NADH molecules produced 6 NADH and 2 FADH2 molecules produced

Glycolysis Vs Kreb’s Cycle


How many NADH are produced by glycolysis ?

atps nadh glycolysis krebs


What are the products of glycolysis ?

end products of glycolysis

Published on August 27th, 2016 by under Diabetes Information.
Article was last reviewed on August 27th, 2016.

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