Glycolysis: The Process Explained

What is the Glycolysis Pathway?

Firstly, any series of linked chemical reactions is known as a pathway. Glycolysis is just one example of a pathway. Each reaction forms a kind of link in a chain, and the reactions link up to cause a series of chemical changes to suit the organism in various ways.

Glycolysis is the first step in a process that goes on in all cells, the process of cellular respiration that releases energy from glucose to power all functions of the cell and every process that is needed for survival.

What does glycolysis do?

It converts one molecule of the raw material of all cellular respiration, glucose, into two molecules of a chemical called pyruvate. Pyruvate is known as an intermediate, because it is not needed for anything in itself, but is used in further pathways, in this case, the link reaction. Pyruvate can also branch into the pathway of anaerobic respiration. 

There are several steps in the pathway of glycolysis, each producing its own intermediates. Each step in the pathway is catalysed by a different enzyme.

The Glycolysis Pathway

The essential function of the glycolysis pathway is to split one glucose molecule (which contains 6 carbon atoms) into 2 pyruvate molecules (each pyruvate contains 3 carbon atoms). This also releases a small amount of energy, in the form of ATP, that can be used in the cell, for example for active transport or for synthesising other biological molecules. 

'Glyco-' refers to the glucose component, and '-lysis' means splitting; think of glycolysis as 'splitting glucose'.

Glycolysis Diagram: Overview of the aerobic glycolysis pathway

The Steps of Glycolysis

Firstly, glucose is phosphorylated (has a phosphate added to it) to form fructose bisphosphate. This step uses two ATP molecules, and is catalysed by its own enzyme. Glucose is very stable, but phosphorylating it allows it to become much more reactive so it is able to take part in further reactions in the pathway. 
An important additional feature is that glucose is trapped in the cell by having been phosphorylated; this stops glucose diffusing out of cells 'by accident' which would be a waste of potential fuel for that cell. 
The fructose bisphosphate molecule is then split into two triose (3-carbon) phosphates, each of which is phosphorylated once again. Further steps in the pathway are oxidation (using the coenzyme NAD, which is itself reduced to NADH) and dephosphorylation, which transfers phosphates to ADP to form ATP. 

Glycolysis FAQs

How much ATP is produced from glycolysis?

Two ATPs are used up in the first stage of glycolysis, whereas two ATPs are released in the final stage that forms pyruvate. However, because the final stage takes place twice, this means that four ATPs are in fact released. Therefore, there is a net release of twoATPs per glucose molecule.

Does glycolysis release energy or use energy up?

Well, in fact, both. The first stage uses twoATPs, whereas the last stage generates four ATPs. This means that glycolysis is regarded as an energy-releasing process because of the new release of two ATPs per molecule of glucose that is respired. 

Where does glycolysis occur?

Glycolysis occurs in every cell, in the cell cytoplasm, catalysed by enzymes that are either free-floating or bound to membranes on the outside of organelles like the mitochondria or endoplasmic reticulum. Glucose that enters the cell cytoplasm can immediately collide with the first enzyme of glycolysis, and the intermediates that are produced can also collide with their respective enzymes that allow the pathway to continue through to pyruvate. 
None of these reactions occurs inside the mitochondria; that is the location of the subsequent steps of aerobic respiration (Link Reaction, Krebs Cycle and Electron Transport Chain). 

What is anaerobic glycolysis?

Glycolysis is often thought of as the first major stage of aerobic respiration (with oxygen). However, in conditions where oxygen is not present, glycolysis can be likened to anaerobic respiration because it does generate some energy, released as ATP, albeit only two ATPs per glucose molecule (as compared to 32 ATPs released aerobically per glucose molecule). 
When there is no oxygen present, the pyruvate is converted (via different pathways) into lactate (in animals) or ethanol (in plants and yeast).

How is Anaerobic Glycolysis Different in Animals and Yeast?

In essence , it does not differ, because pyruvate is the end result. What differs between animals and yeast is how pyruvate reacts further in anaerobic conditions.

In animals, anaerobic respiration occurs at times of limited oxygen availability, for example during vigorous exercise, in order to supply some ATP to cells. Two ATPs being supplied is better than nothing, so cells switch into anaerobic respiration, and the process of glycolysis carries on as a short-term measure. This occurs until lactate builds up to dangerous, toxic levels, and cramp is the result. This causes the exercise to stop and for the oxygen debt to be repaid as lactate is broken down by oxidation.

In yeast, pyruvate is converted to ethanol and carbon dioxide; these are two desirable products in the production of beer, for example, and is why yeast fermentation is used to produce ethanol. The yeast cells grow in number because they are able to generate enough ATP through glycolysis, despite the fact that respiration is much less efficient in anaerobic conditions because only glycolysis can still take place. 

What is aerobic glycolysis?

Aerobic glycolysis is the process outlined above, leading to the further metabolism of pyruvate via and link reaction, Krebs Cycle and the full oxidation of glucose via the electron transport chain; this latter stage is also called oxidative phosphorylation. 

How does glucagon inhibit glycolysis?

The hormone glucagon acts in the opposite sense to insulin, acting to increase blood glucose levels such as by promoting glycogenolysis (breakdown of glycogen stores in the liver and muscles). Another way in which glucagon acts is to suppress glycolysis in certain conditions, allowing blood glucose levels to remain high. This effect is short term because the whole point of increasing blood glucose levels is to promote the supply of glucose to respiring cells, so glycolysis will have to occur at some point to begin the release of energy via respiration. 

Glycolysis Summary

Glycolysis is a biochemical pathway that is the first step in the process of aerobic respiration. Glycolysis takes place in the cell cytoplasm and releases two ATP molecules and as such forms the basis of anaerobic respiration in animal and yeast cells. The end product of glycolysis is pyruvate which in turn, goes on to a variety of different fates depending on the conditions in the cell.