Glycolysis is a metabolic process that consists of the breakdown of glucose into two pyruvate molecules, with the release of energy in the form of ATP and NADH. Glycolysis has several important functions in the body, such as regulating the level of glucose in the blood. This process consists of two phases: the energy investment phase and the energy obtaining phase.
Glucose is an essential molecule for life, as it provides energy to the cells of all living beings. But how is that energy extracted from glucose? The answer is from glycolysis, a metabolic process that consists of 10 chemical reactions that transform glucose into pyruvate, a simpler and more versatile molecule.
You want to know more? In this article we are going to learn what glycolysis is, its functions and phases , how it is carried out and what importance it has for cellular metabolism.
What is glycolysis
Glycolysis is a metabolic pathway that occurs in the cells of all living beings and consists of the breakdown of glucose, a type of carbohydrate, to obtain chemical energy and other useful molecules for the body. It is one of the most studied metabolic pathways and best known for its central role in catabolic metabolism.
It is considered the first stage of cellular respiration and allows us to obtain two molecules of ATP (adenosine triphosphate), which are the main source of energy for cellular chemical reactions, and two molecules of NADH (nicotinamide adenine dinucleotide), which are electron transporters. which are used in other stages of cellular respiration.
As a result of glycolysis, two pyruvate molecules are obtained , which can follow different metabolic routes depending on the availability of oxygen and the type of cell. These routes are the Krebs cycle when oxygen is available, and fermentation when this gas is not available.
This set of chemical reactions that we will talk about in this article, takes place in the cell cytoplasm and does not require oxygen for its operation.
Functions of glycolysis
It is the main means of obtaining energy for cells that lack mitochondria, such as erythrocytes and some bacteria. Discover in this article the Mitochondria: what it is, function and structure .
It is the first stage of aerobic cellular respiration , which allows the complete or partial oxidation of pyruvate and the generation of more ATP and other compounds.
It also represents the starting point for fermentation pathways , which, although less efficient than cellular respiration, also generate chemical energy in the form of ATP.
It is a source of precursors for the synthesis of other biomolecules , such as amino acids, fatty acids and cholesterol.
It is a mechanism for regulating the level of glucose in the blood , since it is activated when there is an excess of glucose and is inhibited when there is a shortage or when other energy sources are available.
Phases of glycolysis
This metabolic process consists of ten reactions catalyzed by enzymes that can be divided into two phases: the energy investment phase, or energy expenditure, and the energy obtaining phase.
The energy investment phase involves the first five reactions of glycolysis, in which two molecules of ATP are invested to convert glucose into two molecules of glyceraldehyde-3-phosphate (G3P). Let’s see their five reactions:
- Glucose is phosphorylated by hexokinase, forming glucose-6-phosphate (G6P).
- G6P is isomerized by phosphoglucose isomerase, forming fructose-6-phosphate (F6P).
- F6P is phosphorylated by phosphofructokinase-1 (PFK-1), forming fructose-1,6-bisphosphate (F1,6BP). This is an irreversible reaction and regulates glycolysis.
- F1,6BP is cleaved by aldolase, forming two triose phosphates: dihydroxyacetone phosphate (DHAP) and G3P.
- DHAP is isomerized by triose phosphate isomerase, forming another G3P molecule.
Next, the energy production phase takes place , which includes the last five reactions of glycolysis, in which four molecules of ATP and two molecules of NADH are produced for each molecule of glucose. These reactions are:
- G3P is oxidized and phosphorylated by glyceraldehyde-3-phosphate dehydrogenase (GAPDH), forming 1,3-bisphosphoglycerate (1,3BPG) and reducing a molecule of NAD+ to NADH.
- 1,3BPG is dephosphorylated by phosphoglycerate kinase (PGK), forming 3-phosphoglycerate (3PG) and generating an ATP molecule by substitution at the substrate level.
- 3PG is isomerized by phosphoglycerate mutase (PGM), forming 2-phosphoglycerate (2PG).
- 2PG is dehydrated by enolase, forming phosphoenolpyruvate (PEP).
- PEP is dephosphorylated by pyruvate kinase (PK), forming pyruvate and generating another ATP molecule by substitution at the substrate level.
Each of these reactions is repeated twice for each molecule of glucose , resulting in a net balance of two molecules of ATP and two molecules of NADH for each molecule of glucose that enters glycolysis.
Additionally, in this video you will be able to learn the example of glycolysis in glucose catabolism.
Importance of glycolysis
The biological importance of glycolysis lies in the fact that it is the first step in energy production in most cells . ATP generated by glycolysis is used to carry out a wide variety of cellular processes, such as the synthesis of new molecules, cell division, muscle contraction, and active transport of molecules across the cell membrane. In addition, glycolysis also provides metabolic precursors for the synthesis of other important compounds, such as fatty acids and amino acids.
In some organisms, such as bacteria and yeast, glycolysis is the main source of energy. In other organisms, such as mammals, glycolysis is an important part of energy production , but is also complemented by other metabolic processes, such as cellular respiration, fermentation, and beta-oxidation of fatty acids.
Furthermore, it has been proven on numerous occasions that glycolysis is a very ancient and conserved metabolic pathway, which means that it originated billions of years ago and that it has remained with few changes in most living organisms.
This indicates the biological importance of glycolysis, since it represents an efficient and versatile way of obtaining energy from an abundant and accessible source.
Glycolysis also has other biological functions, such as providing the precursors for the synthesis of other organic compounds (such as amino acids, fatty acids or nucleotides) or regulating the cellular pH balance by producing lactic acid or ethanol under anaerobic conditions (without oxygen).