The word “GLYCOLYSIS” derives from the Greek language glykýs = sweet and lýsis = splitting, that is, anaerobic (in the absence of oxygen) degradation process of glucose.
Glycolysis (or the Embden-Meyerhof-Parnas pathway-from the names of Gustav Embden, Otto Meyerhof and Jakub Parnas, the three biochemists who contributed most to the explanation of its mechanism), is a central pathway of glucose metabolism for all cells, a source of metabolic energy for erythrocytes, renal medulla, brain, spermatozoa.
It is probably the oldest biological mechanism developed by the first living organisms to obtain energy from organic compounds. Many anaerobic microorganisms are totally dependent on glycolysis to obtain the energy necessary for their survival.
The sequence of reactions that characterizes it, is unique and highly conserved; it is one of the most studied and known metabolic pathways and a reference for many other cellular reactions.
THE DIGESTION OF CARBOHYDRATES
The digestion of carbohydrates begins in the mouth thanks to the ptyalin (enzyme) which demolishes the α-1-4 glycosidic bonds of starch. Hydrolysis is interrupted in the stomach and resumed in the small intestine by the pancreatic alpha amylase enzyme.
Digestion is then completed in the small intestine by enzymes that digest the disaccharides (maltose, lactose, trehalose, sucrose) by breaking them down into the various monosaccharides that pass into the cytoplasm of the intestinal mucosal cells.
The process and speed of absorption of monosaccharides is not the same for everyone. Glucose, for example, is rapidly absorbed through an active sodium-dependent transport process.
From the cells of the intestinal mucosa, glucose and other monosaccharides are transported to the blood and from there to the liver. The liver plays a fundamental role in the collection and sorting of glucose towards the various cellular districts.
THE PHASES OF GLYCOLYSIS
Glycolysis develops in two stages that occur in the cytoplasm of cells:
- the preparatory or energy investment phase
- The energy recovery phase.
In the PREPARATORY PHASE, which involves 5 steps (shown on the side) energy is consumed; energy is present in the cells as an ATP molecule (Adenosine triphosphate or ATP is an acid formed by a nitrogenous base, i.e. adenine, ribose, a pentose sugar, and from three phosphate groups) to obtain more energetic molecules from glucose (glyceraldehyde-3-phosphate).
Some of the enzymes involved in these reactions require the intervention of cofactors (the cofactor is a non-protein molecule or ion that is associated with the enzyme to enhance its activity and the specificity of the enzymatic reaction) such as ATP and magnesium.
At the end of the preparatory phase, one glucose molecule was transformed into two glyceraldehyde-3-phosphate (GAP) molecules and 2 ATP molecules were consumed to synthesize phosphorylated intermediates.
For each glucose molecule that enters the preparatory phase, in the energy recovery phase two molecules of G3P are converted into pyruvate, less energetic, producing more energy than that consumed in the first phase.
The process as a whole is therefore of a catabolic type, in which more complex and energetic molecules are transformed into simpler and less energetic ones, with accumulation of energy.
THE OVERALL ENERGY BALANCE OF GLYCOLYSIS INDICATES A NET GAIN OF ATP molecules.
The reserves that feed it are represented by sources of
- polysaccharides: glycogen (from reserves) starch (from food)
- disaccharides: maltose, lactose, trehalose and sucrose absorbed after hydrolysis
- monosaccharides: fructose, mannose and galactose transformed in liver cells to be channeled in the glycolytic pathway.