We explain what glycolysis is, its phases, functions and importance in metabolism. Also, what is gluconeogenesis.
What is glycolysis?
Glycolysis or glycolysis is a series of ten chemical reactions that make up the initial metabolic pathway for the breakdown of carbohydrates within cells.
Pyruvate is a key compound to be able to make much more ATP within the mitochondria. During glycolysis, glucose molecules (which have a skeleton made up of six carbon atoms) are broken down into pyruvate molecules (made up of three carbon atoms).
So glycolysis transforms each glucose into two pyruvates, through 10 consecutive oxidation reactions. The pyruvate molecule then enters the mitochondria, where it continues to release energy in another series of oxygen-dependent reactions. This last process is known as cellular respiration.
Glycolysis involves some expenditure of chemical energy by the cell, but when completed, there is a gain. ATP (adenosine triphosphate) is a chemical energy storage molecule. To obtain pyruvate from glucose, two ATP molecules need to be broken down, and at the end of the process four are obtained.
Oxygen is not a necessary element for glycolysis. However, it is for the process of cellular respiration, which begins after breaking glucose into two pyruvate molecules and which occurs within the cell's mitochondria.
The reaction rate of glycolysis is so high that it has always been difficult to study. It was formally discovered in 1940 by Otto Meyerhoff. This metabolic route is usually named after the surnames of the major contributors to its discovery: the Embden-Meyerhoff-Parnas route.
The word “glycolysis” comes from the Greek glycos“sugar”, and lysis“breakup.”
- See also: Metabolism
What is glucose?
Glucose is a type of simple carbohydrate. It is said to be a monosaccharide, to differentiate it from polysaccharides, that is, the more complex sugars, composed of many simple molecules linked together.
For example, starch is a polysaccharide made up of many glucose monosaccharides. When we consume starch (in potatoes, rice or flour), it is degraded in the digestive tract until all the monosaccharides that compose it are released. In this way, glucose is available to be absorbed by the intestine. It is then transported through the blood and incorporated by the different cells of our body.
Cells must break down the glucose molecule to obtain energy that they can use for other chemical reactions.
Phases of glycolysis
Glycolysis is a series of ten chemical reactions in which there is a net gain of energy in the form of two molecules of ATP. However, this process requires the cell to invest a certain amount of initial energy.
ATP, or adenosine triphosphate, is a simple molecule that contains high-energy bonds. When these bonds are broken, they release an amount of chemical energy that can be used in other reactions. Most of a cell's energy comes from ATP, which it uses to do different jobs (from making proteins to dividing through mitosis). ATP is considered a unit of biochemical energy.
The chemical reactions that make up glycolysis can be divided into two stages: one with energy expenditure and another with gain.
First stage: energy expenditure
In the first stage, glycolysis reactions involve expenditure of energy in the form of ATP. In this first phase, the glucose molecule goes through a series of reactions until two glucose molecules are obtained. glyceraldehyde-3-phosphate (a low energy yield molecule). Two units of biochemical energy (ATP) are consumed along this path.
During the process six-carbon sugars with phosphate groups are formed, which are unstable. Then they are divided and finally two similar molecules are obtained, with a skeleton of three carbons each.
Despite having the same structure, one of them is different, so it is additionally treated with enzymes (isomerases) to make it identical to the other, and thus obtain two identical compounds. All of this occurs in a five-step chain of reactions. In the next stage, the energy obtained thanks to this initial investment will be doubled.
Second stage: obtaining energy
In the second stage, glycolysis reactions involve obtaining energy in the form of four molecules of ATP and two molecules of pyruvate. In this second phase, each of the two molecules of glyceraldehyde-3-phosphate It goes through a series of reactions until it becomes pyruvate.
In this metabolic pathway, the three-carbon skeleton is preserved, but “phosphate groups” are lost. The two molecules of glyceraldehyde-3 phosphate- They go through a process in which their phosphate groups are transferred to ADP (adenosine diphosphate) molecules, to transform them into ATP.
A total of four ATP molecules are obtained.
Functions of glycolysis
The main function of glycolysis is to take the first step in the catabolism of glucose, in order to obtain chemical energy in the form of an ATP molecule.
Both ATP and pyruvate that arise as a result of glycolysis are compounds with high chemical energy, which can be used within the cell to carry out other processes. When pyruvate enters the mitochondria, it participates in another metabolic pathway, called cellular respiration, in which many more ATP molecules are made. Therefore, glycolysis serves as a supplier of pyruvate for the cellular respiration process of the mitochondria.
Since the energy obtained from glycolysis does not require the presence of oxygen, this process is especially relevant in conditions of anaerobiosis (absence of oxygen). The processes that occur within the mitochondria, however, do require it.
Thanks to ATP, numerous life forms obtain the energy to survive or to trigger much more complex chemical processes. Glycolysis exists in both prokaryotic and eukaryotic organisms. However, the subsequent metabolic pathway occurs differently.
Importance of glycolysis
Glycolysis is a fundamental process in the field of biochemistry. On the one hand, it has great evolutionary importance, since It is the basic reaction for increasingly complex life and for the support of cellular life. On the other hand, its study reveals details about the various existing metabolic pathways and other aspects of the life of our cells.
For example, recent studies at universities in Spain and the University Hospital of Salamanca detected links between neuronal survival in the brain and the increase in glycolysis to which neurons may be subjected. This could be key in understanding diseases such as Parkinson's disease or Alzheimer's disease.
Glycolysis and gluconeogenesis
While glycolysis is the metabolic pathway that breaks down the glucose molecule to obtain energy, gluconeogenesis It is a metabolic route that takes the opposite path: the construction of a glucose molecule from non-glucidic precursors that is, not linked at all to sugars.
This process is almost exclusive to the liver (90%) and kidneys (10%), and takes advantage of resources such as amino acids, lactate, pyruvate, glycerol and any carboxylic acid as a carbon source. In the absence of glucose, such as when we are fasting, the body remains stable and functioning for a reasonable period, as long as glycogen reserves in the liver last.
References
- De Robertis, E.Fundamentals of cellular and molecular biology. 4th Edition. El ateneo (2010).
- Marieb, E. Human anatomy and physiology. 9th edition. PEARSON EDUCATION. (2008).
