From the chemical point of view, glucose is a sugar with six carbon atoms and therefore falls into the category of hexoses.
Most of the complex sugars present in the diet are split and reduced to glucose and other simple carbohydrates.
Glucose, in fact, is obtained by hydrolysis of many carbohydrates, including sucrose, maltose, cellulose, starch and glycogen.
The liver is able to transform other simple sugars, such as fructose, into glucose.
Starting from glucose it is possible to synthesize all the carbohydrates necessary for the survival of the organism.
The level of glucose in the blood and tissues is precisely regulated by some hormones (insulin and glucagon); excess glucose is stored in some tissues, including muscle, in the form of glycogen.
In depth:
- glucose as food (dextrose)
- blood glucose (blood glucose)
- glucose in urine (glycosuria)
- GLUT glucose transporters
- Altered glucose tolerance
- OGTT Oral glucose loading test
- Alanine glucose cycle
- glucose syrup
Glycolysis
Important cellular metabolic pathway, responsible for the conversion of glucose into simpler molecules and energy production in the form of adenosine triphosphate (ATP).
Glycolysis is a chemical process in which a glucose molecule is split into two molecules of pyruvic acid; this reaction leads to the production of energy, stored in 2 molecules of ATP.
Glycolysis has the particularity of being able to take place both in the presence and absence of oxygen, even if, in the second case, a smaller quantity of energy is produced
- In aerobic conditions, the pyruvic acid molecules can enter the Krebs cycle and undergo a series of reactions that determine their complete degradation to carbon dioxide and water
- In anaerobic conditions, on the other hand, the pyruvic acid molecules are degraded into other organic compounds, such as lactic acid or acetic acid, through the fermentation process.
Phases of Glycolysis
The main events that characterize the glycolysis process are:
phosphorylation of glucose: two phosphate groups are added to the glucose molecule, supplied by two ATP molecules which in turn become ADP. Thus glucose 1, 6-diphosphate is formed;
transformation into fructose 1, 6-diphosphate : glucose 1, 6-diphosphate is transformed into fructose 1, 6-diphosphate, an intermediate compound with six carbon atoms, which in turn is split into two simpler compounds, each containing three carbon atoms: dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. The dihydroxyacetone phosphate is converted into another molecule of glyceraldehyde 3-phosphate;
pyruvic acid formation : the two compounds with three carbon atoms are both transformed into 1, 3-diphosphoglycerate acid; then in phosphoglycerate; then in phosphoenolpyruvate; finally, in two molecules of pyruvic acid.
In the course of these reactions four molecules of ATP and 2 of NADH are synthesized.
Balance of the situation
Glycolysis starting from a glucose molecule allows to obtain:
- the net production of 2 ATP molecules
- the formation of 2 molecules of a compound, NADH (nicotinamide adenine dinucleotide), which acts as an energy carrier.
Importance of glycolysis
In living beings, glycolysis is the first stage of the metabolic pathways of energy production; it allows the use of glucose and other simple sugars, such as fructose and galactose. In humans, some tissues that normally have an aerobic metabolism in particular conditions of oxygen deficiency have the ability to derive energy thanks to anaerobic glycolysis. This occurs, for example, in striated muscle tissue subjected to intense and prolonged physical effort. In this way the flexibility of the energy production system, which can follow different chemical pathways, allows the body to satisfy its own needs. However, not all fabrics are able to withstand the absence of oxygen; the heart muscle, for example, has a lower ability to perform glycolysis, therefore it is more difficult to withstand anaerobic conditions.
more about glycolysis »
Anaerobic glycolysis
In anaerobic conditions (lack of oxygen) the pyruvate is transformed into two molecules of lactic acid with the release of energy in the form of ATP.
This process, which produces 2 ATP molecules, cannot persist for more than 1 or 2 minutes because the accumulation of lactic acid produces the sensation of fatigue and hinders muscle contraction.
In the presence of oxygen the lactic acid that has formed is transformed into pyruvic acid which will then be metabolized thanks to the Krebs cycle.
Krebs cycle
Group of chemical reactions that take place inside the cell during the cellular respiration process. These reactions are responsible for transforming the molecules from glycolysis into carbon dioxide, water and energy. This process, favored by seven enzymes, is also called the cycle of tricarboxylic acids or citric acid. The Krebs cycle is active in all animals, in higher plants and in most bacteria. In eukaryotic cells the cycle takes place in a cellular organism called mitochondria. The discovery of this cycle is attributed to the British biochemist Hans Adolf Krebs, who in 1937 described the main steps.
MAIN REACTIONS
At the end of the glycolysis, two pyruvate molecules are formed, which enter the mitochondria and are transformed into acetyl groups. Each acetyl group, containing two carbon atoms, binds to a coenzyme, forming a compound called acetylcoenzyme A.
This, in turn, combines with a molecule with four carbon atoms, oxalacetate, to form a compound with six carbon atoms, citric acid. In the subsequent steps of the cycle, the citric acid molecule is gradually reworked, thus losing two carbon atoms that are eliminated in the form of carbon dioxide. In addition, in these passages four electrons are released which will be used for the last step of cellular respiration, oxidative phosphorylation.
in-depth study of the Krebs cycle »
Oxidative phosphorylation
The third phase of cellular respiration is called oxidative phosphorylation and occurs at the level of mitochondrial crests (folding of the inner membrane of mitochondria). It consists in the transfer of NADH hydrogen electrons to a transport chain (called the respiratory chain), formed by cytochromes, up to oxygen, which represents the final electron acceptor. The passage of electrons involves the release of energy which is stored in the bonds of 36 molecules of adenosine diphosphate (ADP) through the binding of phosphate groups and which leads to the synthesis of 36 molecules of ATP. From the reduction of oxygen and the H + ions that form after the electron transfer from NADH and FADH, water molecules are derived which are added to those produced with the Krebs cycle.
ATP synthesis mechanisms
The protons are passed through the inner mitochondria membrane in a facilitated diffusion process. The enzyme ATP synthase thus obtains sufficient energy to produce ATP molecules, transferring a phosphate group to the ADP.
The transfer of electrons through the respiratory chain requires the intervention of enzymes called dehydrogenases, which have the function of "tearing" the hydrogen from the donor molecules (FADH and NADH), so that H + ions and electrons are produced for the respiratory chain ; moreover, this process requires the presence of some vitamins (in particular, vitamin C, E, K and vitamin B2 or riboflavin).
Point of the situation:
- the demolition of glucose by aerobics (Krebs cycle) leads to the formation of 38 ATP
- the demolition of glucose by anaerobia (glycolysis) leads to the formation of 2 ATPs
