Cell Biology/Energy supply/Glycolysis
Glycolysis, literally meaning "to split sugar," is the initial step in any respiratory system. Glycolysis involves the breaking down of a sugar (generally glucose, although fructose and other sugars may be used) into more manageable compounds in order to produce energy. The net end products of glycolysis are two Pyruvate, two NADH, and two ATP (A special note on the "two" ATP later).
Glycolysis is a process that all organisms undergo; and therefore the most fundamental and primitive of all energy production systems.
The general reaction form for Glycolysis is as follows:
Glucose + 2 ATP ----> 2 Pyruvate + 2 NADH + 4 ATP
The 4 ATP produced at the end means that there is actually a net production of 2 ATP because 2 ATP are consumed during the beginning stages of glycolysis.
(Soon to Come)
Energy Investment Phase
Glycolysis occurs in the cytoplasm of a cell after glucose is ingested through the process of phagocytosis, or "cell eating," in which the cell engulfs a solid compound. Once the glucose is inside the cytoplasm, one molecule of ATP splits and transfers a phosphate group to the glucose, or "phosphorylates" it, so that it becomes an ion which cannot leave the cell because the cell membrane is impermeable to ions. This step effectively allows cells to efficiently absorb and utilize glucose and keep any glucose molecules from escaping. The process of phosphorylation also makes the glucose chemically reactive.
Once the glucose has been phosphorylated, it is called "Glucose-6-Phosphate," which will be abbreviated G6P. The G6P is rearranged into Fructose-6-Phosphate (F6P) by a protein. When the F6P is created, another ATP molecule splits and the F6P is phosphorylated by another phosphate group. This phosphate is attached to an opposite end of the F6P molecule which is now called Fructose-1, 6-Biphosphate. This is done so that the molecule will be not only more chemically reactive but so that it will split due to the tensions placed upon the molecule by the two phosphate groups. (Phosphate groups have a negative charge; this allows them to be added to create incredibly useful energy compounds, ATP among them. Because like charges repel each other, the two phosphate groups are placed in positions that allow the glucose molecule to split roughly in half, although an enzyme called aldolase is still used to perform the division.)
The Fructose-1, 6-Biphosphate, aided by aldolase, is split into Dihydroxyacetone Phosphate (DP) and Glyceraldehyde-3-Phosphate (63P), which are isomers of one another. The enzyme isomerase converts these molecules into one another. Because only 63P is used in the final stages of glycolysis, however, the reaction favors the conversion of DP into 63P. The overall effect of this reaction and the following steps is to send two molecules of 63P into the Energy Payoff phase of glycolysis.
Energy Payoff Phase
The 63P molecules then attach to an enzyme that removes two electrons from each 63P, therefore "oxidizing" the molecules. (Please note that the two 63P molecules do not attach to the enzyme at the same time; rather, two of the same enzyme would be used to carry out this reaction.) One electron from each 63P molecule is transferred to a hydrogen ion (H+; a hydrogen ion is also a proton) and one electron from each 63P molecule is transferred to NAD+, a cellular cofactor whose purpose is to carry electrons. The addition of electrons to the H+ and NAD+ combines the two into NADH. Because there are two 63P molecules, there are 2 NADH produced in this step. Since energy is released from the 63P molecules to allow the for the synthesis of the two NADH, some of the remaining energy attaches an inorganic phosphate group in the cell to each 63P molecule, forming 1, 3 Biphosphoglycerate (1,3B).
The two chemically reactive 1,3B molecules now lose the phosphate group that was just attached to them to an ADP, or Adenosine Diphosphate, molecule. The phosphorylation of ADP creates an ATP molecule; because there are two 1,3B molecules, two ATP are created. With this step, the initial ATP investment is repaid, and the 1,3B molecules become 3-Phosphoglycerate, or 3P. After this step is completed, another enzyme relocates the phosphate group on the 3P, changing the 3P into 2-Phosphoglycerate, or 2P.
After the 2P is created, an enzyme called enolase "hydrolyzes" the 2P molecules, which means that one water molecule is removed from each 2P molecule, which means that 2 H2O have been produced because there are two 2P molecules. In this particular case, the 2P is hydrolyzed so that the resulting Phosphoenolpyruvate (PEP) is very unstable and reactive.
The final step of glycolysis involves a protein called BS. Upon attaching to the Pyruvate Kinase proteins, the two PEP are divided into two Pyruvate and two additional phosphate group which are used to phosphorylate two ADP molecules into two ATP molecules.
In prokaryotes, this all but concludes the respiration sequence. There is one final process that is undergone, however; fermentation. The process of glycolysis coupled with the process of fermentation is known as anaerobic respiration, "anaerobic" literally meaning "without air." There are two main types of prokaryotic fermentation: alcohol fermentation and lactate fermentation.
In alcohol fermentation, the two pyruvate molecules lose two oxygen molecules and one carbon molecule are separated from the pyruvate and merge to become CO2 (Carbon Dioxide). The remaining molecule is called Acetaldehyde. The Acetaldehyde is "reduced" (or, gains electrons) by an NADH molecule. Because it is reduced, it increases in size. In this case, the NADH loses two electrons and two hydrogen ions, or protons, which returns it to its original NAD+ status. The two electrons and protons are then transferred to the Acetaldehyde, and it changes into Ethanol. Ethanol is the "alcoholic" ingredient in alcoholic beverages- in other words, a waste product of bacteria is one of the principal ingredients of beer, wine, and other drinks.
In lactate fermentation, no carbon dioxide is released and the pyruvate is instead reduced by the oxidation of NADH into lactate. In more quantitative terms, two protons and two electrons are removed from two NADH and are instead attached to two pyruvate molecules to create two lactate molecules. This lactate is also known as lactic acid, which happens to cause muscle pain and fatigue in animals.
One might wonder why the NADH would be reverted to its original NAD+ status, since energy was used to create it in the first place. The NADH is actually oxidized into NAD+ so that a "glycolytic cycle" can be preserved: if the NAD+ had not been replenished, the prokaryote would eventually run out of organic materials, and it would no longer synthesize ATP, which would mean its eventual death.
In eukaryotes, glycolysis is only the beginning of respiration. Instead of undergoing fermentation, he products of glycolysis are sent into the Krebs Cycle, explained in the next section.