Cellular Respiration and Fermentation: Energy Production
The process involves the complete oxidation of acetyl-CoA, which is excreted as carbon dioxide. e-/ H + obtained in the successive oxidations are used to form molecules of reducing power and chemical energy in the form of GTP. This formation energy is known as substrate-level phosphorylation. In summary, the Krebs cycle occurs as follows: acetyl-CoA joins oxaloacetic acid to form citric acid, liberating CoA. Then, a series of reactions produce oxaloacetic acid again. This sequence includes the most important reactions: two decarboxylations and four dehydrogenations, 3 with NAD and the other with FAD, leading to free energy in the form of GTP. Electron Transport (Respiratory Chain) corresponds to the final stage of respiration, when the electrons are torn from the molecules that are breathed and stored on the NADH and FADH2 through a series of electron transporters, located in the mitochondrial cristae, forming three large enzyme complexes. The arrangement of conveyors allows electrons to hop from one to another, releasing a certain amount of energy used to form a high-energy bond between ADP and P, which results in a molecule of ATP. The final electron acceptor is molecular oxygen, and another consequence is the formation of water. In the respiratory chain, we can observe: “Of each NADH generated, 3 ATP are produced, and each FADH2 produces 2 ATP. Fermentation is anaerobic energy reactions, essential to regenerate the NAD consumed in glycolysis, from NADH2. Redox reactions occur, producing much less ATP than aerobic respiration. The final acceptor of electrons/protons is not oxygen, but a simple organic molecule. There are two types of fermentation: Lactic Fermentation, which occurs in many organisms and cells of higher organisms under anaerobic conditions to obtain energy from lactose in milk that was previously hydrolyzed to obtain glucose, this after glycolysis. Pyruvate is converted to lactate (Lactobacillus bulgaris…). This process is the basis of cheese and yogurt industries. Alcoholic Fermentation occurs in yeast and many other anaerobes. Pyruvate is decarboxylated and produces acetaldehyde, which is then reduced to ethanol. The last is the electron acceptor acetaldehyde.
1st Phase: Phosphorylation of Glucose and Its Conversion into Glyceraldehyde-3-Phosphate 1 – Glucose is phosphorylated to glucose-6-phosphate due to hydrolysis of one ATP molecule (kinase) 2 – Glucose-6-phosphate is isomerized to fructose-6-phosphate 3 – Fructose-6-phosphate is phosphorylated to fructose-1,6-diphosphate through another molecule of ATP 4 – Fructose-1,6-bisphosphate is broken into 2 molecules of glyceraldehyde-3-phosphate and dihydroxyacetone-phosphate, two 3-carbon molecules each (condensing and lyase) 5 – Dihydroxyacetone-phosphate is transformed into another molecule of glyceraldehyde-3-phosphate 2nd Phase: Oxidation of Glyceraldehyde-3-Phosphate to Pyruvic Acid with ATP and NADH Formation 6 – 1,3-Diphosphoglyceric acid is dephosphorylated into 3-phosphoglyceric acid and one molecule of ATP (oxidation and dehydrogenase) 7 – A group is transferred from phosphoric acid 3-carbon phosphoglyceride 2, by acid 2-phosphoglyceride (substrate phosphorylation and kinase) 8 – Formation of a double bond in 2-phosphoglyceric acid, obtaining phosphoenolpyruvic acid (PEP) and a water molecule (isomerization and isomerase) 9 – Dephosphorylation of the end product PEP, giving pyruvic acid and one molecule of ATP (condensing and lyase) Characteristic: The respiratory chain is not involved, the process does not use oxygen, it is an anaerobic process, the acceptor + is not an inorganic molecule but an organic molecule, ATP synthesis is provided through substrate and not through phosphorylation, ATP occurs by oxidative phosphorylation, energy efficiency is low, the reduced coenzyme formed are initially consumed, in the end these are processes of fermentation in yeasts and bacteria, although lactic acid fermentation occurs in muscle tissue when oxygen is not enough