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Sunday, February 6, 2011
Sunday, February 6, 2011 - 0 Comments
Fermentation is either an evolutionary bypass that some organisms use to keep glycolysis functioning under anaerobic conditions or is a biological remnant that involved very easily in the history of life, when the earth’s atmosphere contained little or no oxygen. As with glycolysis, the presence of fermentation is strong evidence for common descent of organisms from primitive cells in which glycolysis and fermentation first appeared and still persists.
In fermentation hydrogen atoms that glycolysis generates are donated to organic molecules and then reduced compound can be an organic acid like lactic acid, acetic acid, propionic acid or an alcohol in the form of ethanol, butanol. Fermentation regenerates NAD, which is needed to drive glycolysis to ultimately obtain ATP. During fermentation glucose is not completely degraded, so considerable unusable energy still remains in the products. Beyond two ATP molecules formed during glycolysis, no more ATP is produced. Fermentation serves only to regenerate NAD (oxidized from of NADH).
Types of Fermentation:
There are two types of fermentation depending upon the end product obtained. The fermentation in which the end product is an alcohol is known as alcoholic fermentation where as the fermentation in which some acid specially lactic acid is formed is known as Lactic acid fermentation.
Alcoholic fermentation: The pathway from private to ethanol is called alcoholic fermentation and is catalysed by specific microbial enzymes.
Lactic acid fermentation: The pathway in which lactate or lactic acid is produced as end product from private is known as lactic acid fermentation.
There are certain animals cells that are deprived of oxygen, temporarily carry out lactic acid fermentation.
Types of Fermenting Organisms:
Two types of organisms can carry out fermentation, obligative or obligate anaerobic and Facultative anaerobic.
Obligative anaerobic organism:
The organisms that survive only in complete absence of molecular oxygen are termed as obligative or obligate anaerobic organisms. These include certain types of bacteria.
Facultative anaerobic organisms: The organisms that survive only in the absence of molecular oxygen are termed as facultative anaerobic organisms. They are certain bacteria, yeasts, animal muscle cells which can ferment nutrients when oxygen is absent to generate some ATP by providing NAD for glycolysis. Such organisms and tissues carry out more efficient energy harvesting when oxygen is present.
Aerobic respiration: The major source of ATP:
Anaerobic generation of ATP through glycolysis and fermentation is inefficient. The end product of glycolysis (pyruvate) still contains great deal of potential bond energy that can be harvested by further oxidation. Evolution of aerobic respiration in micro-organisms and in the mitochondria of eukaryotic cells became possible only after free oxygen had accumulated in the earth’s atmosphere as a result of photosynthesis. Addition of oxygen requiring stage to energy harvesting mechanisms provided cells with more powerful and efficient way f extracting energy from nutrient molecules. Indeed without mitochondria’s large scale ATP production life would have to be at a “snail’s space” and most animals present on earth today would never have evolved.
In aerobic respiration pyruvate that glycolysis produces is shunted into a metabolic pathway called Kreb’s cycle or citric acid cycle; NADH goes to electron transport chain. During this aerobic metabolism free oxygen accepts electrons and reduces to water together with production of 34 molecules of ATP from each molecule of pyruvate consumed.
Aerobic respiration is organized into Kreb’s cycle and electron transport chain. Two electron carriers’ mitotinamide adenine di-nucleotide (NAD) and falvin adenine di-nucleotide (FAD) act as hydrogen acceptors and reduce to NADH and FADH2.
Most of the remaining energy is in the form of NADH and FADH2. These two molecules are shuttled into electron transport chain. In this chain reduced NADH and FADH2 are oxidized and their electrons are passed along a series of oxidation reduction reaction to the final acceptor oxygen. During this phase of the cycle, three molecules of Co2 are generated from each pyruvate molecule and some energy is harvested in the form of ATP.
It is the initial sequence of catabolic chemical reactions in which six carbon glucose molecules is broken down into two molecules of three carbon compound called pyruvate of pyruvic acid with net production of two molecules of ATP when glucose is completely burned in a test tube it will give about 690,000 calories of energy per mole in the form of heat. In the cell some of this energy is not lost as heat but is retained in the form of ATP.
Steps involved in the process of glycolysis:
(1) Glucose is converted to glucose – 6 – phosphate with the help of an enzyme hexokinase in the presence of ATP.
(2) Glucose – 6 – phosphate is rearranged to form its isomes fructose – 6 – phosphate with the help of an enzyme phosphogluco isomerase.
(3) Fructose – 6 – phosphate reacts with another molecule of ATP to form fructose – 1, 6 – diphosphate or hexose diphosphate with the help of an enzyme phosphor fructokinase.
(4) Fructose – 1, 6 diphosphate is then either converted into 3 – phosphoglyceral dehyde by dihydroxy acetone phosphate under the enzyme aldolase. There is established equilibrium between these compound by inter converting into one another through an enzyme isomerase – 3 – phosphoglyceral dehyde is utilized at a faster rate and when there is deficiency of this compound, then dihydroxy acetone phosphate is converted into 3 – phosphoglyceral dehyde which is processed further by glycolysis.
(5) 3 – phosphoglyceral dehyde is initially oxidized by NAD and then the inorganic phosphate present in the cytoplasm combines to form 1, 3 – phosphoglyceric acid in the presence of enzyme triose phosphate dehydrogenase.
(6) 1, 3 – phosphoglyceric acid is then converted to 3 – phosphoglyceric acid or 3 – phosphoglycerate along with the release two ATP molecules by the enzyme phosphoglycerokinase.
(7) 3 – phosphoglyceric acid or 3 – phosphoglycerate is then converted into 2 – phosphoglyceric acid or 2 – phosphoglycerate in the presence of phosphoglyceromutase.
(8) 2 – phosphoglyceric acid or 2 – phosphoglyceratase is converted into phosphoenol pyruvic acid or phosphoenopyruvate in the presence of enolase.
(9) Finally phosphoenol pyruvic acid or phosphoenylpyruvate is converted into pyruvic acid or pyruvate along with the production of two more ATP molecules by the enzyme pyruvate kinase.
Energy yielding steps are:
(1) During process of glycosis two ATP molecules were used as starter energy to convert Glycosets fructose 1 – 6 biphosphate.
(2) Two ATP molecules are formed during the conversion of 1, 3 – diphosphoglycerate to 3 – phosphoglycerate.
(3) Two additional ATP molecules are released when phosphenolpurate is converted into pyrute the end – product of glycolysis.
(4) In addition to four ATP molecules produced during glycolysis, energy rich compounds NADH2 is also produced which is used to make the ATP by oxidative phosphorylation.
End result of Glycolysis: All the reactions of glycolysis are performed by soluble enzymes that are present in the cytosol and are not found in mitochondrion. Two moles of ATP are needed to start of glycolysis of glucose and during the oxidative reaction of glycolysis these two moles of ATP are regained. The NADH2 formed in above reactions is oxidized via the electron transport chain to form three more ATP molecules. The end result of glycolysis is the formation of two moles of pyruvic acid from each mole of glucose together with two moles of ATP. Although glycolysis does not efficiently harvest all the available energy from glucose, it was the only way most organisms could harvest energy and generate ATP molecules for hundreds of millions of years during the anaerobic stages of early life on earth.
Evolutionary Perspectives on Glycolysis:
All forms of animal life including man carry on glycolysis within their cells, a metabolic memory of animals’ evolutionary past – if glycolysis is such an inefficient method of harvesting energy, why has it persisted?
One reason might be that evolution is slow, incremental process involving change based on past events when glycolysis first evolved the cells possessing it had competitive advantage over these that did not.
Importance of Glycolysis as observed through biochemistry:
Biochemistry of contemporary organisms indicates that only those organisms capable of glycolysis survived the early competition of life on earth. Later on the evolutionary changes in catabolism build on this success. During this building process, glycolysis was not discarded but used as a stepping stone for the evolution of another process for complete breakdown of glucose.
Within their cells, animals make ATP to carry out normal activities of the body. The process of formation of ATP is known as phosphorylation. There are number of processes through which ATP can be obtained. Two of these processes are common namely substrate level phosphorylation and chemiosmossis.
(1) Substrate level phosphorylation:
The generation of ATP by coupling strongly exergonic reaction with ATP synthesis from ADP and phosphate is called substrate level phosphorylation. It appeared very early in the history of organisms because organisms initial use of carbohydrates as an energy source is accomplished by substrate level phosphorylation. The mechanism for substrate level phosphorylation is present in most living animal cells. Substrate level phosphorylation is one of the most fundamental of all ATP generating reactions. ATP formation from ADP and phosphate requires the input of energy of 7.3 K. Cal:
There is still other method of generating ATP that is more efficient and effective and is called chemiosmosis and it takes place in the mitochondrion. In the mitochondrion trans membrane channels are present in the mitochondrial membranes that can pump protons. These proton pumps use flow of electrons to induce a shape change in the protein, which in turn, causes protons to move out of the inner compartment of a mitochondrion. As the proton (H+) concentration in the outer compartment of mitochondrion becomes greater than that of the inside compartment, other protons are driven across the membrane by electrical – chemical proton gradient. As protons move down this gradient between outer and inner mitochondrial compartments, they induce the formation of ATP from ADP, phosphate, and the enzyme ATP synthetase.
The electrons that derive the electron transport system involved in chemiosmosis are obtained form chemical bonds of food molecules in all organisms and from photosynthesis in plants. This electron stripping process is called cellular respiration or aerobic respiration because free oxygen is needed. Basically aerobic respiration is the oxidation of food molecules to obtain energy.