What Is The Krebs Cycle and Why is it Important in The Body?
|The Krebs Cycle - Our Lifes Blood!||Darrell Miller||01/13/11|
January 13, 2011 05:00 PM
Author: Darrell Miller (firstname.lastname@example.org)
Subject: The Krebs Cycle - Our Lifes Blood!
The Krebs Cycle, also known as the Citric Acid Cycle, is an important series of biochemical reactions that are intrinsic to cellular respiration and the generation of energy from oxygen and glucose in aerobic organisms. Although humans can generate energy anaerobically, they cannot do so for long periods and oxygen is essential for life because it fuels the Krebs Cycle. Without this cycle, there would be no available energy to maintain our vital processes.
What is The Krebs Cycle
We shall first discuss exactly what the Krebs Cycle is and then its importance in the body. The process takes place within the mitochondria of the body cells.
A. The cycle begins with the ten-step pathway of glycolysis, during which glucose is converted to two molecules of pyruvate: no oxygen is required for this step. Basically:
Glucose + 2NAD+ + 2P + 2ADP = 2Pyruvate + 2NADH + 2H+ + 2ATP
The pyruvate can then undergo aerobic or anaerobic respiration to generate energy. The Krebs Cycle is the aerobic route, and significantly more efficient that the alternative fermentation.
B. The two pyruvate molecules are then oxidized to two of Acetyl CoA with the release of carbon dioxide. The two acetyl groups of the Acetyl CoA are donated to Oxaloacetate in the presence of water to form Citrate.
C. Citrate then undergoes a number of chemical reactions to arrive back at Oxaloacetate again and the cycle starts all over again by reacting with more Acetyl CoA from Pyruvate and oxygen to form Citrate again. During the cycle more ATP is produced and electrons and other species are sent into the Electron Transfer Chain where most energy is generated.
During this cycle three major events occur:
1. During the various transformations of Citrate, specifically when Succinyl-CoA is converted to Succinate, one Guanosine Triphosphate group (GTP) is generated, which then donates a phosphate to Adenosine Diphosphate (ADP) to create the energy molecule Adenosine Triphosphate (ATP).
2. Over the whole cycle, three molecules of Nicotinamide Adenine Dinucleotide (NAD) are reduced to NADH that donates its electrons to the electron transport chain that is responsible for the generation of large quantities of ATP.
3. A molecule of Flavine Adenosine Dinucleotide (FAD) is reduced to FADH2, again donating its electrons to the electron transport system and the generation of energy.
Factor 2 and 3 are of more significance to energy generation than factor 1, and are the major means by which the Krebs Cycle generates energy.
Electron Transport Chain
The Krebs Cycle takes place within the mitochondria, structures that are contained in each of your body cells. Also within these mitochondria are a series of membranes that are very important in the generation of energy.
The major energy produced in your body cells comes from the Electron Transport Chain, a series of chemical reactions between an electron donor and an electron acceptor. Such reactions drive the transport of hydrogen ions (H+) across the membranes in the mitochondria.
The electron donors are species such as NADH, FADH2 and succinate and the electron acceptors are oxygen molecules. Hence the importance of oxygen in the process of aerobic respiration. The H+ ions are driven across the membranes and result in the conversion of ADP to ATP energy. In essence, the hydrogen atoms and electrons take part in a progressive chain of redox reactions, and at the end react with oxygen molecules and change it to water.
The Ultimate Products
Ultimately, through the whole chain and cycles of:
Glucose to pyruvate (glycolysis) Pyruvate to Acetyl CoA + CO2 (oxidation) Acetyl CoA to Citrate and the entire Krebs Cycle (reduction and oxidation) Krebs Cycle products to the Electron Transfer Chain Electron Transfer Chain to Water and ATP (redox) Final oxidation of hydrogen atoms to water
Basically: Glucose + Oxygen + 30ADP = Carbon Dioxide + Water + 30ATP
This is not balanced of course, and the ATP could be anything from 29 to 38, though 29.85 is the most accurate calculation to date. The entire process of glycolysis, oxidation, Krebs Cycle and Electron Transport Chain is powered by a series of enzymes and a small amount of ATP energy.
Glucose and oxygen are used up as the raw materials and ATP energy molecules are the product. It has been calculated that the total ATP yield obtained from one molecule of glucose lies between 29.5 and 30 molecules of ATP.
The Importance of the Krebs Cycle to Your Body
Were it not for the Krebs Cycle you would not be able to generate energy efficiently from your food. In particular, the carbohydrate content of your food. Carbohydrates are available in your diet from two sources: simple and complex carbohydrate foodstuffs such as cereals and grains, and fats and oils from animal and vegetable sources. They are also available from proteins, which are composed of amino acids at the head of the molecule with a carbohydrate tail.
Carbohydrates by definition contain only carbon, hydrogen and oxygen molecules, and can be converted to glucose within your body. Proteins can be deaminated and then the carbohydrate portion again converted to glucose. In fact, your body will initially use the carbohydrates in your diet as a source of glucose for its glycolysis to pyruvate, then the fatty tissue in your body, then the proteins will be deaminated so your body can get to the carbohydrate they contain.
Were it not for the Krebs Cycle, your body could not use the glucose as described above to generate energy. The only option open to it would be anaerobic respiration, or energy production in the absence of oxygen. ATP is still generated, but much less efficiently.
Anaerobic respiration is basically respiration without oxygen. If there was no Krebs Cycle, then this is the only way your body would have to create energy.
Because no oxygen is actually needed for glycolysis or the Krebs Cycle, anaerobic respiration can proceed right to the end of the Electron Transport Chain. Then instead of oxygen being used as the terminal electron donor, it has to be another species such as nitrate or sulfate. Because the Reduction Potential of these species is much lower than that of oxygen, the amount of ATP energy produced is also much lower.
Your body tries to compensate by producing even more pyruvate and the excess is removed through lactic acid fermentation. Although this also generates ATP energy enough for short-term use, it leads ultimately to lactic acidosis and a reduction in pH, causing pain and vomiting.
Ultimately, if oxygen is not forthcoming, the brain runs short of the energy needed for it to work and you die. There is insufficient ATP generated from anaerobic respiration to maintain human life.
The Krebs Cycle is of critical importance for the production of energy: not just the energy to enable you to run fast, but to enable your metabolic processes to continue. It is needed for both aerobic and anaerobic respiration, and while lactic acid fermentation can be used by your muscle cells as a brief but effective means of generating short-term energy, it is insufficient to maintain the needs of your metabolism.
Without the Krebs Cycle, mammalian and most other animal life would not be viable and the world would be populated by anaerobic bacteria.