Fungi, plants and animals conduct the process of cellular respiration, also called oxidative metabolism. In this process, energy stored in nutrients is metabolically converted to cellular energy in the form of ATP. Cells use the energy in ATP or adenosine triphosphate to perform cellular work, such as enzyme reactions, the transport of molecules or protein synthesis. During cellular respiration, a stepwise series of metabolic reactions controls the release of energy so that it can be captured and used by the cell. An example of the uncontrolled release of energy is burning wood. Cellular respiration requires oxygen and releases the waste products carbon dioxide and water. It occurs in three stages--glycolysis, the citric acid cycle and the electron transport chain.
Glycolysis
The metabolic process of glycolysis splits a six-carbon glucose molecule into a three-carbon molecule called pyruvate. It does not require oxygen and takes place in the cytoplasm of cells. During this process, two ATPs are produced from each glucose molecule that is split. Also, electrons are removed from the glucose molecule and captured by the electron carrier. The transfer of electrons releases energy and can be compared to the energy released when a stretched rubber band flies across the room after being released. In this example, the stored energy in the stretched rubber band is converted to kinetic energy so that the rubber band moves a certain distance.
Electron Carriers
The molecules nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH2) carry the high-energy electrons to the next stages of cellular respiration. These carriers are shuttled from the cytoplasm to the mitochondria, where they will transfer the captured electrons from glucose to other carriers.
Citric Acid Cycle
The end-product of glycolysis, pyruvate, moves into the mitochondria and prior to the first metabolic reaction in the citric acid cycle, a carbon dioxide group is removed. This carbon dioxide is a waste product and is transported out of the cell and into the blood. It then travels to the lungs and is exhaled. The now two-carbon molecule, called acetyl-CoA, enters the cycle and for each acetyl-CoA that enters, three NADH, one FADH2 and one ATP are produced.
Electron Transport
The electron transport chain is the final stage of cellular respiration. It consists of a system of protein electron carriers that are located in the matrix of mitochondria in cells. The electrons harvested from glucose in the earlier stages of respiration are passed from one electron carrier to another. Hydrogen ions move with the electrons and a charge and chemical gradient are created in the inner membrane of the mitochondria. The separation of charge and difference in concentration of hydrogen ions are forms of potential energy, and this energy is utilized to drive the formation of ATP. In the electron transport chain, each NADH transfers enough electrons to produce approximately three molecules of ATP, and each FADH2 produces about two molecules of ATP.
Energy Yield
The entire process of cellular respiration produces between 32 and 38 ATP per molecule of glucose. The overall efficiency level of cellular respiration is reported in the "Science Encyclopedia" as approximately 40 percent. The other 60 percent of glucose energy is released as heat, which is important for maintaining body temperature.
Anaerobic Respiration
If oxygen is not present,, glucose energy can still be harvested by glycolysis and fermentation. In the process of fermentation, the electron that was transferred to NADH during glycolysis is transferred back to pyruvate. This metabolic reaction converts pyruvate to lactic acid. Fermentation only yields two ATP per glucose molecule, but it also regenerates NAD+ so that it can now accept more electrons in another round of glycolysis. In humans, anaerobic respiration occurs in skeletal muscle when oxygen levels are low, which can result from intense exercise. Scientists reported in the American Journal of Physiology, Regulatory, Integrative and Comparative Physiology that in skeletal muscle, the process of fermentation increases proportionally to exercise intensity. However, contrary to popular opinion, the increase in lactic acid production does not cause muscle fatigue and in fact if lactic acid were not produced, muscle fatigue would have a more rapid onset.
Cellular Respiration in Cancer Cells
The Warburg Hypothesis proposed in 1924 says cancer cells process energy primarily by fermentation rather than oxidative metabolism. Scientists report in a January 2010 issue of the journal Current Medicinal Chemistry that drugs that target enzymes in glycolysis may decrease tumor growth and prove beneficial in the treatment of cancer.
References
- Kimball's Biology Pages: Cellular Respiration
- Biology.clc.uc.edu: Cellular Respiration and Fermentation
- Science.jrank.org: Respiration--Efficiency of Cellular Respiration
- American Journal of Physiology: Biochemistry of exercise-induced metabolic acidosis
- Pubmed.gov: Current Medicinal Chemistry": Manipulation of glycolysis in malignant tumors: fantasy or therapy?
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