Which electron carrier is used to harvest high energy electrons from glucose during glycolysis?
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Chapter 6. Cellular Respiration BI 101 Davison Fall 2008 Show Respiration is about breathing, yes, but physiologically why do you need to breathe? The answer lies in an understanding of the role of the respiratory gases CO2and O2. Each organism �breathes� that is exchanges those respiratory gases with its environment. For terrestrial animals the exchange occurs between some body organ (lungs-humans; skin-earthworms) and the atmosphere. For some aquatic animals the exchange occurs between an aquatic environment and the body surface. For example, all parts of a jellyfish�s exposed body takes in O2 and releases CO2 . For other aquatic animals (e.g. bony fish such as bass) specialized body surfaces called gills are the sites of gas exchange. Gills with their greater efficiency in exchanging respiratory gases are required in order to meet the metabolic needs of the fish whereas in jellyfish no specialized respiratory organ is needed to meet the needs of the lowly jellyfish. As you learn the chemistry of cellular respiration, keep in mind these questions: �Why do we need O2 or what happens to the O2 once it enters your body?� and �Where in our body�s physiology does the CO2 we exhale come from?� The purpose of cellular respiration is however not to intake O2 and release CO2. Those events are necessary sideline events supporting the main goal of ATP production. You will learn that glucose is the fuel broken down producing 38 molecules of ATP for every single glucose molecule processed. Cellular Respiration - has 3 series of reactions, each consisting of many reactions, or each is an example of a specific metabolic pathway mediated by a series of enzymes. 1. Glycolysis � occurs in cytoplasm; splits glucose into two molecules of the three-carbon molecule pyruvate; transfers energy to produce two molecules of ATP and loads high energy electrons onto electron carriers (NADH). 2. Krebs Cycle � occurs in the mitochondria; [as a simplification we will ignore the transition reaction that occurs between glycolysis and the Krebs cycle] breaks down the organic residue of pyruvate into CO2 and therein transfers energy to produce two molecules of ATP; high-energy electrons taken from pyruvate are transferred to electron carriers (NAD+ & FAD - special molecules that act as transports, much like a wheelbarrow acts as a transport when carrying electrons these molecules occur as NADH & FADH2). Electron carriers shuttle electrons taken from pyruvate to the final series of respiratory reactions, the electron transfer chain--see below. The Krebs Cycle is intimately linked to the electron transfer chain and is dependent, albeit indirectly, on the presence of O2. [note: the CO2 produced by the Krebs Cycle makes its way out of the cell and into the blood stream via diffusion; once in the lungs, CO2 is eliminated with exhalation. The meager amount of ATP produced by the Krebs cycle moves out of the mitochondrion and is available as an energy source needed to drive energy-requiring metabolism.] 3. Electron Transfer Chain = Electron Transfer Phorsphorylation � occurs in mitochondria; takes the electrons delivered by electron carriers (NADH & FADH2) and converts the potential energy of those high-energy electrons into many ATP molecules. For each glucose molecule that begins cellular respiration, 34 molecules of ATP are produced from the high-energy electrons removed during the preceding glycolysis and the Krebs Cycle events. Of course at the end of the electron transport system there are electrons whose useable potential energy has been spent in ATP production. These little negative "waste" particles must be removed. Enter O2! Good ole O2 will serve as the final electron acceptor and will take the electrons out of "harms way." Oxygen, O2, molecules accept the electrons and combine with H+ 's and become water molecules (H2O), a stable and safe combination for sequestering "spent" electrons. The above constitutes what is called aerobic cellular respiration. (aerobic is in reference to the presence of O2). But what if O2 is not present? Our bodies can deal with that, at least temporarily. If O2 is not available as during brief periods of intense exercise in which the blood supply simply can�t meet the O2 demands of the activity, our cells switch to an anaerobic (w/out O2) method of respiration. Plants and bacteria can also switch to anaerobic methods. Some species of bacteria actually require anaerobic environments. (more on anaerobic respiration below) Anaerobic Cellular RespirationATP production can continue (briefly) in most organisms without the presence of O2 and without the Krebs Cycle and Electron Transport System. These anaerobic routes of respiration provide for only limited ATP production as produced by glycolysis only; O2 is soon required to sustain the tremendous turnover in ATP production required for most life forms. Thus, look upon these anaerobic routes of respiration as temporary means of limited ATP production during temporary, brief episodes of O2 shortages in higher plants and animals. A number of microbes (e.g. yeast & certain bacteria) are anaerobic to a greater degree, some are even obligate anaerobes, i.e. they are killed by the presence of O2. Anaerobic respiration consists of only 2 reaction series and only the first of these, glycolysis, produces ATP: 1. Glycolysis � occurs exactly the same as in aerobic cellular respiration. Two molecules of pyruvate result from the breakdown of glucose. Two ATP are synthesized. As in aerobic respiration, Glycolysis also delivers high-energy electrons taken from glucose to electron carriers. In order for glycolysis to occur, that is in order to split a molecule of glucose into 2 molecules of pyruvate, some electrons must be removed from glucose. Removing electrons from glucose results in glucose falling apart forming two molecules of pyruvate. Electron carriers (NAD+) must be available to accept these removed electrons. Electron carriers that have accepted electrons (NADH) must unload those electrons before accepting additional electrons. When the wheelbarrow is full it must be emptied before it can accept more load. Under aerobic conditions the electron carriers deliver the high-energy electrons to the Electron Transport System in the mitochondria and the electrons are �milked� of their energy to produce more ATP and O2 serves as the final electron acceptor and becomes H2O. The electron carriers, once they have dropped off the electrons to the electron transport chain are free to return to the cytoplasm and aid the process of glycolysis. (I can see that empty wheelbarrow hustling back to that big pile of sand! The sand can�t be moved if there aren�t any empty wheelbarrows to move it.) Glycolysis cannot continue without electron carriers available to take away electrons. So, in the absence of O2, the reactions below occur for one reason: to make electron carriers available so that glycolyis and the ATP production inherent to glycolysis can continue. 2. Fermentation � pyruvate is chemically altered in one of two ways. Each of these alterations requires additional electrons. The additional electrons are delivered to the fermentation process by electron carriers (NADH) that have been loaded with electrons during glycolysis. As electrons are delivered to the fermentation process NADH is converted back into NAD+ and these electron carriers are now capable of accepting electrons as needed to help drive glycolysis and ATP production therein. A. Alcoholic fermentation � occurs in germinating seeds when they are waterlogged and without O2.
(the seeds will die if they don�t get O2 eventually). Also, yeast perform this producing the CO2 and ethanol, both important to the baking and brewing industries. The conversion of pyruvate into waste ethanol & CO2 requires the addition of electrons. B. Lactate fermentation � occurs in our muscle cells during periods of O2 shortage as when muscle activity surpasses
O2 delivery . Also, certain bacteria perform this. The conversion of pyruvate into lactate requires the addition of electrons. In both fermentation examples above, the addition of electrons during the fermentation of pyruvate has the sole benefit of this (i.e. the whole point of fermentation is this final point!): electron carriers delivering electrons to the fermentation process are freed of their electron load and are thus available for use in glycolysis--glycolysis is the only a process that produces ATP in anaerobic respiration. Without electron carriers available to accept electrons glycolysis and all ATP production would cease. Click Here to Return to BI 101 Home Page Which 2 carriers for high energy electrons are used during cellular respiration?There are two types of electron carriers that are particularly important in cellular respiration: NAD +start superscript, plus, end superscript (nicotinamide adenine dinucleotide, shown below) and FAD (flavin adenine dinucleotide).
What energy carrier molecules are produced by glycolysis?During glycolysis, a glucose molecule with six carbon atoms is converted into two molecules of pyruvate, each of which contains three carbon atoms. For each molecule of glucose, two molecules of ATP are hydrolyzed to provide energy to drive the early steps, but four molecules of ATP are produced in the later steps.
What is the most common mobile electron carrier used in glycolysis?Glycolysis. Glycolysis starts by bringing glucose into the cell. From there, a little bit of energy has to be spent to activate the glucose and split it into two molecules of glyceraldehyde 3-phosphate (G3P). Next, an electron carrier comes in called NAD + , which is the oxidized form.
Does glycolysis have electron carriers?As in aerobic respiration, Glycolysis also delivers high-energy electrons taken from glucose to electron carriers. In order for glycolysis to occur, that is in order to split a molecule of glucose into 2 molecules of pyruvate, some electrons must be removed from glucose.
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