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Section 9-2
Section 9-1
Chemical Pathways
The Krebs Cycle and Electron Transport
Food provides living things with the chemical building blocks they need to grow and reproduce. Food serves as a source of raw materials for the cells of the body. One gram of the sugar glucose (C6H12O6), when burned in the presence of oxygen, releases 3811 calories of heat energy. A calorie is the amount of energy needed to raise the temperature of 1 gram of water 1 degree Celsius. The Calorie (capital “C”) that is used on food labels is a kilocalorie, or 1000 calories. cells gradually release the energy from glucose and other food compounds.This process begins with a pathway called glycolysis . In the presence of oxygen, glycolysis is followed by the Krebs cycle and the electron transport chain. Glycolysis, the Krebs cycle, and the electron transport chain make up a process called cellular respiration. Cellular respiration is the process that releases energy by breaking down glucose and other food molecules in the presence of oxygen. Glycolysis is the process in which one molecule of glucose is broken in half, producing two molecules of pyruvic acid, a 3-carbon compound.
At the end of glycolysis, about 90 percent of the chemical energy that was available in glucose is still unused, Oxygen is required for the final steps of cellular respiration. Because the pathways of cellular respiration require oxygen, they are said to be aerobic. The second stage of cellular respiration is the kreb cycle. During the Krebs cycle, pyruvic acid is broken down into carbon dioxide in a series of energy-extracting reactions. The Krebs cycle generates high-energy electrons that are passed to NADH and FADH2. The electrons are then passed from those carriers to the electron transport chain. The electron transport chain uses the high-energy electrons from the Krebs cycle to convert ADP into ATP. H+ ions rush across the membrane, producing enough force to spin the ATP synthase and generate enormous amounts of ATP. Although glycolysis produces just 2 ATP molecules per molecule of glucose, in the presence of oxygen, everything changes. As the figure below shows, the Krebs cycle and electron transport enable the cell to produce roughly 36 ATP molecules per glucose molecule, The 36 ATP molecules represent about 38 percent of the total energy of glucose,the remaining 62 percent is released as heat. To obtain energy, the body uses ATP already in muscles and new ATP made by lactic acid fermentation and cellular respiration. For short term energy, the body relize on lactic acid to produce ATP. But long term energy, cellular respiration is the only was to produce ATP to last for a long period of time.
When oxygen is not present, glycolysis is followed by a different pathway. The combined process of this pathway and glycolysis is called fermentation. Fermentation releases energy from food molecules by producing ATP in the absence of oxygen. Because fermentation does not require oxygen, it is said to be anaerobic. The two main types of fermentation are alcoholic fermentation and lactic acid fermentation.Alcoholic fermentation produces carbon dioxide as well as alcohol. Lactid acid fermentation produces lactic acid, This process regenerates NAD+ so that glycolysis can continue. Lactic acid is produced in your muscles during rapid exercise when the body cannot supply enough oxygen to the tissues.
