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6: Glycolysis, the Krebs Cycle and the Atkins Diet
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6.1: Introduction
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We used to get metabolic pathways charts like the one you see here free from vendors of biochemical reagents. This one is a high-resolution image; if you zoom in, you can actually read the content…, but don’t feel you must! The big picture is correct in macro- detail, but the chart is likely out of date in small new details. In this chapter, we ‘zoom in’ on the small region in the middle of the chart, encompassing glycolysis and the Krebs cycle.
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6.2: Glycolysis (from the Greek glyco (sugar) lysis (Separation), or Sugar Breakdown
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To begin with, the most common intracellular energy currency with which livings things “pay” for cellular work is ATP. The energy to make ATP on planet earth ultimately comes from the sun via photosynthesis. Recall that light energy fuels the formation of glucose and O2 from CO2 and water in green plants, algae, cyanobacteria and a few other bacteria. Photosynthesis even produces some ATP directly, but not enough to fuel all cellular and organismic growth and metabolism.
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6.3: Some Details of Glycolysis
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Recall that ATP hydrolysis is an exergonic reaction, releasing ~7 Kcal/mole (rounding down!) in a closed system under standard conditions. The condensation reaction of glucose phosphorylation occurs with a DGo of +3 Kcal/mole. This is an endergonic reaction under standard conditions. Summing up the free energy changes of the two reactions, we can calculate the overall DGo of -4 Kcal/mole for the coupled reaction under standard conditions in a closed system.
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6.4: Gluconeogenesis
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In a well-fed animal, most cells can store a small amount of glucose as glycogen. All cells break glycogen down as needed to retrieve nutrient energy as G-6-P. Glycogen hydrolysis, or glycogenolysis, produces G-1-P that is converted to G-6-P, as we saw at the top of Stage 1 of glycolysis. But, glycogen in most cells is quickly used up between meals. Therefore, most cells depend on a different, external source of glucose other than diet.
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6.5: The Atkins Diet and Gluconeogenesis
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You may know that the Atkins Diet is an ultra-low carb diet. It is one of several low-carb ketogenic diets. The glucocorticoid hormones released on a low carb diet trick the body into a constant gluconeogenic state.
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6.6: The Krebs/TCA/Citric acid cycle
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Glycolysis through fermentative reactions produces ATP anaerobically. The evolution of respiration (the aerobic use of oxygen to efficiently burn nutrient fuels) had to wait until photosynthesis created the oxygenic atmosphere we live in now. Read more about the source of our oxygenic atmosphere in Dismukes GC et al.
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6.7: Key Words and Terms
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