Chapter 9 Cellular Respiration and Fermentation

Glucose doesnt burn in cells but is oxidized through

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Glucose doesn't burn in cells, but is oxidized through a long series of carefully controlled redox reactions. Cellular respiration is a four-step process for producing ATP from a starting material with high potential energy- usually glucose. 1. Glycolysis: one molecule of glucose is broken into 2 molecules of the 3-carbon compound. Two ATP molecules are produced from ADP and one molecule NAD + is reduced to form NADH. 2. Pyruvate Processing: pyruvates processed to form the compound acetyl-CoA. Another NADH is produced. 3. Citric Acid Cycle: Acetyl-CoA is oxidized to 2 molecules of CO 2 . More ATP and NADH are produced and flavine adenine dinucleotide (FAD) is reduced to form FADH 2 . 4. Electron transport and chemiosmosis: electrons from NADH and FADH 2 move through a series of proteins called an elctron transport chain (ETC). The flow of protons back across the membrane is used to make ATP. Cellular respiration is defined as any suite of reactions that produces ATP in an electron transport chain. All 10 reactions of glycolysis occur in the cytosol. o Glycolysis starts by using ATP- glucose is phosphorylated to form glucose-6-phosphate. o After an enzyme rearranges this molecule to fructose-6-phosphate, the third reaction adds a second phosphate group, forming fructose-1,6-biphosphate. o The sixth reaction results in the reduction of two molecules of NAD + and the seventh produces two molecules of ATP. o The final reaction produces another two ATPs. o For each molecule of glucose processed, the net yield is two molecules of NADH, two ATP and two pyruvate. How is Glycolysis Regulated?
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High levels of ATP inhibit a key glycolytic enzyme called phosphofructokinase. Phosphofructokinase catalyzes the reaction that synthesizes fructose-1,6-biphosphate from fructose-6- phosphate. When an enzyme in a pathway is inhibited by the product of the reaction sequence, feedback inhibition occurs. The product molecule "feeds back" to stop the reaction sequence when the product is abundant. Phosphofructokinase has two distinct binding sites for ATP. ATP can bind at the enzyme's active site or at a location that changes the enzyme's activity- a regulatory site. At the active site, ATP is converted to ADP and the phosphate group is transferred to fructose-6- phosphate. This reaction results in the synthesis of fructios-1,6-biphosphate. If ATP concentrations are high, however, the molecule also binds at the regulatory site on phosphofructokinase. When ATP binds at this second location, the enzyme's conformation changes in a way that dramatically lowers the reaction rate at the active site. In this case, ATP acts as an allosteric regulator. In eukaryotes, pyruvate produced by glycolysis is transported from the cytosol to mitochondria. A mitochondrion has two membranes, called the inner membrane and outer membrane.
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