Study GuideBiochemistry I–Organization of Metabolism1.Energy Production in CellsLiving cells need energy to grow, repair themselves, and carry out daily activities. This energy comesfrom metabolism—the set of chemical reactions that break down and build up molecules inside thecell.Main Energy-Releasing PathwaysThe main pathways that release energy involve:•Carbohydrates•Lipids (fats)•Amino acidsWhen these molecules are broken down, their energy is captured and used by the cell.1.1The Central Pathway: The TCA (Krebs) CycleAt the center of energy production is thetricarboxylic acid (TCA) cycle, also known as theKrebscycle. This cycle is considered the core metabolic pathway of the cell.Earlier scientists knew many metabolic pathways, but most of them had a clearstarting point(substrate)and a clearending point (product).The Krebs cycle is different. It works as acycle, meaning the starting molecule is regenerated at theend and can be used again.1.2Entry into the Krebs CycleEnergy-rich molecules enter the Krebs cycle in a special way:•A2-carbon compoundenters the cycle.•It combines with a4-carbon compound.•Together, they form a6-carbon compound called citric acid.Preview Mode
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