Biochemistry I - Organization of Metabolism - Quick Guides | Biochemistry

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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.

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Study GuideBecause citric acid is formed at the start, the cycle is also called thecitric acid cycle. This step isvery important because it allows carbon-containing compounds from food to enter the cycle.1.3Carbon Changes During the Cycle4 + 2 = 6; 6 = 5 + 1; 5 = 4 + 1Explain what happens to carbon atoms during the cycle:•A4-carbonmolecule combines with a2-carbonmolecule to form a6-carboncompound.•The6-carboncompound loses one carbon to become a5-carboncompound.•The5-carboncompound loses another carbon to return to a4-carboncompound.This simple number pattern is ashorthandway to track carbon atoms.You will see this type of shorthand used often in metabolism to make complex steps easier tounderstand.1.4Oxidation and Energy TransferAs citric acid is broken down:•Carbon atoms are released ascarbon dioxide (CO₂).•At the same time, a cofactor calledNADis reduced (it gains electrons).NAD carries these high-energy electrons through a series of reactions. Eventually, the electrons arepassed to aterminal electron acceptor.•In animals and plants, the final acceptor isoxygen, which formswater.•Some organisms, especiallybacteria, use other acceptors such assulfuror certainorganiccompounds.1.5How Different Nutrients Feed into the CycleDifferent nutrients enter the Krebs cycle in different forms:

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Study Guide•Carbohydrates and lipidsare broken down into2-carbon compoundsthat enter the cycle.•Amino acidscan be converted into2-carbon or 4-carbon compounds, depending on theirtype.•Purines and pyrimidines(components of nucleic acids) are usuallyrecycledin animals andplants rather than broken down.oHowever, in plants and bacteria, theycanbe broken down intocarbon dioxide andammonia.Key TakeawayThe Krebs cycle acts as acentral meeting pointfor the breakdown of carbohydrates, fats, andproteins.By carefully breaking down carbon compounds and transferring electrons, the cell efficiently producesthe energy it needs to survive and function.2.Biosynthetic vs. Catabolic ReactionsCells constantly carry out two major types of chemical reactions:•Biosynthetic (anabolic) reactions, which build molecules•Catabolic reactions, which break molecules downUnderstanding the difference between these two is essential for understanding metabolism.2.1Building Molecules: Biosynthetic (Anabolic) ReactionsBiosynthetic reactions involve makinglarge, complex moleculesfromsmaller, simpler ones.During this process, carbon atoms usually becomemore reduced(they gain electrons).Some common examples include:•Makingglucose from carbon dioxideduring photosynthesis•Makinglipids (fats) from carbohydratesin animalsThese reactions do not happen automatically. They require energy and careful control by the cell.

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