Overview of Energy Metabolism

In the first stage, metabolic fiiels are hydrolyzed in the gastrointestinal (GI) trad to a diverse set of monomeric building blocks (glucose, amino acids, and fatty acids) and absorbed. 

In the second stage, the building blocks are degraded by various pathways in tissues to a common metabolic intermediate, acetyl CoA. Most of the energy contained in metabolic fuels is conserved in the chemical bonds (electrons) of acetyl CoA. A smaller portion is conserved in reducing nicotinamide adenine dinucleotide (NAD) to NADH or flavin adenine dinucleotide (FAD) to FADH. Reduction indicates the addition of electrons that may be free, part of a hydrogen atom (H), or a hydride ion (H ). 

In stage three, the citric add (Krebs, or tricarboxylic acid [TCA]) cycle oxidizes acetyl CoA to COj- The energy released in this process is primarily conserved by reducing NAD to NADH or FAD to FADHj. 

The final stage in the extraction of energy from food is oxidative phosphorylation, in which the energy of NADH and FADHj is released via the electron transport chain (ETC) and used by an ATP synthase to produce ATP. This process requires O.  

In addition to energy reserves, many other types of biochemicals are required to maintain an organism. Cholesterol is required for cell membrane structure, proteins for muscle contraction, and polysaccharides for the intracellular matrix, to name just a few examples. These substances may be produced from transformed dietary components. 

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