Urea processing uses the following steps {urea cycle}. Ammonia builds up in cells from various deaminations. Ammonia is toxic, because it blocks TCA cycle. In mitochondria, ammonia reacts with two ATP, carbon dioxide, and water molecule to make one carbamyl phosphate. In cytoplasm, carbamyl phosphate reacts with ornithine to make citrulline. Citrulline diffuses to cytosol and reacts with aspartic acid, which splits to give arginine and fumarate. Water reacts with arginine to make nitrogen compounds {urea}| {ornithine}. Urea is not toxic, can cross membranes, and excretes in urine.
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Date Modified: 2022.0224