All plant and animal groups existing together and the physical factors with which they interact compose an ecosystem<span>.</span>
<span>All four nucleotides (adenine, guanine, thymine, cytosine) are synthesized in the liver. The nucleotides can be synthesized de novo or recycled through salvage pathways. The synthesis occurs in the cytoplasm of the cell, not within a specific organelle. The components which are used for the nucleotide synthesis are derived from biosynthetic precursors of carbohydrate and amino acid metabolism, and from ammonia and CO2.</span>
