Living organisms difference from

The molecules and ions contained within a living organism differ in kind and in concentration from those in the organism s surroundings. A Paramecium in a pond, a shark in the ocean, an erythrocyte in the human bloodstream, an apple tree in an orchard—all are different in composition from their surroundings and, once they have reached maturity, all (to a first approximation) maintain a constant composition in the face of constantly changing surroundings. 

Living organisms differ in their capacity to synthesize the amino acids required for protein synthesis. Although plants and many microorganisms can produce all their amino acids from readily available precursors, other organisms must obtain some preformed amino acids from their environment. For example, mammalian tissues can synthesize NAA (Table 14.1) by relatively simple reaction pathways. In contrast, EAA must be obtained from the diet because mammals lack the long and complex reaction pathways required for their synthesis. 

In the Chapter 6 Focus On, "Terpenes Naturally Occurring Alkenes," we looked briefly at terpenoids, a vast and diverse group of lipids found in all living organisms. Despite their apparent structural differences, all terpenoids are related. All contain a multiple of five carbons and are derived biosynthetically from the five-carbon precursor isopentenyl diphosphate (Figure 27.6). Note that formally, a... 

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