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The synthesis of more complex molecules from simpler ones in cells by a series of reactions mediated by enzymes. The overall economy and survival of the cell is governed by the interplay between the energy gained from the breakdown of compounds and that supplied to biosynthetic reaction pathways for the synthesis of compounds having a functional role, such as deoxyribonucleic acid (DNA), ribonucleic acid (RNA), and enzymes. Biosynthetic pathways give rise to two distinct classes of metabolite, primary and secondary. Primary metabolites (DNA, RNA, fatty acids, α-amino acids, chlorophyll in green plants, and so forth) are essential to the metabolic functioning of the cells. Secondary metabolites (antibiotics, alkaloids, pheromones, and so forth) aid the functioning and survival of the whole organism more generally. Unlike primary metabolites, secondary metabolites are often unique to individual organisms or classes of organisms. See Enzyme, Metabolism
The selective pressures that drive evolution have ensured a diverse array of secondary metabolite structures. Secondary metabolites can be grouped to some extent by virtue of their origin from key biosynthetic pathways. It is often in the latter stages of these pathways that the structural diversity is introduced. All terpenes, for example, originate from the C5 (five-carbon) intermediate isopentenyl pyrophosphate via mevalonic acid. The mammalian steroids, such as cholesterol, derive from the C30 steroid lanosterol, which is constructed from six C5 units. Alternatively, C10 terpenes (for example, menthol from peppermint leaves) and C15 terpenes (for example, juvenile hormone III from the silk worm) are derived after the condensation of two and three C5 units, respectively, and then with further enzymatic customization in each case. See Cholesterol, Organic evolution, Steroid
the formation of organic substances from simpler compounds, occurring within living organisms or outside them under the action of biocatalysts—enzymes. Biosynthesis is part of the metabolic process of plants, animals, and microorganisms. Compounds rich in energy serve as the immediate source of energy for biosynthesis, but ultimately (for all organisms except bacteria, which themselves accomplish biosynthesis), that source is the energy of solar radiation accumulated by green plants. Every unicellular organism, as well as every cell of a multicellular organism, synthesizes the substances that constitute it. The type of biosynthesis accomplished in the cell is determined by the hereditary information “coded” in its genetic apparatus. Biosynthesis accomplished outside organisms is used widely as a method (sometimes the only possible one) for commercially obtaining biologically important substances—vitamins, certain hormones, antibiotics, amino acids, proteins, and other compounds.
S. E. SEVERIN