The compounds present in plants are conveniently divided into two major groups: primary and secondary metabolites. Primary metabolites are those produced by and involved in primary metabolic pathways such as respiration and photosynthesis. Secondary metabolites are clearly derived by biosynthesis from primary metabolites and are generally much more variable in their distribution patterns within the plant kingdom.
Primary metabolites include the components of processes such as gly-colysis, the Calvin-Benson cycle, and the Krebs cycle. Primary metabolites are virtually identical throughout the plant kingdom: they are mainly sugars, amino acids, and organic acids. As intermediates in metabolic pathways, these molecules may be present in some activated form. Glucose, for example, when taking part in metabolism, occurs in an energy-rich form as glucose 1-phosphate or as uridine diphosphoglucose. Other primary metabolites are the proteins, nucleic acids, and polysaccharides of plant cells. These have universal functions as enzymes, structural elements, storage forms of energy, and hereditary materials.
Secondary metabolites are produced by biosynthetic pathways, beginning with primary metabolites as starting materials. It has been estimated that about one hundred thousand secondary metabolites have been characterized in plants, and additional substances are continually being discovered. The amount of any secondary compound present in a plant is the result of an equilibrium between synthesis, storage, and metabolic turnover. Regulation of secondary metabolism is complex, and production may be limited to certain organs of the plant and may only take place during a single phase of the life cycle (e.g., during flowering or fruit formation).
Secondary metabolites are conveniently divided into three main chemical classes: the phenolics, the terpenoids, and the nitrogen-containing substances. The phenolics include the lignins, which are the aromatic materials of cell walls, and the anthocyanins, the colorful red to blue pigments of angiosperm flowers. Another phenolic class are the plant tannins, mainly present in woody plants, which have the special property of being able to bind to protein. They impart an astringent taste to plant tissues containing them and are significant flavor components in tea, wine, and other plant beverages.
The terpenoids are probably the most numerous of secondary substances. They are subdivided into monoterpenoids and sesquiterpenoids (essential oils); diterpenoids, including resin acids; triterpenoids (phytosterols, cardenolides, limonoids, etc.); and tetraterpenoids (carotenoids). The most visible terpenoids are the yellow to red carotenoid pigments present in flowers and fruits. Limonin gives lemon its characteristic taste. By contrast, volatile terpenoids give caraway and carrot their characteristic scents.
The nitrogen-based secondary metabolites are variously classified as amines, alkaloids, cyanogenic glycosides, and mustard oil glycosides. In general they have only limited occurrences. Alkaloids are the best known compounds of this type and are found in 20 percent of all plant families. Some alkaloids, such as morphine, because of their physiological activities in humans, have been used extensively in medicine. Other alkaloids, such as coni-ine from the hemlock, have been used as poisoning agents.
While the role of primary metabolites is clear, the functions of secondary substances are still uncertain. The anthocyanin and carotenoid pigments, together with the floral essential oils, are necessary to attract animals to flowers. The gibberellins, auxins, and cytokinins, together with abscisic acid and ethylene, control plant growth and development. Alkaloids and tannins deter animals from feeding on green tissues and thus are valuable to plants for limiting the extent of insect herbivory and animal grazing. see also Alkaloids; Cacao; Carbohydrates; Carotenoids; Cellulose; Coca; Defenses, Chemical; Flavonoids; Lipids; Opium Poppy; Photosynthesis, Carbon Fixation and; Photosynthesis, Light Reactions and; Physiologist; Psychoactive Plants; Terpenes.
Jeffrey B. Harborne
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