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Nutrients are any chemicals that are required for life. Nutrients can be of two basic types: (1) inorganic substances absorbed by autotrophic organisms such as plants and certain microorganisms for use in their synthetic reactions and metabolism; or (2) biomass ingested as nourishment by animals and heterotrophic microorganisms.

Plants absorb a wide range of mineral nutrients, which they utilize in their photosynthetic reactions and other metabolic processes to manufacture all of the biochemicals that they require for growth and reproduction. Some nutrients are required by plants in relatively large quantities. These are called macronutrients and include compounds of the following elements: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. Of these, carbon is required in the largest quantity, typically making up about one-half of the dry weight of plant tissues, while hydrogen, oxygen, nitrogen, and calcium occur in concentrations of one to several percent. Phosphorus, potassium, magnesium, and sulfur typically account for 0.1% to 1% of the dry weight of plants. Micronutrients are required by plants in much smaller quantities and include metals such as copper, iron, and zinc.

Animals must eat plants or other animals to obtain virtually all of their nutrients. After ingestion, animals typically digest their food and thereby break it down into relatively simple biochemicals and inorganic chemicals, which are then absorbed through the gut and used in the animal's metabolism. The largest nutritional need of animals is for energy to support their respiration and growth, and sources of fixed energy such as plant or animal biomass are ingested for this purpose. There are also a few micronutrients that animals require in small quantities, but they cannot synthesize. These include biochemicals called vitamins, which must be ingested with food. Some animals can also utilize mineral forms of certain nutrients, which they may take directly from the inorganic environment without eating biomass. For example, many large grazing mammals will utilize salt licks when they are available, because these animals crave sodium, which is not usually present in sufficiently large concentrations in the plants that they eat.

Appropriate nutrition for all organisms is a matter of both quantity and balance. For good nutritional health, all of the essential inorganic and organic nutrients must be available, but they must be obtainable in an appropriate balance. A severe shortage of even a micronutrient required in trace quantities can result in severe metabolic dysfunctions, and even death of organisms.

See also Vitamin.

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