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Nutrients and the body
A nutrient is any element or compound that is necessary for or contributes to an organism's metabolism, growth, or other functioning. There are six nutrient groups and these can
be divided into those that provide energy and those that otherwise support metabolic processes in the body. These are:
Substances that provide energy
The energy content of fat is 9 kcal/g; of
proteins and carbohydrates 4
kcal/g. Ethanol (grain
alcohol) has an energy content of 7 kcal/g.
Substances that support metabolism
- Minerals: generally trace
elements, salts, or ions such as copper and iron essential to normal metabolism
- Vitamins: organic compounds essential to the body's functioning, usually acting
as coenzymes
- Water: absolute requirement for normal growth and metabolism directly involved in all
the chemical reactions of life—sometimes referred to as the forgotten nutrient
The division of "nutrients" into these two groups is somewhat of a false dichotomy. All useful substances taken into the body
will be either metabolzed for their energy content, utilized for structural purposes (growth or replacement of living
structures), or participate in chemical reactions necessary for life. Any particular substance can play more than one role in the
body.
Not considered to be nutrients by some are the phytonutrients which
are organic compounds from plants not required for normal functioning of a body but having potential beneficial effects on health
or playing an active role in amelioration of disease. Any organic compound that can be metabolized by the body (including
ethanol) has at least some nutrient value as a source of energy.
Nutrients and the environment
While in essence true to the definition above, the term nutrients has a more limited meaning within the
subjects of water quality and water pollution, referring specifically to plant fertilizers. In this sense, certain
mineral compounds can have an adverse impact on water quality because of their ability to promote plant and algae growth. An excessive growth of aquatic plants can clog waterways (see Giant salvinia for an example) and over-stimulation of algae and microbes leads to an ecological
process called eutrophication.
The number of elements of interest or concern in this context is surprisingly
small: really just nitrogen and phosphorus in most aquatic systems. Mineral compounds involved are
ammonia, nitrites, nitrates, and orthophosphates. Organic
compounds also may contribute, in as much as they also contain nitrogen and phosphorus. The reason only a few chemicals are of
concern has to do with the fact that plants are made up mostly of compounds of carbon (C), hydrogen (H), oxygen (O), nitrogen
(N), and phosphorus (P), and lesser amounts of sulfur (S), potassium (K), magnesium (Mg), and calcium (Ca). These elements
constitute the macronutrients. Many other elements are necessary for growth, yet because these others are needed
in very small amounts they are classified as micronutrients. Plants obtain carbon, hydrogen, and oxygen (elements most needed for growth) from
the air and water, where all three elements are very abundant as water and carbon dioxide. So nutrients having greatest potential to influence plant
growth in aquatic environments would be those elements necessary for plant growth, but likely to be limiting — that is, once used up, plant growth stops. Of the nine macronutrients, nitrogen and
phosphorus are most likely to become limiting. The others are always present in great abundance (C, H, O) or usually present in
amounts that exceed the requirements of aquatic plants or algae.
Farmers apply fertilizer nutrients in the form of nitrogen, phosphorus, and potassium (N, P, and K with perhaps
micronutrients) to prevent these elements from becoming limiting in the soil. These elements are concentrated in wastewaters from
animal pens and septic or sewage systems. And these elements (especially N and P) in
runoff or wastewater discharges reaching streams, lakes, or seas will promote aquatic
plant growth. Abundant plant growth itself is a concern in assessing water quality. The most abundant "plants" in most aquatic
environments are algae. When essential nutrients are plentiful, algae multiply. If these algae are microscopic phytoplankton, their growth increases the turbidity of the water. The water then becomes cloudy and colored a shade of green, yellow, or brown (sometimes
red). An abundance in an aquatic system of any algae, or of higher plants, can signal excessive inputs of nutrients.
Nutrients are recycled at the ecosystem and biosphere system level through biogeochemical
cycles:
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