The macro-nutrients (excluding fiber and water) provide structural material (amino acids from which proteins are built, and lipids from which cell membranes and some signaling molecules are built) and energy.
Some of the structural material can be used to generate energy internally, and in either case it is measured in joules or calories (sometimes called “kilocalories” and on other rare occasions written with a capital C to distinguish them from little ‘c’ calories). Carbohydrates and proteins provide 17 kJ approximately (4 kcal) of energy per gram, while fats provide 37 kJ (9 kcal) per gram., though the net energy from either depends on such factors as absorption and digestive effort, which vary substantially from instance to instance. Vitamins, minerals, fiber, and water do not provide energy, but are required for other reasons.
A third class dietary material, fiber (i.e., non-digestible material such as cellulose), seems also to be required, for both mechanical and biochemical reasons, though the exact reasons remain unclear.
Molecules of carbohydrates and fats consist of carbon, hydrogen, and oxygen atoms. Carbohydrates range from simple monosaccharides (glucose, fructose, galactose) to complex polysaccharides (starch). Fats are triglycerides, made of assorted fatty acid monomers bound to glycerol backbone. Some fatty acids, but not all, are essential in the diet: they cannot be synthesized in the body. Protein molecules contain nitrogen atoms in addition to carbon, oxygen, and hydrogen.
fundamental components of protein are nitrogen-containing amino acids, some of which are essential in the sense that humans cannot make them internally. Some of the amino acids are convertible (with the expenditure of energy) to glucose and can be used for energy production just as ordinary glucose. By breaking down existing protein, some glucose can be produced internally; the remaining amino acids are discarded, primarily as urea in urine. This occurs normally only during prolonged starvation.
Other micro-nutrients include antioxidants and phytochemicals which are said to influence (or protect) some body systems. Their necessity is not as well established as in the case of, for instance, vitamins.
Most foods contain a mix of some or all of the nutrient classes, together with other substances such as toxins or various sorts. Some nutrients can be stored internally (e.g., the fat soluble vitamins), while others are required more or less continuously. Poor health can be caused by a lack of required nutrients or, in extreme cases, too much of a required nutrient. For example, both salt and water (both absolutely required) will cause illness or even death in too large amounts.
The nutritional requirements of most animals are relatively extensive and complex compared with the simple requirements of plants. The nutrients used by animals include carbohydrates, lipids, nucleic acids, proteins, minerals, and vitamins.
Lipids are used to form cellular and organelle membranes, the sheaths surrounding nerve fibers, and certain hormones. One type of lipid, the fats, are extremely useful energy sources.
Vitamins are organic compounds essential in trace amounts to the health of animals. Vitamins can be water soluble or fat soluble. Water-soluble vitamins must be consumed frequently, while fat-soluble vitamins are stored in the liver in fat droplets. Among the many essential vitamins are vitamin A for good vision, vitamin B for substances used in cellular respiration (FAD, NAD, and coenzyme A), and vitamin D to assist calcium absorption in the body.
Animals obtain their nutrients through a broad variety of feeding patterns. Many animal species, such as sponges, feed on small particles of food that enter their pores. Other aquatic organisms, such as sea cucumbers, wave their tentacles about and trap food on their sticky surfaces. Mollusks, such as clams and oysters, feed by filtering materials through a layer of mucus in their gills. Other animal species, such as certain arthropods, feed exclusively on fluids.
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Some animals feed on food masses, and they usually have organs for seizing, chewing, and consuming food. Herbivores are animals that eat plants, while carnivores are animals that eat other animals. Omnivores, which consume both plants and animals, are typified by humans.
The mechanism by which organisms obtain food is referred to as the modes of nutrition. The organisms either synthesize their own food or obtain food prepared by other organisms in various ways. There are basically two modes of nutrition – autotrophic and heterotrophic.
‘auto’ means self and ‘trophic’ refers to food. So, the organisms, which synthesize their own food using CO2, are called the autotrophs and the process is called autotrophic nutrition.
Autotrophs include all green plants and some bacteria such as the nitrifying bacteria.
‘hetero’ refers to other or different and ‘trophic’ refers to food. Thus, the organisms that obtain their food from other organisms are called heterotrophs and the process of obtaining the food from other organisms is called heterotrophic nutrition. All heterotrophs depend directly or indirectly on the autotrophic organisms for their food and energy requirements. Heterotrophs include most of the bacteria, fungi and all animals.