Nutrition is the biological process by which an organism, such as a plant or animal, takes in and utilizes food. This food is converted to energy which is used to keep the organism alive. While plants and animals have distinctly different nutritional requirements, there are some commonalities.
Theories about nutrition have been posed throughout history. During the time of the ancient Greeks, Anaxagoras (c.500-c.428 B.C.) suggested that nutrition was a result of the fact that everything contained a small amount of everything else. For example, he believed that when food was eaten, the part of the food that was hair would become part of the hair while the part of it that was muscle would become muscle. Theories about plant nutrition were also advanced. For example, Aristotle believed that plants got all their nutritional needs from the soil. This theory was widely accepted until the seventeenth century when a Belgian physician named Johannes Baptista van Helmont ran an experiment to see whether that was true. He thought that if plants grew by absorbing soil, then the amount of soil that he started with would become less overtime as the plant grew. After five years he found that almost no soil was lost. He concluded that the main compound responsible for the increase in the plant's mass was the water that he gave the plant. In the eighteenth century, Stephen Hales (1677-1761) postulated the notion that plants were also nourished by the air. Later, all of these scientists were found to be at least partially correct.
Plants grow mainly by accumulating water in their cells. In this case, water is a nutrient because it supplies the hydrogen and some oxygen that is incorporated into compounds for photosynthesis. Most of the water that enters a plant is lost by transpiration. The water that is left behind works as a solvent and helps maintain the form of soft tissue. The dry weight of a plant is about 95% organic material and 5% inorganic minerals. Carbon , oxygen, and hydrogen are the most abundant elements found in the dry weight of a plant. Nitrogen, sulfur, and phosphorus are also relatively abundant.
Since plants and other photosynthetic organisms can produce many of their own nutrition requirements they are known as autotrophs. This means they can readily transform inorganic compounds into biologically useful organic compounds. And while plants can produce much of their own nutritional needs such as starch, they still require at least seventeen essential nutrients.
To some extent the minerals in a plant reflect the mineral content of the soil they are growing in. Some nutrients are essential. Elements required in large amounts by plants are known as macronutrients. Nine have been identified. These include carbon, oxygen and hydrogen which are the major components of the plant's organic compounds. Nitrogen and phosphorus are important components of the plant's genetic material, the nucleic acids. Nitrogen is also a key component of proteins, hormones, and coenzymes. Phosphorus is found in the phospholipid bilayer of the cell membranes. Other macronutrients include potassium which helps maintain water balance and is involved in protein synthesis. Calcium is an important part of the formation of cell walls. It also helps activate some enzymes and is involved in the way plant cells respond to stimuli. Magnesium is a component of chlorophyll, the energy producing organelle in plant cells.
In addition to the macronutrients, plants also require smaller amounts of micronutrients to grow properly. Most of these function as cofactors in biological reactions. These include chlorine which is involved in photosynthesis and water balance control. Iron makes up part of the cytochromes, the organelles which control electron transport. Other micronutrients include boron, manganese, zinc, copper, molybdenum, and nickel. Mineral deficiency can weaken or even kill a plant. Typically, when a plant is deficient in a nutrient, a variety of symptoms might result. For example, if a plant is deficient in magnesium, a key part of chlorophyll, the leaves turn yellow.
Plants take in their nutrient requirements in different ways. Some nutrients are absorbed from the soil by the roots. Others are absorbed through the air. Carbon dioxide, the most important, is taken in through the air. Some plants get nutrition by being predators. For example, mistletoe grows on trees and supplements its nutrition by absorbing nutrients from the tree. Carnivorous plants have specialized leaves that can act as traps for insects.
Unlike plants, animals are heterotrophs which means they are unable to live on inorganic nutrients alone. They need a supply of organic compounds for energy and growth. There are different types of animals and they can be classified by their nutrition requirements. Herbivores eat plants, algae and autotrophic bacteria. Carnivores eat other animals. Omnivores eat both plants and other animals. Animals are also categorized by the way in which they get there food. Suspension feeders sift food particles from the water. This includes animals such as clams and whales. Substrate feeders live in their food source and eat their way out. This is the strategy of many insect larvae. Fluid feeders suck nutrients from a host. Aphids or mosquitoes are examples. Bulk feeders eat large pieces of food.
Eating is called ingestion and it is the first step in food processing. Digestion is the second step and this involves the breaking down of food into molecules. Digestion breaks down chemical bonds in a process called enzymatic hydrolysis. Absorption is the third step. Here the appropriate compounds are taken in by the animal's cells. Elimination is the final stage in which unused and waste material are passed out of the digestive system.
Food has three primary functions for animals. First, it acts as fuel for various reactions. This energy runs all the chemical reactions and provides energy for locomotion. Food also provides the organic raw materials for the body to make new molecules. This is how animals get their source of carbon and nitrogen. And finally food provides the essential nutrients which animals can not make for themselves. Essential nutrients include some amino acids. There are eight amino acids that must be ingested because the human body can not produce them themselves. Certain fatty acids are also essential. These are unsaturated fatty acids, an example of which is linoleic acid. Vitamins are another required nutrient. There are 13 vitamins required in a typical human diet. They act as coenzymes and are required in small amounts. Minerals are inorganic materials that are essential to proper nutrition. For example, they are involved in such things as bone and tooth formation, iron metabolism and muscle activity. Minerals that are required include such things as calcium, phosphorus, and sodium. There are over 17 different minerals that are required in a human diet.
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