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nutrition,study of the materials that nourish an organism and of the manner in which the separate components are used for maintenance, repair, growth, and reproduction. Nutrition is achieved in various ways by different forms of life. Plants that contain the green pigment chlorophyllchlorophyll
, green pigment that gives most plants their color and enables them to carry on the process of photosynthesis. Chemically, chlorophyll has several similar forms, each containing a complex ring structure and a long hydrocarbon tail.
..... Click the link for more information. can synthesize their food from inorganic substances in the process called photosynthesisphotosynthesis
, process in which green plants, algae, and cyanobacteria utilize the energy of sunlight to manufacture carbohydrates from carbon dioxide and water in the presence of chlorophyll. Some of the plants that lack chlorophyll, e.g.
..... Click the link for more information. . Organisms such as plants that can thus manufacture complex organic compounds from simple inorganic nutrients are termed autotrophic. Organisms that must obtain "prefabricated" organic compounds from their environment are heterotrophic, and these include the fungi, some other plants, and animals. Heterotrophic plants may be saprophytic (obtaining nutrients from dead organisms) or parasitic (obtaining nutrients from living organisms while living on or in them). Heterotrophic animals may be parasites, herbivores (plant eaters), carnivores (meat eaters), or omnivores (obtaining nutrition from both plants and animals).
Humans require food substances to supply the components necessary to build tissues, to repair tissues as they wear out and die, to keep the body in good working condition, and to supply fuel for energy. For good nutrition a person should eat a well-balanced diet, that is, one that provides an adequate amount of each of the classes of nutrients each day, furnishing at the same time an adequate but not excessive number of calories for the body's energy needs. Children require relatively larger amounts of nutrients and calories because of their rapid growth. The foods required for proper nutrition fall roughly into three major groups: proteins, carbohydrates, and fats; vitamins, minerals, and water are also important.
any of the group of highly complex organic compounds found in all living cells and comprising the most abundant class of all biological molecules. Protein comprises approximately 50% of cellular dry weight.
..... Click the link for more information. in the diet provides amino acidsamino acid
, any one of a class of simple organic compounds containing carbon, hydrogen, oxygen, nitrogen, and in certain cases sulfur. These compounds are the building blocks of proteins.
..... Click the link for more information. for forming body proteins, including the structural proteins for building and repairing tissues, and the enzymes for carrying out the metabolic processes. In addition, protein may be used as a source of energy when the preferred fat and carbohydrate supply runs low. A body that is in the process of building itself (such as that of a growing child or an adult recovering from illness) will need a greater proportion of protein to weight than one that is fully grown and utilizes protein merely for repair of worn-out tissues. The average adult requires 1 gram of protein per kilogram of body weight per day; children may require two to three times this amount. Human proteins consist of up to 22 different amino acids, of which 9 (called essential amino acids) must be supplied by food protein; the other 13 are synthesized by human cells. Complete protein sources—those foods containing all 22 amino acids—include animal products such as meat, eggs, cheese, and milk. Incomplete protein sources, such as vegetables, beans, and grains, may be combined to create complete proteins.
any member of a large class of chemical compounds that includes sugars, starches, cellulose, and related compounds. These compounds are produced naturally by green plants from carbon dioxide and water (see photosynthesis).
..... Click the link for more information. (starches and sugars) provide a readily available energy source. Surplus carbohydrates are also converted by the body to glycogenglycogen
, starchlike polysaccharide (see carbohydrate) that is found in the liver and muscles of humans and the higher animals and in the cells of the lower animals. Chemically it is a highly branched condensation polymer of glucose; it is readily hydrolyzed to glucose.
..... Click the link for more information. and fat, the storage forms of caloriescalorie,
abbr. cal, unit of heat energy in the metric system. The measurement of heat is called calorimetry. The calorie, or gram calorie, is the quantity of heat required to raise the temperature of 1 gram of pure water 1°C;.
..... Click the link for more information. for energy, and to some of the amino acids used in protein synthesis. Most health professionals recommend that carbohydrates comprise 50% to 60% of the dietary calories, of which most (c.80% of all carbohydrates eaten) should be complex carbohydrates, such as cereals and vegetables. Complex carbohydrates are preferred because the fast-acting simple carbohydrates, such as honey and sugar, are difficult for the body (especially the pancreas) to handle in large doses. Simple carbohydrates also lack the vitamins, minerals, proteins, and fiberfiber, dietary,
bulky part of food that cannot be broken down by enzymes in the small intestine of the digestive system. Almost all natural fiber comes from plants. Although fiber has little nutritional value, it offers other health benefits.
..... Click the link for more information. that generally accompany foods rich in complex carbohydrates. Cereals, fruits, vegetables, legumes, and pasta are good sources of complex carbohydrates.
Fats (see fats and oilsfats and oils,
group of organic substances that form an important part of the diet and also are useful in many industries. The fats are usually solid, the oils generally liquid at ordinary room temperatures.
..... Click the link for more information. ) in the diet provide a concentrated source of energy; 1 gram of fat supplies about 9 calories as opposed to only 4 calories per gram of carbohydrates and protein. Fats in the body, in addition to acting as a source of stored energy, supply physical protection and insulation for tissues and form important portions of cell membrane structure. Fats also aid in the absorption of the fat-soluble vitamins (vitamins A, D, E, and K) from the intestine. Milk, butter, meat, and oils are important sources of fat.
Vitamins, Minerals, and Water
To keep the body functioning properly it is necessary to have, in addition to the basic foods, a sufficient intake of accessory substances such as vitaminsvitamin,
group of organic substances that are required in the diet of humans and animals for normal growth, maintenance of life, and normal reproduction. Vitamins act as catalysts; very often either the vitamins themselves are coenzymes, or they form integral parts of coenzymes.
..... Click the link for more information. , minerals (see mineral, dietarymineral, dietary,
any of a group of inorganic elements that are essential to humans and animals for normal body function. In nutrition, minerals are those elements for which the body's requirement is at least 100 mg per day, and trace minerals
..... Click the link for more information. ), and enough water to carry nutrients to the tissues and waste products away from them. A minimum of about 2 liters of liquid per day are recommended for the average adult. Vitamins function as coenzymescoenzyme
, any one of a group of relatively small organic molecules required for the catalytic function of certain enzymes. A coenzyme may either be attached by covalent bonds to a particular enzyme or exist freely in solution, but in either case it participates intimately in
..... Click the link for more information. in important body processes, with the exception of vitamin D, which is synthesized upon exposure to sunlight. A large variety of minerals are required, some in trace amounts and others, such as calcium and iron, in relatively large amounts. Milk, cheese, and dark, leafy green vegetables are excellent sources of calcium; liver, meat, and egg yolks are good sources of iron. Minerals are vital to the development of teeth and bones (calcium, phosphorus, and fluoride) and to the functioning of a number of the body's metabolic systems. Iron is a necessary part of hemoglobin in the blood; various metals are required in many enzymes; sodium and potassium are essential to maintenance of fluid balance and functioning of the nervous system; magnesium is needed for the normal functioning of nerves and muscles; and iodine is required for thyroid hormone. The usefulness of vitamin and mineral supplements for a person of good health who eats a well-balanced diet continues to provoke debate among health experts.
Importance of Good Nutrition
Good nutrition is reflected not only in the growth and function of the body but also in its appearance. The eyes, skin, hair, and teeth indicate whether body nourishment is good or poor. A poorly nourished child will fail to grow properly; a poorly nourished adult will have a decreased resistance to infection and disease. A diet deficient in proteins causes a disease called kwashiorkor in children; a diet deficient in both protein and calories results in marasmus, with lethargy, abdominal enlargement, and wasting—the classical malnutrition syndrome. Poor nutrition may result from excesses in the diet as well as deficiencies; excess of certain vitamins or minerals can produce potentially lethal disease states, and excess of carbohydrates or fat can result in obesity.
The Food Guide Pyramid
The U.S. Dept. of Agriculture (USDA) illustrates a well-balanced diet with the Food Guide Pyramid (1992), which emphasizes the need to eat less fat and proportionally more complex carbohydrates. At the base of the pyramid is the grains group, which should be eaten in the largest quantity (6–11 servings per day). Just above are the two groups fruits (2–4 servings) and vegetables (3–5 servings). Near the top are the meat products (2–3 servings) and dairy products (2–3 servings). At the apex are the fats, oils, and sweets, which are not considered a food group and should be consumed sparingly. See also food pyramidfood pyramid
or Food Guide Pyramid,
diagram used in nutrition education that fits food groups into a triangle and notes that, for a healthful diet, those at the base should be eaten more frequently than those at the top.
..... Click the link for more information. .
Specialized diets are useful in the treatment of certain disease states; the most common is a low-calorie diet to produce weight loss in obese persons. A diet low in phenylalaninephenylalanine
, organic compound, one of the 22 α-amino acids commonly found in animal proteins. Only the l-stereoisomer appears in mammalian protein.
..... Click the link for more information. is used to treat phenylketonuriaphenylketonuria
(PKU), inherited metabolic disorder caused by a deficiency in a specific enzyme (phenylalanine hydroxylase). The absence of this enzyme, a recessive trait, prevents the body from making use of phenylalanine, one of the amino acids in most protein-rich foods, and
..... Click the link for more information. . A diet low in cholesterolcholesterol
, fatty lipid found in the body tissues and blood plasma of vertebrates; it is only sparingly soluble in water, but much more soluble in some organic solvents. A steroid, cholesterol can be found in large concentrations in the brain, spinal cord, and liver.
..... Click the link for more information. and saturated fats seems to be useful in the treatment and prevention of heart disease. Elimination of certain foods from the diet may be necessary to control allergies in some individuals. In all cases, however, specialized diets must provide all classes of essential nutrients in adequate amounts to maintain health in adults and support growth in children.
See J. Brody, Jane Brody's Nutrition Book (1981); S. Gershoff, The Tufts University Guide to Total Nutrition (1990); J. Mayer, Jean Mayer's Diet and Nutrition Guide (1990).
the process of ingesting and assimilating substances that an organism needs to meet energy requirements, to build and renew body tissues, and to regulate functions. Nutrition is an important part of metabolism. The science of nutrition deals with many problems in human physiology and in the physiology of wild and domesticated animals.
In animals. The food relationships between different animal species are the basis of the biogenous cycle of matter. They define the species’ community ties, they constitute the most important factor in population control, and they are the principal link between animals and the environment. Endogenous nutrition—feeding on food reserves in the body, for example, during famine or hibernation—is distinguished from exogenous nutrition— feeding on sustances that enter from the external environment. Food substances that are obtained by the body decompose into relatively simple chemical compounds, which after absorption are used to build tissues and organs. Nutrition is essential to the realization of all living processes, for example, muscle contraction, acclimatization to temperature and altitude changes, reproduction, and milk formation. Malnutrition lowers the body’s resistance to cold and infection and disrupts the physiology of reproduction, including the reproductive cycle, sexual arousal, and lactation; it can even lead to cessation of reproduction, as for example, in a number of rodent species. In predators, for example, the sable, ingestion of fats and lipoids is necessary to maintain reproductive capacity.
The process of nutrition is conventionally divided into four phases: food gathering, digestion, absorption, and assimilation. Nutrition is effected by various means in protozoans. Many actively ingest food through phagocytosis. Complex adaptations arise for capturing the food, for example, special systems of ciliary structures called membranelles and a mouth equipped with a threadlike apparatus. Some protozoans, mainly parasites, such as trypanosomes, live in mediums that are rich in nutrient matter and consequently feed by absorption, mainly through pinocytosis.
Nutrition determines a species’ choice of habitat and the size of the population. Animals are classified as being euryphagous —feeding on many diverse foods; stenophagous—feeding on a limited set of foods; or monophagous—feeding on only one type of food. For the most part, the fauna of tropical forests is stenophagous; to a lesser extent, so are animals that live at high latitudes.
The type of nutrition induces specific reflexes and determines the morphology of dentition, of the gastrointestinal tract, and of the sense organs. Examples of nutritionally induced reflexes are hoarding of food by certain rodents, which ensures a plentiful winter supply; the sucking reflex and the reflexive attraction to fur and warm surfaces in animals that are born blind; the reflexive brushing aside of a moving object by ungulates that are born sighted; and shaking of the nest in rooks. Except for hoarding, all these reflexes are present at birth or hatching. As the individual develops, numerous conditioned feeding reflexes are formed on the basis of the unconditioned reflexes. Nutrition is influenced by season, which affects migration, reproduction, and consequently population dynamics.
In humans. Nutrition is an external factor that substantially influences health, the ability to work, and longevity in humans. Nutritional hygiene deals with the study of the fundamentals of a sensible diet for healthy humans; dietetics studies nutrition for the ill. The science of nutrition investigates not only the requirements of a full diet but also attempts to determine the optimal conditions for synthesizing essential nutrients within the body itself.
Great emphasis is placed on investigating and evaluating nutritional conditions of populations throughout the world. Research is being conducted to increase food productivity; to create new food products, especially protein-containing foods; and to increase the nutritional value of foods by enriching with biologically active substances, for example, mineral salts and vitamins.
Nutritional deficiencies lead to disturbances of function in specific organs and systems and to cachexia, or a general weakening of the body. An adult human who performs work of average difficulty requires 3,000 kilocalories (kcal) daily (1 kcal =4.19 kilojoules). Table 1 presents the daily requirements for adult males who live in populated areas with developed community services.
Inadequate nutrition has an especially deleterious effect on children, retarding growth and physical and mental development and lowering resistance to various diseases. Vitamin deficiency produces hypovitaminoses and avitaminoses. Excessive feeding can result in many pathological conditions, including obesity, atherosclerosis, diabetes mellitus, and metabolic disturbances. Protein deficiency in the diet of children may produce the severe dystrophy kwashiorkor. The etiology of some serious diseases, for example, dysentery and food poisoning, is nutrition related.
A sound diet must meet both quality and quantity requirements.
|Table 1. Value and distribution of daily caloric requirement for adult males in the USSR according to type of work1|
|Type of work||Age range(years)||Caloric requirement (kcal/day)||Distribution (grams)|
|1The lesser metabolic intensity and body weight decrease the requirements in calories, proteins, fats, and carbohydrates for women by 15 percent. The daily caloric requirement increases 200 kcal for persons who reside in populated areas with underdeveloped community services.|
|Entails little or no physical exertion||2,800|
|Mechanized industrial, entails little physical exertion||3,000|
|Nonmechanized industrial, entails significant physical exertion||3,200|
|Nonmechanized industrial, entails moderate to great physical exertion||3,700|
|Table 2. Caloric content and chemical composition of some foods (per 100g)|
|Food||Caloric content (kcal)||Chemical composition|
|Proteins (g)||Fats (9)||Carbohydrates (g)||Vitamins (mg)||Minerals (mg)|
|Milk (yogurt, sour milk) .............||62||3.0||3.5||4.5||0.05||0.05||1.0||120.0||127.0|
|Sour cream, grade I .............||285||2.1||28.0||3.0||0.30||0.05||-||86.0||91.0|
|Cheese, cottage, whole............||230||11.0||19.0||3.0||-||-||-||140.0||-|
|Cheese, cottage, fat free..........||75||14.0||0.5||3.5||-||-||-||164.0||-|
|Cheese, Sovetskii .............||380||21.0||30.0||2.5||0.22||0.06||-||700.0||-|
|Beef, grade I ................||154||15.0||10.0||-||0.01||0.08||-||8.0||238.0|
|Beef, grade II..................||106||18.0||4.0||-||-||-||-||9.0||259.0|
|Mutton, grade I ...................||206||14.0||16.0||-||-||0.13||-||7.0||217.0|
|Sausage, Liubitel’skaia .............||290||12.0||26.0||-||-||0.33||-||7.0||213.0|
|Pike perch ...................||72||16.0||1.0||-||-||0.02||-||11.0||162.0|
|Herring, Atlantic spring, salted ‖.........||120||16.0||6.0||-||Trace||0.02||-||58.0||144.0|
|Bread, rye ..............||240||5.1||1.0||42.5||-||0.15||-||29.0||249.0|
|Milk chocolate ............||568||5.8||37.0||47.0||-||-||-||175.0||487.0|
|Groats, wheat ............||335||10.1||2.3||66.5||-||0.30||-||30.0||286.0|
|Cabbage, white ..........||27||1.5||-||5.2||Trace||0.05||24.0||38.0||148.0|
|Carrot, red .............||36||1.3||-||7.6||9.0||0.05||4.0||34.0||129.0|
|Tomato, red ............||18||0.5||-||4.0||2.0||0.05||34.0||10.0||150.0|
|Strawberries, wild .........||43||1.5||-||8.9||Trace||0.02||51.0||19.0||137.0|
|Mushrooms, cèpe .........||32||4.6||0.5||3.0||-||-||-||20.0||-|
|Mushrooms, honey agaric ............||23||1.7||0.5||3.8||-||-||-||-||-|
|Table 3. Value and distribution of average daily caloric intake by country|
|Average daily caloric intake (kcal/inhabitant/ day)||Protein content (g/day)||Distribution among various foods (percent)|
|Grains||Potato andother root crops||Vegetables, legumes, nuts, fruits||Sugar, sugar products||Meat, eggs||Fish||Milk, milk products|
|Arab Republic of Egypt ......||2,940||85.1||72.6||1.0||6.4||9.2||1.9||0.7||8.5|
It must contain optimal ratios of the components of food, for example, essential and nonessential amino acids, polyunsaturated fatty acids, phosphatides, and sterols, as well as fats, sugars, vitamins, minerals, and organic acids. More than 60 nutritional substances must be present in the correct ratios. A sound diet provides an organism with the structural, energy, and regulatory substances that are necessary for normal functioning. Structural substances are used to build new cells and tissues and to replace old ones; they include proteins, some fats, and some minerals, for example, calcium and phosphorus. The energy food substances are carbohydrates, fats, and, to some extent, proteins. Regulatory substances, including trace elements and vitamins, participate in metabolism and perform catalytic and other regulatory functions.
A diet that excludes certain groups of foods over a long period of time disturbs the balance of food components and adversely affects the processes of assimilation and synthesis. The ideal human diet should include proteins, fats, and carbohydrates in a ratio of 1:1:4. Fifteen percent of the daily caloric intake should be accounted for by proteins, 30 percent by fats, and 55 percent by carbohydrates. At least half the amount of proteins should be of animal origin, while the fats should be 75–80 percent animal fat and 20–25 percent vegetable oil. A healthy diet must include as the principal sources of proteins and fats meat, fish, and milk products. Vegetables and fruits should also be included as the sources of carbohydrates, minerals, and vitamins (see Table 2).
Well-balanced diets are typical of the more developed countries. This applies first of all to the developed socialist countries of Europe, where the attainment of a high standard of living is one of the major goals of a planned economic policy. The general degree of food distribution is fairly high in the developed countries of Western Europe and in the USA, Canada, and Australia.
In many Asian and African countries the average daily caloric intake is less than 2,500 kcal; often, it is even less than 2,000 kcal. At the same time, the food is low in protein and fats (see Table 3). According to UN estimates, about one-half of the world’s population is underfed—in many cases, starving—and that half is concentrated in the developing countries.
Milk is the most widely consumed component of the human diet. The carbohydrates, proteins, and fats that it contains are in almost optimal proportions and are easily assimilated. However, some very populated nations, such as China, where the raising of milk-producing animals is underdeveloped, use practically no milk or milk products. The consumption of milk is also very low in India.
Daily food requirements differ in composition, depending on the climatic region (see Table 4). For example, in the European USSR the daily fat intake should be lower for inhabitants of southern regions than for inhabitants of northern regions; the proportion of protein in the diet is constant for all climatic regions. Recommended norms with respect to carbohydrates vary, being generally higher for southerners than for northerners.
Special norms that take into account the needs of a growing body have been established for children (see). Nutrition for middle-aged and older persons is recommended with the requirements of the aging body in mind. Aging is characterized by progressive atrophy and by decrease in the intensity of oxidative processes, in cellular activity, in metabolic rate, and in the functions of all systems, including the digestive system. With aging, some degree of limitation of food intake is recomended (see Table 5).
|Table 4. Value and distribution of daily caloric requirement by climatic region|
|Caloric requirement (kcal/day)||Distribution|
It is important that meals be eaten at a fixed hour each day. For an adult human, four meals per day at intervals of four to five hours is optimal. Under these conditions, the strain on the digestive tract is uniform and the most complete processing of food by completely active digestive juices is ensured. Distribution of caloric intake over four meals depends on the individual’s schedule. It is recommended that breakfast provide 25 percent of the daily caloric intake; lunch 15 percent; dinner 35 percent; and supper 25 percent. For older persons or persons whose work is mental, caloric intake may be more evenly distributed, omitting large differences between the caloric value of breakfast and dinner. At present, the food service industry is an important nutritional concern.
REFERENCESPokrovskii, A. A. Besedy o pitanii, 2nd ed. Moscow, 1968.
Gigiena pitaniia, vols. 1–2. Edited by K. S. Petrovskii. Moscow, 1971.
Lechebnoe pitanie. Edited by I. S. Savoshchenko. Moscow, 1971.
V. A. KUDASHEVA and K. S. PETROVSKII
in botany, the process by which plants absorb and assimilate from the environment the chemical elements necessary for their existence. It involves the movement of matter from the environment to the cytoplasm of the plant cells and the chemical transformation of the matter into compounds characterstic
|Table 5. Value and distribution of daily caloric requirement for the elderly in the USSR1|
|Age range (years)||Caloric requirement (kcal/day)||Distribution (grams)|
1The caloric requirement increases 100–150 kcal for persons who reside in populated areas with underdeveloped community services and for persons who engage in active forms of recreation.
of the given plant species. The absorption and assimilation of nutrient matter (anabolism) together with the decomposition and excretion of the matter (catabolism) are the two components of metabolism—the basis of the life processes of an organism.
Almost all chemical elements have been found in plants. However, only the following elements are essential for plant nutrition: carbon (C), oxygen (O), hydrogen (H), nitrogen (N), phosphorus (P), sulfur (S), potassium (K), calcium (Ca), magnesium (Mg), and iron (Fe). Essential trace elements include boron (B), manganese (Mn), zinc (Zn), copper (Cu), and molybdenum (Mo). The nutritional elements are absorbed from the air as carbon dioxide gas (CO2) and from the soil as water (H2o) and ions of mineral salts. In higher terrestrial plants a distinction is made between atmospheric, or leaf, nutrition (see PHOTOSYNTHESIS) and soil, or root, nutrition (seeMINERAL NUTRITION OF PLANTS). The entire surface of lower plants (bacteria, fungi, algae) absorbs CO2, H2O, and salts.
A plant requires different amounts of each element. Most essential in large amounts are oxygen and hydrogen, since a plant consists of 80–90 percent water. Hence, in plants the weight ratio of oxygen to hydrogen is similar to that of water, that is, 8:1. Moreover, a plant expends in transpiration hundreds of times more than its own weight in water during its lifetime (for the conversion of excess heat). The dry matter of a plant consists principally of carbon (45 percent) in combination with oxygen (42 percent), and hydrogen (6–7 percent). Mineral elements, among which nitrogen and potassium predominate, constitute only 5–7 percent of the dry matter of the plant.
No one element can be replaced by another (the principle of irreplaceability of nutritional elements). The absence or severe deficiency of any element leads to the cessation of growth and the death of the plant. Each element performs a particular function in the plant tissues, which is indissolubly related to all other functions of the organism. Thus, all organic molecules generally consist of carbon in combination with hydrogen and oxygen (see BIOGENIC ELEMENTS). Substances consisting only of these three elements (carbohydrates) are the main foods used in respiration. The membranes of plant cells also consist of polymeric carbohydrates. Each plant species or variety predominantly absorbs those elements that are essential in the largest quantities for its characteristic metabolism. For example, the potassium content in plants is usually dozens of times greater than the sodium content, although the ratio of these elements is reversed in the soil. Some plant species accumulate rare elements in their tissues. Such an element is lanthanum, and this phenomenon is used in geological exploration (see INDICATOR PLANTS).
There are several types of plant nutrition, each having a different source of carbon. Some lower plants—all fungi and most bacteria—can use carbon only from organic compounds in which it is contained in reduced form. After the plants oxidize the compounds during respiration, the chemical energy stored in the compounds is released and expended on such endergonic processes as the synthesis of more complex compounds and the movement of substances in the plant. Nutrition of this type and plants requiring.an organic source of carbon are said to be heterotrophic (see HETEROTROPHIC ORGANISMS).
Nutrition from dead organic remains is called saprophytic, and plants that feed on such remains are called saprophytes. Saprophytic nutrition characterizes all putrefactive fungi and bacteria.
Heterotrophic plants that metabolize the organic compounds of other living organisms are parasites. They include all fungi and bacteria that are causative agents of animal and plant diseases, as well as such higher plants as broom rape that feed on the juices of other plants by means of special suckers. Parasitic plant nutrition differs from symbiosis, which is characterized by a constant, mutually beneficial association between two organisms. Symbiotic plant nutrition is observed in nitrogen-fixing bacteria that settle in nodules on the roots of leguminous plants (seeNITROGEN FIXATION); in hymenomycetous fungi, whose hyphae penetrate the root tissues of woody plants (seeMYCORRHIZA); and in lichens, which consist of a group of fungi that are in permanent cohabitation with algae.
A large number of plants can assimilate carbon from carbon dioxide, reducing the gas to organic compounds. This type of nutrition, known as autotrophic nutrition (seeAUTOTROPHIC ORGANISMS), is characteristic of all higher green plants, all algae, and some bacteria. The reduction of CO2 to organic compounds requires an expenditure of energy by means of photosynthesis or chemosynthesis.
Plant nutrition is essential to the biogeochemical cycle of matter in nature. Autotrophic plants—mainly photosynthetic autotrophs—give rise to the cycle, removing CO2 from the atmosphere and creating organic substances that are rich in energy. Heterotrophic plants, mainly saprophytes, close the cycle by breaking down dead organic remains into the original mineral substances.
During photosynthesis, plants not only absorb matter but accumulate energy. One of the primary products of photosynthesis is sugar. When 6 gram molecules of CO2 unite with the same quantity of H2O, 1 gram molecule of glucose (180 g) is formed. This process occurs with the absorption of 674 kilocalories (1 kcal = 4.19 kJ) of radiant energy, which is stored in the chemical bonds of the sugar. Together with the molecules of sugar, the stored chemical energy may then be moved to other, nonphotosynthesizing plant organs, such as the root. The energy may then be released during respiration to synthesize more complex compounds and to make possible other cellular processes. Although only CO2 and H2o participate directly in photosynthesis, all other elements of plant nutrition, however small their quantity, are essential for the process and, especially, for the subsequent conversion of the process’s primary products.
The conversions of nutritional substances occur in various organs and tissues and are associated with one another in a continuous cycle of matter within the plant. During photosynthesis, the primary organic products (assimilates) are formed in the leaves from the CO2 of the air and the H2O entering from the roots. One of these assimilates—sucrose—is the universal form of carbohydrate transport. From the photosynthesizing cells of the leaves, sucrose enters a special transport system, the sieve tubes of the phloem, which ensure the descending movement of matter first through the leaf veins and then through the conducting bundles of the stem into the root. Here the assimilates leave the sieve tubes and are distributed throughout the root tissues. Water and ions of mineral salts, which first are bound by the surfaces of root cells and then permeate the cells through the cell membranes, meet the assimilates flowing in from the leaves. At the same time, some elements (potassium, sodium, calcium, magnesium) enter the sap and reach aboveground organs in an immutable state. Other elements, such as nitrogen, upon meeting the centrifugal flow of assimilates, interact with them. They become part of the composition of organic componds (amino acids and amides) and enter the sap in this changed form. A third group of elements, which includes phosphorus, upon passing through the root tissues, is also included in organic compounds (nucleotides and phosphoric esters of sugars), but the elements break off again and enter the sap mainly in the form of free ions.
One way or another, the elements of root nutrition, together with water, enter the vessels of the xylem—the second transport system of the plant. The xylem provides for the ascending movement of matter to aboveground organs. The movement of water and the dissolved elements in the vessels is due to root pressure and transpiration. In the leaf, the substances from the vessels enter the photosynthesizing cells, where the subtances’ secondary interaction with assimilates occurs. At this time, the most diverse organic and organomineral compounds are formed. After a series of complex reactions, new organs of the plant are formed from the compounds.
Plant nutrition furnishes substances and energy for the following processes: maintenance of vital activity (replacement of nutrient loss during respiration and excretion to the environment), growth of organs, deposit of matter for storage, and reproduction (formation of fruits and seeds).
When plants receive inadequate amounts of nutrient substances, the processes of vital activity and reproduction are the first to receive nutrients. If the deficiency is moderate, the growth of young parts of the plant (upper leaves, root tips) still continues by means of reutilization, that is, the repeated use of nutritive elements by means of their effluence from older leaves. With severe nutrient deficiency, growth ceases and all nutrient resources are directed to the principal function of a plant organism —reproduction. Nutrient-deficient barley, for example, grows only to a height of 4–5 cm but forms two or three normal caryopses. An excess of any one nutritive element is as harmful as a deficiency.
The most effective means of maintaining the yield of agricultural plants is the creation of the best conditions of soil nutrition through the use of irrigation and fertilization. In hotbeds and hothouses, the atmospheric nutrition of plants may be regulated by changing the CO2 content of the air and by using supplementary lighting. The creation of the optimal set of conditions for plant nutrition is one of the principal goals of horticulture. Also important is the implementation of measures for reclaiming saline soils (removal of excess salts, which are harmful to plant nutrition) and agrotechnical measures of improving the soil (compacting and aeration). Weeds, which compete with crops for nutrients, must be controlled.
REFERENCESTimiriazev, K. A. Zhizn’ rastenii (collection of articles), vol. 3. Moscow, 1949.
Sabinin, D. A. Fiziologicheskie osnovy pitaniia rastenii. Moscow, 1965.
Maksimov, N. A. Kak zhivet rastenie, 4th ed. [Moscow, 1966.]
D. B. VAKHMISTROV