STRUCTURE AND ACTIVITIES OF CELLS 



the living state in the cell. The enzymes of the cell maintain the steady state 

 of utilization of food, transfer of energy, and synthesis of cellular components. 

 They control the pace and the pattern of cellular life. 



Food 



Food is necessary for the maintenance of the structure and function of cells; 

 it provides the chemical units needed to replace the ones that are constantly 

 being destroyed in metabolic processes and lost by excretion (p. 82). In 

 young organisms growth depends on proper food; and food is the fuel that 

 provides the energy required to do the mechanical, electrical, and chemical 

 work of the organism. If the chemical nature of the cell contents is recalled, 

 it will be evident that food must contain water, inorganic salts, carbohydrates, 

 lipids, and proteins. In addition, a group of compounds known as vitamins is 

 necessary. 



Water, which is the most abundant constituent of the cell and body, is also, 

 with the exception of oxygen, the most important. It is present as such in the 

 so-called solid foods which are consumed and is produced in the cell by the 

 oxidation of organic foods. Water is taken in as such by terrestrial vertebrates, 

 and man supplements his intake with a variety of beverages. 



Although inorganic salts constitute only a small percentage of the chemical 

 compounds of the organism, they are of widespread importance. Inorganic 

 ions are components of various enzyme systems, of several vitamins and 

 hormones, and of respiratory pigments; they are required for such processes 

 as the conduction of nerve impulses, the contraction of muscle, and the 

 clotting of blood. The, diet must contain calcium, phosphorus, sodium, 

 potassium, magnesium, sulfur, and chlorine, which together comprise 60 to 

 80 per cent of the total inorganic material of the human body. In addition, 

 traces of iron, copper, iodine, manganese, cobalt, and zinc are required. 

 Milk is a source of calcium and phosphorus, both of which are necessary in 

 proper proportion for formation and maintenance of bone. Phosphorus is 

 also exceedingly important in cellular metabolism, as we shall see. Other 

 inorganic substances are obtained from milk, from drinking water, and from 

 plants grown on soils containing adequate amounts of minerals. 



To determine the energy value of various food constituents, the chemist 

 burns or oxidizes them in a bomb calorimeter; the amount of heat given oflT 

 in this complete combustion is measured in calories. A calorie is the quan- 

 tity of heat required to raise the temperature of one gram of water one 

 degree centigrade. Some of the energy which is released from food during 

 cellular metabolism (p. 37) is transformed into heat which aflfects the body 

 temperature. Special heat-regulatory mechanisms in warm-blooded animals, 

 the birds and mammals, control loss of heat and make possible a relatively 

 constant body temperature (p. 125). 



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