GENERAL ZOOLOGY 



The hexose carbohydrates are the most efficient source of energy available 

 in the diet. Glucose in excess of the immediate requirements of cellular 

 metabolism can be built up into the polysaccharide glycogen and stored for 

 future use. Some of the polysaccharides, such as the cellulose occurring in 

 plants, cannot be digested by man and comprise part of the roughage which 

 gives necessary bulk to the feces. 



Lipids in the diet provide a major source of energy for the animal. When 

 oxidized, fats yield over twice as much energy per unit weight as other foods. 

 Also, the oxidation of fat releases about twice as much water as other food- 

 stuffs. This is an important factor in providing some animals with a supply 

 of water under adverse conditions. For example, the desert-dwelling camel 

 stores much fat and has a high level of fat metabolism. Also, the developing 

 chick embryo, shut off for 21 days from supplies outside its eggshell, gets its 

 necessary water from the oxidation of the lipid which makes up about 90 per 

 cent of the dry weight of the yolk of the hen's egg. About 1 per cent of the 

 dietary lipid must contain what are called essential fatty acids, of which there 

 are three. These are unsaturated fatty acids found in many food oils. In 

 their absence, various skin and nervous disorders may occur in man; the 

 cellular basis for these disturbances is not known. Both carbohydrates and 

 proteins can be converted to fat in cellular metabolism, and such excess or 

 reserve food is stored in cells as fat. 



Proteins are an indispensable part of the diet for young and old. Growth 

 and restoration of body proteins lost in daily activities can be accomplished 

 only when a supply of amino acids is available. Of the 25 amino acids known 

 to be constituents of proteins, only 8 are essential for human nutrition. This 

 means that they must be supplied to man in his dietary protein because he is 

 unable to synthesize them from other foodstuffs at a rate necessary for normal 

 function. It is important that all the essential amino acids be provided in 

 the required amounts at approximately the same time. If all the amino acids 

 required for the synthesis of a particular protein are not simultaneously 

 present, those that are present will be degraded and lost from the so-called 

 protein pool. Amino acids, whether essential or non-essential, cannot be 

 stored in cells. Protein foods are not equally efficient in supplying essential 

 amino acids; eggs, dairy products, liver, and kidney contain all of them. 



Vitamins are accessory dietary components which play important roles in 

 cellular metabolism, although required in very small amounts. Members 

 of this heterogeneous group of organic compounds are not sources of energy, 

 nor are they used to replace worn-out cellular components. Instead, they 

 make possible the utilization of other foodstuffs in normal metabolic sequences; 

 some have been demonstrated to be coenzymes. Thus, inadequate amounts 

 of vitamins may result in metabolic defects which in time give rise to clinically 

 recognizable diseases. 



To be classified as a vitamin for a particular kind of animal, the substance 

 must be one which the animal requires in its food. This definition differen- 

 tiates vitamins from hormones and enzymes, which also act in very small 



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