526 VERTEBRATE LIFE AND ORGANIZATION 



our diet contains more carbohydrates than fats or proteins, and the 

 carbohychates are the prime source of energy for the cells. 



The various kinds of single sugars that are absorbed are carried to 

 the li\er where most of them are converted to glycogen (animal starch) 

 for storage. When needed, liver glycogen is broken down and released 

 into the blood stream as glucose. The role of the liver in maintaining 

 a constant level of glucose in the blood was discussed in section 26. 

 The glucose molecules are carried to all the cells of the body where 

 they are oxidized via the Krebs citric acid cycle to carbon dioxide and 

 water and their energy is released. If sugars are absorbed in great 

 excess, not all are converted to glycogen and released as glucose. Some 

 are converted by the liver and other cells to fat and then transported 

 to the subcutaneous connective tissue and other sites for storage. It is a 

 common observation that an excessive intake of carbohydrates or pro- 

 teins is just as fattening as an excessive intake of fats. 



Absorbed fats may be metabolized in the citric acid cycle to yield 

 energy for cellular activities, but fats are also an important raw ma- 

 terial in the synthesis of components of protoplasm. The plasma and 

 nuclear membranes and the membranes around mitochondria contain 

 many lipids. 



Most of the amino acids are used as raw materials for the synthesis 

 of proteins— the major constituents of protoplasm. A small amount of 

 amino acids may be stored as such in the liver and other organs but 

 most of those not used as raw materials undergo various conversions. If 

 the amino group is stripped off (deamination), the rest of the molecule 

 can enter the citric acid cycle to be used immediately as a source of 

 energy, or it can be converted to glycogen or fat. Deamination occurs 

 principally in the cells of the liver, but it can take place in any of the 

 cells of the body. After deamination, the amino group is converted to 

 ammonia, a toxic substance that would be injurious if it accumulated 

 in the cells. In mammals, ammonia is combined with carbon dioxide to 

 form the less toxic urea, which is excreted by the kidney. Urea syn- 

 thesis takes place in the liver and kidney cells and involves a number 

 of intermediate steps, including the temporary combination of am- 

 monia and carbon dioxide with ornithine and the eventual release of 

 ornithine. This series of reactions is known as the urea cycle. 



Other absorbed materials include minerals, steroids, nucleotides, 

 water and vitamins. Most of these substances are involved in the syn- 

 thesis of protoplasm, and have been discussed in Chapter 2. However, 

 a bit more should be said concerning the vitamins at this time. By 

 definition, the vitamins are organic substances that an animal needs in 

 minute amounts and must obtain from its environment, for they can- 

 not be synthesized by the animal in question, at least not in adequate 

 quantity. In so far as their specific role in metabolism is understood, 

 they are constituents of coenzymes. If they are lacking in the diet, the 

 reservoir of vitamins that can be stored in the body cells (chiefly liver 

 cells) is used up, metabolic processes dependent on these coenzymes 

 are impaired, and deficiency diseases result. A list of the more common 



