56 



THE INDIVIDUAL ORGANISM 



passage through cell membranes. In consequence they require a digestive 

 process before they can be taken into the cells or blood stream and become 

 available for use. 



Carbohydrates contain carbon, hydrogen, and oxygen, usually with 

 the hydrogen and oxygen in the same proportion as in water (H 2 0). 

 Glucose (dextrose or grape sugar), C 6 Hi20 6 , is an example. It is readily 

 soluble in water and will pass through a cell membrane. Carbohydrates 

 with this chemical formula are known as single sugars, or monosaccharides. 

 Disaccharides — for example, cane sugar — have the formula C12H22O11 

 and may, under suitable conditions, be split into two molecules of a 



Fig. 4.1. Diagram of a cell in relation to artery, capillary, vein, and lymph vessel, to show 

 intake of oxygen and food, and outgo of carbon dioxide and metabolic wastes, via the tissue 

 fluid (black). A leucocyte is shown leaving a capillary and entering a lymph vessel. 



monosaccharide after a molecule of water has been added. Various 

 polysaccharides exist, the formulas of which may be expressed by the 

 generalized formula w(C 6 Hio0 5 ). Starch, glycogen, and cellulose are 

 examples. Under suitable conditions, polysaccharides may be split into 

 monosaccharides. Carbohydrates are primarily energy foods. When a 

 monosaccharide is oxidized in the body, energy is released, and CO2 

 (carbon dioxide) and H 2 (water) are the end products. 



C 6 H 12 6 + 60 2 -> 6CO2 + 6H 2 + energy 



Fats, like carbohydrates, are composed of carbon, hydrogen and 

 oxygen, but much less oxygen is present in proportion to the carbon and 

 hydrogen. As in the case of carbohydrates, oxidation of fats results in 

 the release of energy and production of carbon dioxide and water. How- 

 ever, more oxygen is required to oxidize a fat than is necessary to oxidize 



