THE BIOLOGY OF THE CELL SURFACE 



comprise the carbohydrates. Simple sugars Hke dextrose 

 have the formula, C6.H12.O6, secondary sugars 2(C6Hi206) 

 — HoO, or CioHooO]], whilst more complex carbohydrates 

 are only more molecules of simple sugars piled up. 



Whereas plants because of their photosynthetic power 

 can in sunlight or in artificial light produce sugars from COo 

 and HoO in the presence of the green substance, chlorophyl, 

 and from sugar thus formed synthesize lipins and proteins 

 when N is present, most animals ingest food as protein, 

 carbohydrate and lipin. Before utilization by the animal 

 organism, these compounds must be broken down, i.e., 

 digested, into their component parts: proteins to amino 

 acids, carbohydrates to dextrose and laevulose and lipins 

 to fatty acids and glycerol. These fractions are built up 

 again by the organism into the more complex molecules. 

 Special significance is attached to the synthesis of amino 

 acids, for the proteins thus formed by the organism are 

 peculiar to it and not to the organism which furnished them; 

 thus herbivorous animals, cattle for example, convert the 

 plant proteins of their food into proteins peculiar to beef. 



The break-down of these compounds is accomplished by 

 enzymes, chemical bodies which act much as catalysts — 

 i.e., by accelerating chemical changes. Enzymes show a 

 high degree of specificity. For proteins, there exist the 

 enzymes like pepsin found in the human stomach and 

 trypsinogen, produced by the pancreas; for the starches 

 is the enzyme, amylase; for the sugars, sucrose, maltose, 

 lactose, the enzymes are sucrase, maltase, and lactase 

 respectively; for the lipins, lipase. The break-down is 

 one in which water is used, a so-called hydrolytic cleavage. 

 Enzymes also synthesize the end-products of digestion into 

 proteins, carbohydrates and lipins in which case water is 

 lost. Some enzyme-reactions are reversible: thus, the 

 reaction lipin plus lipase = fatty acid plus glycerol, or 



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