118 GENERAL BIOCHEMISTRY 



rates great enough for the needs of living cells. Life appears to depend 

 upon continuously and rapidly available supplies of many substances. 

 Thus cells require a large variety of effective reaction catalysts, and as 

 a result they contain many different enzymes. The particular proper- 

 ties of enzymes are discussed separately in Chapter 6. 



Proteins perform special transport functions. Many compounds 

 penetrate cells and accumulate inside at much higher concentrations 

 than expected from diffusion alone. These transfers require the input 

 of energy needed to produce a higher concentration inside than out- 

 side the cell and are usually termed active transport. The mechanism 

 of active transport is still not clear, but it does involve proteins either 

 as catalysts or as adsorbents or both. In some cases of active transport, 

 enzymes convert a substrate to some other derivative which may then 

 pass through the cell membranes. However, if only free diffusion con- 

 trolled the passage, the derivative ought to diffuse out into the en- 

 vironment at the same time. Usually such products are not found out- 

 side the cells, so perhaps they are adsorbed on proteins too large to 

 escape from the cell and accumulate inside in the form of protein 

 complexes. 



In higher animals there are circulatory systems which basically are 

 colloidal systems containing suspended cells in many species but with- 

 out cells in other species. In either case, the respiratory pigments 

 present form complexes with oxygen in regions like the lungs where 

 the partial pressure of the gas is high. These complexes circulate in 

 the blood to organs where the demand for oxygen has reduced the 

 partial pressure. Satisfaction of the equilibrium constant then leads to 

 dissociation of the complex and a supply of oxygen for the tissues 

 utilizing it. Other gases, including the toxic carbon monoxide, may 

 be transported in the same way. Likewise, non-gaseous solutes appear 

 to be transported as adsorption complexes with blood proteins, partic- 

 ularly with serum albumin. Certain vitamins, for example, associate 

 with serum albumin as blood circulates past the digestive tract. Dis- 

 sociation of these complexes then supplies required vitamins to those 

 cells not directly in contact with the food supply. 



Proteins serve as fibrous components of the tissues holding the rigid 

 skeletal elements together. Collagen is the fundamental material of 

 connective tissue; it serves primarily in a structural role. Muscle, also 

 largely protein, fills structural needs, but its principal function is 

 mechanical, being the tissue in which chemical energy is converted 

 into mechanical energy necessary for physical activity. 



Certain of the nucleoproteins serve as carriers of genetic traits and 



