176 - The Cell 



maltase) into the sm rounding bread or other 

 food. Thus digestion occurs externally, and 

 the resulting glucose is absorbed by the 

 rhizoids. The rhizoids serve also lor the ab- 

 sorption of the other essential nutrients, in- 

 cluding water, salts, and oxygen. The distri- 

 bution of the food substances proceeds by 

 diffusion and protoplasmic streaming, via the 

 rhizoids to the stolons, and via the stolons 

 to the sporangiophores. The metabolism of 

 the mold closely resembles that of the yeast, 

 except that anaerobic growth does not occur. 

 Also the mold (to a greater extent than yeast) 

 utilizes nitrate ( — NO :! ) salts for the synthesis 

 of amino acids and proteins. All energy is de- 

 rived from preformed organic substances 

 (chiefly carbohydrate) that are absorbed. 

 Respiration and excretion are localized 

 mainly in the rhizoids, because the upper 

 parts of the mycelium are too dry to permit 

 exchanges with the air. The only significant 

 excretory waste is water, since carbohydrate 

 is the main fuel. 



Economic Importance of the Yeasts and 

 Molds. A wide variety of yeasts and molds 

 have been known to biologists for many 

 years, and investigations on the metabolism 

 of these organisms have gradually revealed a 

 number of by-products that are very im- 

 portant in medicine and industry. More than 

 100,000 tons of yeasts are produced annually 

 in the United States, and various kinds of 

 fermentation are used in the production of 

 citric acid, glycerol, acetone, purified fats, 

 and a number of other valuable organic com- 

 pounds. Likewise, yeasts are widely used in 

 the production of vitamins, and massive yeast 

 cultures are beginning to be used as direct 

 sources of man's food. 



Molds and bacteria are also very impor- 

 tant as sources of antibiotic compounds, such 

 as penicillin, streptomycin, and aureomycin, 

 compounds that now are widely used as 

 therapeutic agents in many serious infectious 

 diseases (p. 580). Various molds are also use- 

 ful in the curing of hides, tobacco, coffee, 

 cocoa, paper pulp, etc.; and to some extent 

 the yeasts and molds cooperate with the bac- 



teria in achieving the decay of organic mat- 

 ter (see below). 



Nutrition of the Saprophytic Bacteria: 

 Decay and Putrefaction. Although bacteria 

 are among the smallest and, from the view- 

 point of visible structure, the simplest of 

 organisms, they display a remarkable diver- 

 sity in their nutritive processes. Among the 

 saprophytic forms, which constitute a large 

 majority ol the bacteria, many variations 

 exist as to the chemical nature of the organic 

 substances required by the different species 

 for the fulfillment of their metabolic needs. 



Bacteria lack chlorophyll, although one 

 small group, the purple bacteria, possesses a 

 chlorophyll-like pigment, and can utilize 

 light to energize some phases of their metab- 

 olism (p. 187). Bacteria possess relatively 

 rigid cell walls, composed of organic sub- 

 stances other than cellulose. Accordingly, 

 it is upon rather arbitrary grounds that 

 the bacteria are classified among the 

 plants. 



Although the nutritional variation among 

 bacteria is very great, the variation in form 

 is generally limited to three common shapes. 

 Among bacterial species there are: (1) spher- 

 ical forms, called cocci, (2) rod-shaped forms, 

 called bacilli, and (3) spiral forms, called 

 spirilla (Fig. 10-1). Man) bacteria are strictly 

 unicellular, but others form colonial aggre- 

 gates of greater or lesser permanence; and 

 bacterial colonies are usually named accord- 

 ing to the arrangement of the cells in the 

 aggregate. Irregular clusters are designated 

 by the prefix staphylo-, as in staphylococcus; 

 chainlike forms are specified b\ the prefix 

 strepto-, as in streptococcus or streptoba- 

 cillus. 



The great importance of saprophytic bac- 

 teria is that their digestive and metabolic 

 processes effect the decomposition of a vast 

 quantity and a rich variety of organic mate- 

 rials that otherwise would tend to accumu- 

 late on earth in a form not suitable for use 

 by other organisms. Without the saprophytic 

 bacteria all decay would be eliminated, ex- 

 cept for that relatively small fraction -which 



