174 - The Cell 



mycelium displays a syncytial organization; 

 in others it consists of distinct cells, indi- 

 vidually separated by transverse cell walls 

 (Fig. 10-3). 



Fig. 10-2. Yeast cells, budding. The light spots are fat 

 droplets and cell-sap vacuoles; the nuclei are not visi- 

 ble in living, unstained cells. 



SAPROPHYTIC NUTRITION 



The saprophytic mode of nutrition is dis- 

 played by most of the yeasts, molds, and 

 bacteria. Saprophytic organisms, like ani- 

 mals, require at least a minimum of pre- 

 formed organic food; but lacking a digestive 

 cavity, saprophytes must absorb their organic 

 nutrients directly from the environment. 

 Therefore the localities favorable for the 

 growth of saprophytes are very limited. Such 

 forms are found only in places where con- 

 siderable quantities of organic materials have 

 accumulated, such as upon soil richly laden 

 with humus (decomposing plant material), 

 or directly upon the remnants of other or- 

 ganisms. 



Nutrition of Yeasts. Many wild yeasts 

 grow saprophytically upon sweet fruits that 

 have been crushed in falling to the ground. 

 The most important yeasts, however, are the 

 kinds that are cultivated by man because of 

 their usefulness in brewing and in bread- 

 making (Fig. 10-1). 



Yeast cannot groiv like a green plant in a 

 medium containing only inorganic sub- 

 stances. But yeast will thrive vigorously in 

 a solution containing one or more sugars 

 (glucose, fructose, mannose, sucrose, or malt- 

 ose) oxygen, and a suitable variety of inor- 

 ganic salts. Lacking chlorophyll, the yeast 



ASPERGILLUS 



MUCOR / 



Fig. 10-3. Small portions of the mycelia of two com- 

 mon mold fungi. The black spots are nuclei. Note that 

 Mucor is a syncytial organism, whereas Aspergillus 

 is typically cellular. 



cannot synthesize its own sugar; but given 

 "ready-made" sugar as a source of energy and 

 matter, it can carry on metabolism quite like 

 a green plant. Like the green plant it can 

 utilize inorganic nitrogen to synthesize all 

 essential amino acids and proteins, although 

 yeasts grow better when ammonium (NH+) 

 salts, rather than nitrates (NO.7), are avail- 

 able. 



When glucose or some other monosaccha- 

 ride is present in the surrounding medium, 

 the yeast absorbs the sugar directly, without 

 recourse to any sort of digestive process. But 

 when the available sugar is a disaccharide, 

 such as sucrose (or maltose), digestion must 

 precede absorption. In this case, the enzyme 

 sucrase (or maltase) is extruded from the 

 yeast cells, and digestion occurs externally, 



