. Principles of modern biology. Biology. 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 form
. Principles of modern biology. Biology. 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 (), are avail- able. When glucose or some other m
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