. Principles of modern biology. Biology. 582 - Heredity and Evolution matter. If protoplasm freezes,2 the formation of ice crystals tends to isolate the water as a separate phase—and without water, the protoplasmic structure disintegrates beyond repair. Conversely, if protoplasm continues to absorb heat, even though the heat capac- ity is high, a point is finally reached where the enzymes and other protein components of the cell become denatured, and the proto- plasmic structure is destroyed. Although living organisms can remain ac- tive only when the body temperature keeps within certain limi
. Principles of modern biology. Biology. 582 - Heredity and Evolution matter. If protoplasm freezes,2 the formation of ice crystals tends to isolate the water as a separate phase—and without water, the protoplasmic structure disintegrates beyond repair. Conversely, if protoplasm continues to absorb heat, even though the heat capac- ity is high, a point is finally reached where the enzymes and other protein components of the cell become denatured, and the proto- plasmic structure is destroyed. Although living organisms can remain ac- tive only when the body temperature keeps within certain limits, many species possess adaptations that permit them to inhabit regions of unusual heat or cold. This is especially true of homeothermic animals— namely the birds and mammals—which pos- sess mechanisms for controlling the body temperature at a relatively steady level even when there are great changes in the environ- mental temperature. But even the birds and mammals vary widely as to their capacity to endure the extremes of cold and heat. Cold- adapted species (Figs. 30-8 and 30-9) tend to possess specialized integumentary coverings (feathers or fur) and thickened layers of sub- cutaneous fatty tissue—which are effective in insulating the body against the loss of heat; and polar species are apt to maintain a very high basal metabolism—which produces heat as a by-product (p. 436). Conversely, tropical species tend to limit their basal metabolism to a minimum, and such organisms usually display a well-developed capacity for dissipat- ing heat from the body surfaces. Poikilothermic organisms, in which the body temperature changes constantly with the surrounding temperature, likewise are variously adapted in relation to the tempera- 2 By the "quick-freezing" technique, which involves a very rapid reduction to subfreezing temperatures, protoplasm may solidify without the separation of ice crystals, and without a disruption of structure. How- ever, the conditions
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