Dqs

physical energy stream physical energy stream

reproduction developmental program

Figure 5.17 a: Modular growth of an accreting organism is governed by the positive feedback inherent in diffusion-limited accretion. b: In more conventional organisms, growth and development are brought under the control of negative feedback systems driven by the chemical energy from food.

reproduction developmental program

Figure 5.17 a: Modular growth of an accreting organism is governed by the positive feedback inherent in diffusion-limited accretion. b: In more conventional organisms, growth and development are brought under the control of negative feedback systems driven by the chemical energy from food.

form, and the energetics of glucose breakdown are pretty much set by its thermodynamic properties, unaffected by the vagaries of a chaotic and unpredictable environment. Perhaps the increasing degree of physiological "in-sourcing" among the higher animals reflects the conversion from a physical energy economy for powering physiology to a chemical energy economy. The greater predictability of glucose fuel may, in turn, have promoted the engineering of the internal environment to ensure the conditions for its effective utilization. And indeed, most of the major physiological functions operate to provide a degree of stability to the internal environments of animals. The phenomenon is known as homeostasis, the process of ensuring the proper conditions of temperature, pH, and solute concentration and the organized delivery and distribution of nutrients, fuel, oxidant, and wastes throughout the body.

We will be delving into homeostasis in earnest in the later chapters of this book, in particular in Chapters 11 and 12. For now, suffice it to say that many homeostatic processes operate through another type of feedback system, so-called negative feedback (Fig. 5.18). As the name implies, negative feedback is the opposite of positive feedback: it is the consequence of a process that negates the process itself. A thermostat in a room is a familiar example of negative feedback: if the room gets too cold, the thermostat activates a heater, which reverses the decline of room temperature.

Homeostasis does a lot of good things for organisms, among them enabling animals to exploit a wider range of environments than would be exploited if they did not have it. So, for example, if a rat's body temperature could not deviate far from 38oC without fatal consequence, a rat with unregulated body temperatures would be limited to a narrow selection of fairly warm environments. Give the rat a mechanism that keeps body temperature constant, though, and the rat could move into much colder or warmer environments. The benefits do not come free, of course: homeostasis car-

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