Endotherms

Endotherms As you learned in Unit 2, the mitochondria in the cells produce the energy for all of the biochemical processes. However, all of the energy that is available in the glucose molecules does not go into the production of ATP. Some of it is converted into thermal energy. Scientists have estimated that up to 25 % of the basal metabolic rate of most endotherms can be attributed to the energy that is consumed to offset this loss of thermal energy. The body cells of endotherms contain far more mitochondria, and they are proportionately larger than the body cells of ectotherms. Birds and mammals make up the majority of endotherms, and they have the most elaborate thermoregulatory mechanisms of all animals. Their temperature set points lie within a narrow range: 39 to 42 °C for birds and 36 to 39 °C for mammals. A given species may experience seasonal environmental variations of 70 °C or more, depending on its habitat, but can survive these variations and thrive in almost any environment. emergence from burrow 6 8 10 12 14 16 18 20 24 28 32 36 40 Time of day (h) Body temperature (°C) Figure 3 This graph shows the rapid change in a lizard’s body temperature when it moves from its cool burrow into the sunlight. thermal acclimatization the process by which an animal gradually adjusts to temperature changes in its environment 438 Chapter 9 • Homeostasis: A Fine Balance NEL Thermoreceptors on the skin and within the body alert the nervous system to any changes in external or internal temperature. If the core temperature drops below the set point, the body responds by constricting the arteries that supply blood to the skin. This reduces the amount of thermal energy that is lost to the environment by radiation. Shivering and “goose bumps” also regulate the internal temperature. Muscle contractions during shivering generate thermal energy, and the raised hairs in the skin trap air, helping to maintain warmth. If the core temperature rises above the set point, the body relaxes arteries to allow excess thermal energy to exit via the skin. In a small number of mammals, including humans, the skin contains large numbers of sweat glands. These secrete sweat onto the skin, which removes thermal energy as it evaporates. Figure 4 summarizes the thermoregulatory responses in humans.

Summary of Endotherms

Endotherms, primarily birds and mammals, possess specialized mechanisms for thermoregulation, allowing them to maintain a stable internal temperature despite external environmental fluctuations. Key points include:

  • Energy Production: Mitochondria in endotherm cells generate energy, but approximately 25% of the basal metabolic rate is used to counteract thermal energy loss.

  • Cellular Structure: Endotherm cells contain more and larger mitochondria compared to ectotherms.

  • Temperature Set Points: Birds maintain a body temperature of 39 to 42 °C, while mammals range from 36 to 39 °C. They can endure seasonal temperature variations of over 70 °C.

  • Thermoregulatory Mechanisms:

    • Thermoreceptors: Detect changes in internal and external temperatures, signaling the nervous system.

    • Response to Cold: When core temperature drops, blood vessels constrict to minimize heat loss, and mechanisms like shivering generate heat.

    • Response to Heat: When core temperature rises, blood vessels dilate to release heat, and sweat glands (in some mammals) secrete sweat to cool the body through evaporation.

These adaptations enable endotherms to thrive in diverse environments while maintaining homeostasis.