Organisms utilize various strategies for regulation of body temperature.

Define the term endotherm.

Suggest one physiological advantage and one physiological disadvantage that ectotherms have compared to endotherms.

During an evolutionary physiology symposium, two researchers made the following claims regarding thermoregulation:

Researcher X: "Endothermic organisms rely solely on increasing metabolic respiration to maintain a constant core temperature during cold exposure, without employing any behavioral adaptations."

Researcher Y: "Ectothermic organisms possess no physiological capacity to alter heat exchange with their environment and must rely exclusively on behavioral postures and microclimate selection."

Discuss, using examples, whether these two claims are scientifically accurate.

In response to prolonged exposure to cold climates, the mammalian thyroid gland is stimulated to secrete the hormone thyroxine. Thyroxine enters hepatocytes (liver cells) by crossing the lipid bilayer and binds to intracellular thyroid hormone receptors. This stimulates the transcription of genes encoding mitochondrial proteins, which increases the basal metabolic rate (BMR) to generate heat.

Suggest two physiological advantages of using thyroxine-mediated metabolic upregulation over muscular shivering for long-term thermoregulation in a cold environment.

Thyroxine is a lipid-soluble hormone that crosses the cell surface membrane of hepatocytes, whereas peptide hormones are polar and must bind to cell surface receptors. State two properties or functions of the cell surface membrane that are illustrated by these differing mechanisms of hormone action.