14 2 Body Temperature Strategies As environmental temperature fluctuates heat transfer between the environment and an animal will also fluctuate Some habitats have fairly stable temperatures such as the deep sea or areas around Earth s equator Most habitats experience moderate fluctuations in temperature over the course of a day and from season to season Animals must be able to maintain physiological function despite these fluctuations which affect rates of chemical reactions and thus physiology Some animals do this by maintaining a steady body temperature These animals are called homeotherms Other animals allow their body temperature to fluctuate with the environmental temperature Instead of keeping Tb constant to maintain bodily functions they use other mechanisms These animals are called poikilotherms All animals generate heat When cells catabolize molecules they break chemical bonds which releases heat As the heat radiates out of the body it will warm the animal s tissues Some animals are able to regulate the amount of heat produced by metabolism These animals are endotherms because they regulate their body temperature by regulating heat that is generated internally Other animals while they still generate heat via chemical reactions are unable to control this to regulate their body temperature These are ectotherms because they are dependent on heat from external sources The figure below shows where major animal groups fall among these categories However these are not absolute categorizations Most animals are not 100 homeothermic vs poikilothermic nor are they 100 endothermic vs ectothermic These descriptions can vary depending on the region of the animal s body regionally and in different times of day or seasons temporally Most animals are best described by a combination of these terms Homeothermic animals have a thermoneutral zone that differs from species to species This is a range of temperatures bounded by a lower critical temperature and an upper critical temperature Within this range physiological processes are able to proceed without any thermoregulatory mechanisms The animal s metabolic rate is normal basal metabolic rate However below the lower critical temperature the animal must increase its heat production H to compensate for the heat lost H to the environment An increase in metabolic rate and thus exothermic catabolic reactions like glucose metabolism during cellular respiration can compensate down to a certain point beyond this point the animal s Tb will fall hypothermia which will ultimately be fatal Above the upper critical temperature the animal must actively cool itself to compensate for the heat absorbed from the environment This will also require an increase in metabolic rate A homeotherm is able to actively cool itself to a point beyond this point the animal s Tb will increase hyperthermia which will ultimately be fatal Homeotherms use a variety of ectothermic and endothermic processes to generate heat and actively cool in order to maintain Tb within the thermoneutral zone Unlike homeotherms poikilotherms do not attempt to maintain a steady Tb they do not attempt to keep H as close to zero as possible Thus poikilotherms do not have a thermoneutral zone or upper or lower critical temperatures Like all living organisms their cells are still affected by temperature so they have preferred Ta s at which their physiological processes function optimally Poikilotherms are able to tolerate a range of temperatures around their preferred temperature this range is bounded by the animal s incipient lower lethal temperature and incipient upper lethal temperature which are Ta values at which 50 of individuals die because it is too cold or too hot respectively If an individual animal is kept at a particular temperature for some period of time it can become acclimated to that temperature Its body can undergo temporary modifications that make it more efficient at a given temperature If an animal is acclimatized to a warmer temperature its IULT increases in other words it can tolerate a higher temperature before dying At some point the animal s ILLT will also increase in other words it will lose the ability to tolerate lower temperatures before dying Poikilotherms can be roughly grouped based on whether the range of ambient temperatures they can tolerate is wide or narrow Eurytherms can tolerate a wide range of ambient temperatures before dying while stenotherms can tolerate a narrow range These concepts are illustrated in the graphs below Recall that across habitats of different temperatures Km values of enzymes are conserved One would expect that at colder temperatures chemical reactions occur more slowly and at higher temperatures intermolecular forces change As molecules get hotter hydrogen bonds and van der Waals forces are disrupted and hydrophobic interactions become more stable As they get colder most of the weak bonds that hold proteins together are strengthened This decreases the ability of the protein to breathe which describes the conformational changes enzymes undergo as they catalyze reactions Thus enzymes are less able to catalyze reactions as they get colder How do poikilotherms conserve Km and maintain chemical interactions at rates that allow them to function in the face of changes in Ta Notice in the graph that illustrates Km conservation below all of these animals are poikilothermic In poikilotherms this conservation is not facilitated by maintenance of body temperature Some of the mechanisms that allow poikilotherms to maintain cellular function include Remodel membranes to maintain fluidity homeoviscous adaptations Cold adapted enzymes in psychrotrophs Heat shock proteins Freeze tolerance extracellular nucleators intracellular solute concentrations Freeze avoidance antifreeze proteins The lipid tails within the phospholipid bilayer of the cell membrane are held together by van der Waals forces which are temperature dependent The interactions between lipid tails are essential to maintaining the fluidity of the membrane If the membrane becomes too cold it will stiffen Its shape will change it can become brittle and the cell will be unable to regulate traffic in and out of the cell as the membrane becomes a solid layer encasing the cytosol If the membrane becomes too hot and the van der Waals forces are disrupted it will become too fluid and the proteins embedded in it will not be held in place a disordered measure called anisotropy The membrane will
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