Discuss the Advantages and Disadvantages of being Ectothermic and Endothermic for Vertebrates.

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Discuss the Advantages and Disadvantages of being Ectothermic and Endothermic for Vertebrates.

Vertebrates can be found all over the world, from the freezing poles to the hot deserts. The normal air temperature in these regions varies from -40?C to 50?C respectively. The majority of living organisms exist within confined limits of temperature, (approximately 10-35?C), but various organisms show adaptations enabling them to exploit geographical areas at both extremes of temperature.

Temperature indicates the amount of heat energy in a system, and is a major factor determining the rate of chemical reactions. The most important reactions which are inhibited by inappropriate temperature are those that are catalysed by enzymes. Below freezing point, cells may freeze, and the cell structure destroyed by formation of ice crystals. Above 45?C enzymes usually become denatured, ceasing to function; in both of these cases, the organism dies. Therefore, if vertebrates did not regulate their body temperature they would be unable to survive outside a narrow range of temperatures.

All animals derive heat from two sources: the external environment and from the release of chemical energy within their cells. The extent to which animals are able to generate and conserve this heat depends upon physiological mechanisms associated with their phylogenetic position. The possible evolution of the endotherms will be discussed later.

Until recently, animals were classified as poikilotherms or homeotherms; reference to their being respectively cold or warm-blooded animals. The word poikilothermic (Greek poikilos = changeable) refers to the fact that the temperature of a cold-blooded animal fluctuates with that of its surroundings. For example, a fish has the temperature of the water it swims in. However, a deep-sea fish that spends its entire life in water that has barely measurable temperature fluctuations is really an animal with a constant body temperature - hence it would be fair to describe it as a homeotherm, a term reserved for birds and mammals. Consequently, the new terms of ectothermy and endothermy are used. An ectotherm has a high rate of thermal conductance and a low rate of heat production; the body temperature is therefore determined by the environment. However, endotherms are capable of raising the temperature of their tissues above that of the environment, due to the heat from metabolism.

Heat can be transferred by a number of means (Adams & Iampietro, 1968):

* "Conduction...thermal energy exchange through a medium or between objects in a physical contact by the transfer of intramolecular energy, not involving the transfer of material."

* "Convection...route of thermal energy flow depending on the movement of a fluid over a surface which is at a different temperature."

* "Radiation...heat transfer by the exchange of electromagnetic energies between facing surfaces..."

* "Evaporation...depends upon thermal transfer in the conversion of a material from a liquid to a gas phase."

These different factors are utilised by both endotherms and ectotherms, but ectotherms depend on them more than the endotherms (as the endotherms can control their internal temperature).

It is believed that endotherms evolved from ectotherms, but the intermediate steps are not clear. Endotherms are an inverse of ectotherms; where ectotherms rely upon thermal conductance to increase body temperature, endotherms control their body temperature by heat produced from metabolism, and they have a low thermal conductance to retain this heat. Endotherms have up to five times more mitochondria than ectotherms, and in addition, Akhmerov found that in certain endotherms oxidative phosphorylation in the mitochondria was uncoupled, thereby producing heat and not ATP (1986). It has been proposed that insulation evolved at first, as this would have limited a fluctuation in body temperature (Crawshaw et al., 1982), aiding specialisation of biochemical processes.

Ectothermy

The majority of animals are ectothermic, and their activity is determined by the prevailing environmental temperature. The metabolic rate of ectotherms is relatively low and as previously mentioned, they lack mechanisms for conserving heat. As a result, aquatic vertebrates such as fish usually have a body temperature which is at thermal equilibrium with that of the water. Fish cannot maintain a temperature below that of the water but may in some cases, such as that of the tuna, retain heat by means of a countercurrent heat exchanger system. This can raise the temperature of the 'red' swimming muscle to about 12?C above that of the sea water.
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Terrestrial ectotherms have to contend with greater temperature fluctuations than those of the aquatic ectotherms, but they have the benefit of living at higher environmental temperatures. This allows them to be more active, and show a variety of complex behavioural patterns based upon prevailing temperature conditions. Many species are capable of maintaining temperatures slightly above or below that of the air, and thereby avoid extremes. The relatively poor thermal conductivity of air reduces the rate of heat loss from organisms, whilst water loss by evaporation may be used to cool the organism.

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