Essay C, Student 12, Marker 5

Describe and Discuss the Influence of Environmental Temperature on the Metabolism of Poikilotherms and Homeotherms

Poikilotherms are animals whose body temperature tends to fluctuate, more or less following the temperature of the environment. By contrast homeotherms are animals that regulate their own body temperature within a narrow range, regardless of the environmental temperature (Campbell et al. 2008). Therefore environmental temperature has a very different effect on each of their metabolisms, which are catalysed by enzymes and therefore temperature dependent. This essay will explore the effect of environmental temperature on the metabolism of animals, looking first at poikilotherms and then homeotherms.

The temperature of the environment has a direct effect on the metabolic rate of poikilotherms as their body temperature more or less follows environmental temperature. As environmental temperature increases, their metabolic rate increases exponentially as the enzymes which catalyse the reactions have increased rates of reaction (Randall, Burggren, French. 2002). The amount metabolic rate increases as temperature increases differs in different poikilothermic animals.[e1] It can be measured by a temperature quotient (Q10), where the change in metabolic rate of an animal is measured for a 10°C change in temperature (Randall, Burggren, French. 2002). The Q10 for the majority of poikilotherms is between 1-3, for example most fish have a Q10 of 1.86, which means their metabolic rate is multiplied by 1.86 with every 10°C increase in temperature. However, if the environmental temperature increases too much it passes the critical thermal maximum (CTM) of the poikilotherm and the metabolic rate of the animal starts to decrease as its enzymes are denatured by the high temperatures, which can be fatal (Randall, Burggren, French. 2002).[e2]

Many poikilothermic species can become acclimatised to environmental temperature if they have sustained exposure to it, for example a change in seasons (Purves et al. 2004). This acclimatisation includes metabolic compensation, where the organism alters its metabolic rate to counter the effects of temperature change, for example many fish in cold temperatures over winter increase their metabolic rate (Bullock. 1955). This is possible by the organism producing variants of their enzymes that have the same function but have different optimal temperatures (Somereo. 1969).

In contrast to poikilotherms, homeotherms body temperature does not follow the temperature of the environment, which means environmental temperature does not affect their metabolism directly. However homeotherms maintain a constant temperature because the majority are endothermic, meaning they use their metabolism to generate their own heat, which means the environmental temperature does affect homeotherms metabolism indirectly. At moderate temperatures within the thermal neutral zone (TNZ) a homeotherm's metabolism is at the basal metabolic rate (BMR) as this produces enough heat to balance heat loss to the environment. It can regulate its body temperature within the TNZ through adjusting the thermal conductance of its body surface, which is metabolically inexpensive (Randall, Burggren, French. 2002).

However if the environmental temperature decreases below the TNZ it reaches the homeotherm's lower critical temperature (LCT). Below the LCT the BMR of the animal is not sufficient in balancing heat production with heat loss, and so in order to maintain its body temperature the animal needs to increase its heat production by using metabolic energy (Randall, Burggren, French. 2002). This means as environmental temperature decreases below an animals LCT, the animals metabolism increases. The ways it can produce heat metabolically, known as thermogenesis, are by shivering and by non-shivering thermogenesis. Shivering uses the contraction of skeletal muscles to convert ATP into ADP, which releases energy in the form of heat.[e3] In non-shivering thermogenesis certain hormones cause mitochondria to increase their metabolic activity and produce heat instead of ATP. This non-shivering thermogenesis can occur throughout the body, but mostly in specialised adipose tissue called brown fat which is specialised for rapid heat production by having abundant mitochondria and a rich blood supply (Purves et al. 2004). If environmental temperature falls too far the animal enters a state of hypothermia where its regulating mechanisms fail, its body temperature falls and its metabolic rate drops, which can eventually lead to death (Randall, Burggren, French. 2002).

If the environmental temperature increases above the TNZ of an homeotherm it reaches its upper critical temperature (UCT). Above the UCT heat loss in the animal is not balanced with heat production by the BMR, and so the animal must actively dissipate heat which uses metabolic energy (Randall, Burggren, French. 2002). This means as temperature increases above the UCT of an animal, the animals metabolism increases. Homeotherm's active heat-dissipating mechanisms are sweating and panting. These both use the evaporation of water to cool the body, as one gram of water absorbs about 285 calories (2448 J), and so is a highly effective way of removing excess body heat if enough water is available (Randall, Burggren, French. 2002). If the environmental temperature rises too far, not enough heat can be lost and the animal's metabolic rate drops as it enters a state of hyperthermia which can lead to death (Randall, Burggren, French. 2002).

In conclusion, the temperature of the environment has a direct effect on the metabolism of poikilotherms, with an increase in environmental temperature causing an exponential increase in metabolic rate in the animal. However poikilotherms can become acclimatised to environmental temperature if exposed for a sustained time, adjusting their metabolic rate. In homeotherms, if the environmental temperature falls within the TNZ of the animal, its metabolism remains at BMR. However if it falls below this, it causes the metabolic rate to rise as the animal uses thermogenesis to keep warm, and if the environmental temperature rises above the TNZ, the metabolism also rises, as the animal uses active heat-dissipating mechanisms to cool down. So all in all environmental temperature has a very important and significant influence on the metabolism of both poikilotherms and homeotherms.

Word count: 956

References

Bullock, T.H. (1955) Compensation for temperature in metabolism and activity of poikilotherms. Biological Reviews, 30, (3) 311-342

Campbell, N. A., Reece, J.B., Urry, L.A., Cain, M. L., Wasserman, S.A., Minorsky, P. V., Jackson, R. B. (2008) Biology (Eighth edition). Pearson Benjamin Cummings, San Fransisco.

Purves, W. K., Sadava, D., Orians, G. H., Heller, H. C. (2004) Life -The Science of Biology (Seventh edition). Sinauer Associates, Inc., Sunderland, Massachusetts.

Randall, D., Burggren, W., French, K. (2002) Ekert Animal Physiology – Mechanisms and Adaptations (Fifth Edition). W.H. Freeman and Company, New York.

Somero, G. N. (1969) Enzymic mechanisms of temperature compensation: immediate and evolutionary effects of temperature on enzymes of aquatic poikilotherms. The American Naturalist, 103, (933) 517-532

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